Roles of Antiferromagnetic Fluctuation in Vortex States in Superconductors with Strong Pauli-Paramagnetic Eﬀect

Kazushi Aoyama,∗ Ryusuke Ikeda †

Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

The heavy fermion material CeCoIn5 is a spin singlet d-wave superconductor with strong Pauli-paramagnetic effect. In both cases with a magnetic field parallel and perpendicular to the two-dimensional plane of CeCoIn5, a high field and low temperature (HFLT) superconducting phase, surrounded by a discontinuous Hc2(T) transition and a continuous structural transition from the familiar Abrikosov phase in lower fields, appears1. Theoretical studies taking account both of the paramagnetic and orbital depairings indicate that the HFLT phase corresponds to a Fulde-Ferrell-Larkin-Ovchinikov vortex lattice modulating along the magnetic field2. On the other hand, according to transport data suggesting the presence of quantum critical antiferromagnetic (AF) fluctuation near Hc2(0), the HFLT phase of CeCoIn5 should be described by assuming both the Pauli-paramagnetic and AF fluctuation effects to be strong. Based on such background, we study the superconducting vortex lattice in the presence of field-induced AF fluctuations as well as Pauli-paramagnetic effect. The field- induced AF fluctuation is important even in lower fields below the HFLT phase. Neutron scattering data on CeCoIn5 in the perpendicular field shows that the vortex lattice form factor increases with field, which is in contrast to the typical field dependence of the form factor in type superconductors3. Although such an anomalous behavior is qualitatively explained as a result of strong Pauli-paramagnetic effect4, it is not sufficiently explained in the high field region. At a glance, it seems difficult that one attribute the high-field anomaly to the AF fluctuation because the quasiparticle damping induced by the AF fluctuation su- presses the paramagnetic effect. In this study, we investigate the superconducting vortex lattice based on the microscopic calculation of Ginzburg-Landau approach. We find that, in the case of sufficiently strong Pauli-paramagnetic effect, the AF fluctuations rather enhances the Pauli-paramagnetic effect in the flux distribution, and that the observation in Ref.3 may be explained by taking account both of the paramagnetic effect and the AF fluctuation.

[1] A. D. Bianchi et al., Phys. Rev. Lett. 91, 187004 (2003); T. Watanabe et al., Phys. Rev. B 70, 020506(R) (2004). [2] R. Ikeda, Phys. Rev. B 76 134504 (2007). [3] A. D. Bianchi et al., Science 319, 177 (2008). [4] M. Ichioka and K. Machida, Phys. Rev. B 76 064502 (2007). —————– ∗E-mail: [email protected] †E-mail: [email protected] Nernst eﬀect in amorphous superconducting ﬁlms A. Pourret, P. Spathis, H. Aubin,∗ K. Behnia

Laboratoire Photons et Mati`ere (CNRS), ESPCI, 10 rue Vauquelin, 75231 Paris, France

1 2 In amorphous superconducting thin films of Nb0.15Si0.85 and InOx , a finite Nernst coefficient can be detected in a wide range of temperature and magnetic field3. Due to the neg- ligible contribution of normal quasi-particles, superconducting fluctuations easily dominate the Nernst response in the entire range of study. In the vicinity of the critical temperature and in the zero-field limit, the magnitude of the signal is in quantitative agreement with what is theoretically expected for the Gaussian fluctuations of the superconducting order parameter. Even at higher temperatures and finite magnetic field, the Nernst coefficient is set by the size of superconducting fluctuations. The Nernst coefficient emerges as a direct probe of the ghost critical field, the normal-state mirror of the upper critical field4. Moreover, upon leaving the normal state with fluctuating Cooper pairs, we show that the temperature evolution of the Nernst coefficient is different whether the system enters a vortex solid, a vortex liquid or a phase-fluctuating superconducting regime5.

[1] H. Aubin, C. A. Marrache-Kikuchi, A. Pourret, K. Behnia, L. Berge, L. Dumoulin, and J. Lesueur. Magnetic-field-induced quantum superconductor-insulator transition in nb0.15si0.85. Phys. Rev. B, 73(9):094521, 2006. [2] P. Spathis, H. Aubin, A. Pourret, and K. Behnia. Nernst effect in the phase-fluctuating superconductor inox. Eur. Phys. Lett., 83(5):57005, 2008. [3] A. Pourret, H. Aubin, J. Lesueur, C. A. Marrache-Kikuchi, L. Berge, L. Dumoulin, and K. Behnia. Observation of the nernst signal generated by fluctuating cooper pairs. Nat. Phys., 2(10):683–686, 2006. [4] A. Pourret, H. Aubin, J. Lesueur, C. A. Marrache-Kikuchi, L. Berge, L. Dumoulin, and K. Behnia. Length scale for the superconducting nernst signal above tc in nb0.15si0.85. Phys. Rev. B, 76(21):214504, 2007. [5] A. Pourret, P. Spathis, H. Aubin, and K. Behnia. Nernst effect as a probe of superconducting fluctuations in disordered thin films. New J. Phys., 11:18, 2009.

∗E-mail: [email protected] Anisotropy and the irreversibility line in 1111 Fe arsenide single-crystals

Luis BalicasA∗

ANational High Magnetic Field Laboratory, Florida State University, Tallahassee-FL 32306, USA

Transport measurements indicate that the superconducting anisotropy in 1111 Fe ar- ab c senides, as estimated through the ratio of upper critical ﬁelds γH = Hc2 /Hc2, is relatively modest when compared to that of the high-Tc cuprates. But in several cases we found it to be temperature dependent1, 2. Nevertheless, we show that a proper description of the angular dependence of the magnetic torque in SmFeAsO0.8F0.2 and SmFeAsO0.9F0.1 single crystals requires, i) a proper procedure to subtract the superimposed magnetic signal and ii) the introduction of a term describing the anisotropy of the penetration depth γλ which 3, 4 is distinct and considerably larger than γH as well as strongly temperature dependent . Both observations are consistent with a multi-gap pairing scenario. Our estimations of the irreversibility ﬁeld Hm(T ), separating the vortex-solid from the vortex-liquid phase in SmFeAsO0.9F0.1 single crystals, indicates that it could be described by the melting of a vortex lattice in a moderately anisotropic uniaxial superconductor. Most importantly, the area occupied by the vortex liquid phase within the H −T phase diagram of the 1111 compounds is rather modest when compared to that of the cuprates. This opens perhaps interesting practical opportunities. Remarkably, indications for a vortex lock-in transition or a kinked vortex structure, common to very anisotropic superconductors, are also clearly observed.

[1] Y. J. Jo, J. Jaroszynski, A. Yamamoto, A. Gurevich, S. C. Riggs, G. S. Boebinger, D. Larbalestier, H. H. Wen, N. D. Zhigadlo, S. Katrych, Z. Bukowski, J. Karpinski, R. H. Liu, H. Chen, X. H. Chen, L. Balicas, Physica C 469, 566 (2009) [2] J. Jaroszynski, F. Hunte, L. Balicas, Youn-jung Jo, I. Raicevic, A. Gurevich, D. C. Larbalestier, F. F. Balakirev, L. Fang, P. Cheng, Y. Jia, and H. H. Wen, Phys. Rev. B 78, 174523 (2008); A. Yamamoto, J. Jaroszynski, C. Tarantini, L. Balicas, J. Jiang, A. Gurevich, D. C. Larbalestier, R. Jin, A. S. Sefat, M. A. McGuire, B. C. Sales, D. K. Christen, and D. Mandrus, Appl. Phys. Lett. 94, 062511 (2009) [3] L. Balicas, A. Gurevich, Y. J. Jo, J. Jaroszynski, D. C. Larbalestier, R. H. Liu, H. Chen, X. H. Chen, N. D. Zhigadlo, S. Katrych, Z. Bukow, arXiv:0809.4223 (2008) [4] L. Balicas, Y. J. Jo, and A. Gurevich, S. Weyeneth and H. Keller, N. D. Zhigadlo, S. Katrych, Z. Bukowski, and J. Karpinski (unpublished)

∗E-mail: [email protected] Large amplitude low frequency velocity ﬂuctuations and its evolution with diﬀerent phases of the driven vortex state

S. S. BanerjeeA,∗ Shyam MohanA,Jaivardhan SinhaA,A. K. SoodB, S. RamakrishnanC, A. K. GroverC

ADepartment of Physics, Indian Institute of Technology, Kanpur 208016, India B Department of Physics, Indian Institute of Science, Bangalore 560012, India C Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India

There have been few studies into the nature of the vortex state prior to the well studied Peak effect (PE) phenomenon. A heterogeneous vortex phase exists in the PE regime, with the fractions of ordered and disordered vortex phases changing as one goes across this regime. We have investigated the nature of the driven and the quasi - static vortex state in weakly pinned single crystals of 2H- NbSe2. The nature of the driven vortex state was probed via transport measurements by studying the time series of the voltage (equivalent to vortex velocity) fluctuations. Our results show an interesting evolution of slow velocity fluctuations in the voltage time series as one sweeps across different phases of the driven vortex matter. The power spectrum of the fluctuations shows peaks at characteristic low frequencies, which evolve with the different phases of the driven vortex state. The nonlinear - nature of the velocity fluctuations with characteristic frequencies was probed via an ac drive superimposed on a dc drive. The amplitude of the fluctuations exhibit a spectacular resonant like behavior when excited with the ac drive at harmonics of the characteristic frequency. We now propose the existence of a regime with coherent dynamics prior to the onset PE, which leads to the unexpected velocity fluctuations in the vortex state prior to PE1. Our earlier investigations into the behavior of dissipation, probed via ac-susceptibility measurements, had revealed some characteristic changes in the vortex state well prior to the PE. We had found evidence of the possible coexistence and transformations between weakly collective and strong pinning phases deep inside the so called homogenous elastic vortex solid phase, well before the PE2.

[1] S. Mohan et al. (submitted). [2] S. Mohan, J. Sinha, S. S. Banerjee and Y. Myasoedov, Phys. Rev. Lett. 98, 027003 (2007).

∗E-mail: satyajit@@iitk.ac.in Matching eﬀects in nanoperforated ultrathin TiN ﬁlms

Tatyana I. Baturina∗

Institute of Semiconductor Physics, 13 Lavrentjev Ave., Novosibirsk, 630090 Russia

We present the results on low-temperature transport measurements of critically disordered (kF l ≈ 1 − 2) nanoperforated films of thickness 5 nm, less than the superconducting coherence length1. Our films comprise of an ultrathin superconducting titanium nitride, patterned with a nanometer square array of holes by means of electron beam lithography and subsequent plasma etching (a center to center spacing is a = 80 nm and 200 nm for different samples). The nanopatterned structures cover the 50 ×100 µm2 area and thus contain about 780000 and 125000 elemental SNS units for the 80 nm and 200 nm period samples, respectively. The magnetoresistance of nanoperforated films exhibit a rich structure including oscillations at low fields, with the period corresponding to the magnetic flux quantum 2 Φ0 = πh/e¯ per unit cell, ∆B = Φ0/a , that merge into a giant peak at higher fields. There is a notable difference between our results on perforated TiN films (with kF l ≈ 1 − 2) and preceeding findings on less disordered perforated films with kF l ≫ 1 (see, for example Refs [2,3]). Namely, we observe the magnetoresistive oscillations in the wide temperature region below Tc, contrasting results of Refs [2,3], where the oscillations were seen near Tc only. Futhermore, the temperature dependence of the amplitude of the oscillations does not follow the corresponding dR/dT behavior proposed in Refs [4,5]. The above oscillations are due to geometric commensurability effects locking the phase of the order parameter. Matching between applied voltage and the amplitude of the order parameter gives rise to yet another remarkable effect: the sharp dips in the differential resistance dV/dI at voltages which com- mensurate with the magnitude of the superconducting gap. This implies the anomalously high charge transmission at matching voltages. The origin of this effect is the nonlocal nature of the charge transfer in multiply connected SNS systems. We propose a mechanism of the correlated transmission of the Cooper pairs in large arrays of SNS junctions based on the simultaneous Andreev conversion processes at many NS-interfaces.

[1] T.I. Baturina et al, JETP Lett. 79 (2004) 337; Physica B 359 (2005) 500; Phys. Rev. Lett. 99 (2007) 257003; JETP Lett. 88 (2008) 752 [2] T.I. Baturina, D.W. Horsell, D.R. Islamov, I.V. Drebushchak, Yu.A. Tsaplin, A.A. Babenko, Z.D. Kvon, A.K. Savchenko, A.E. Plotnikov, Physica B 329 (2003) 1496 [3] Ajay D. Thakur, Shuuichi Ooi, Subbaiah P. Chockalingam, John Jesudasan, Pratap Raychaudhuri, and Kazuto Hirata, App. Phys. Lett. 94 (2009) 262501 [4] M. Tinkham, D.W. Abraham, C.J. Lobb, Phys. Rev. B 28 (1983) 6578 [5] R.S. Newrock, C.J. Lobb, U. Geigenm¨uller,M. Octavio, in: H. Ehrenreich, F. Spaepen (Eds.), Solid State Physics, Academic Press, San Diego (2000) 266

∗E-mail: [email protected] Transport Properties Governed by the Edge Inductance in Bi2Sr2CaCu2O8 H. BeidenkopfA,∗ Y. MyasoedovA, E. ZeldovA, E.H. BrandtB, G.P. MikitikB,C, T. TamegaiD, T. SasagawaE, C.J. van der BeekF, M. KonczykowskiF

ADepartment of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel BMax-Planck-Institut f¨urMetallforschung, Heisenbergstr. 3, D-70506 Stuttgart, Germany C B. Verkin Institute for Low temperature Physics & Engineering, Kharkov 61103, Ukraine D Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan E Materials and Structures Laboratory, Tokyo Institute of Technology, Kanagawa 226-8503, Japan F Laboratoire des Solides Irradi´ees,CNRS UMR 7642 & CEA/IRAMIS/DRECAM, Ecole Polytechnique, 91128 Palaiseau cedex, France

At high temperatures transport current flows uniformly in the bulk of superconducting Bi2Sr2CaCu2O8 samples. At lower temperatures, however, it flows on the sample edges due to dominant edge barriers that impede vortex entry and exit1. These barriers define an effective edge resistance, Re(T ), that becomes smaller than the bulk resistance below a certain temperature. We find that in the presence of an ac current at yet lower temperatures, below the so called Tx transition, the edge resistance becomes even smaller than the effective geometrical inductance of the sample edges, Re(Tx) = ωLe, which thus dominate the sample 2 impedance . Accordingly, below the electrodynamic Tx transition the majority of vortices redistribute from side to side within each half cycle inside the sample to comply with the magnetic field profile self-induced by the ac current rather than cross the edges and flow across the sample bulk. Consequently, the flow of vortices essentially inside a ‘closed box’ becomes insusceptible to the inherent asymmetries of the surface barrier, as manifested in the antisymmetric current-induced magnetic field profile that we measure. The frequency dependence of the transition, Tx(ω), is used to extract the edge resistance that agrees with the simultaneously measured transport resistance after eliminating the c-axis contribution by a novel ion-irradiation scheme, and extends it by three orders of magnitude below transport noise. We further use this method to study the effect vortex melting has on the edge resistance.

[1] D.T. Fuchs et al., Nature 391, 373 (1998). [2] H. Beidenkopf et al., cond-mat 0907.3649 (submitted to Phys. Rev. Lett.).

∗E-mail: [email protected] Geometry-driven Flux Structures in 3D Superconducting Mesostructures

M.A. EngbarthA, A. M¨ullerA, S. DaleA, S.J. BendingA,∗ M.V. Miloˇsevi´cB

ADepartment of Physics,University of Bath, Bath, BA2 7AY, UK BDepartement Fysica, Universiteit Antwerpen, 2020 Antwerpen, Belgium

One of the key goals of contemporary research in the field of mesoscopic superconductivity is achieving “quantum control” of the properties of a superconducting sample via manipulation of its size and shape.1 The influence of the sample geometry is particularly important for vortex matter, as individual vortices in type-II superconductors strongly interact with edge currents. As a result the geometry profoundly influences the stability of fluxon states with different vorticity. It has even been predicted that this can lead to the spontaneous generation of antivortices that allow vortex configurations to comply with the symmetry group of the sample.2 The intermediate state of type I mesostructures is also strongly influenced by geometry due to the role played by surface superconductivity, demagnetisation factors and surface barriers. We report here investigations of three dimensional geometrical control of flux in superconducting mesostructures achieved by exploiting recent breakthroughs in electrochemical deposition on graphite and boron-doped diamond substrates. These have made it possible to tailor the shape of metallic nanostructures by careful control of electrodeposition parameters.3 We have realised a range of highly facetted Pb, Sn and Pb/Sn alloy and core-shell mesocrystals with various shapes, symmetries and sizes, and investigated their magnetic properties using Hall array magnetometry as a function of temperature and applied field. We find that our nominally type I samples exhibit a rich variety of behaviours including the formation of giant vortex and multi-vortex intermediate states. Their properties result from a subtle interplay between the nucleation of surface superconductivity, geometrical shape, temperature-dependent Ginzburg-Landau parameter and surface barriers. Our experiments demonstrate that flux structures formed can be strongly influenced by the sample symmetry and geometry, in good agreement with theoretical Ginzburg-Landau calculations. In particular, our results illustrate the largely unexplored potential of electrochemical deposition for engineering the magnetic properties of superconducting nanostructures.

[1] V.V. Moshchalkov et al., Nature 373, 319 (1995). A.K. Geim, S.V. Dubonos, I.V. Grig- orieva et al., Nature 407, 55(2000). [2] L.F. Chibotaru, A. Ceulemans, V. Bruyndoncx and V.V. Moshchalkov, Nature 408, 833 (2000). [3] Z.L. Xiao, C.Y. Han et al., J. Am. Chem. Soc. 126, 2317 (2004).

∗E-mail: [email protected] The (1 + 1)-dimensional random directed polymer problem

Gianni BlatterA,∗ Vadim GeshkenbeinA,† Victor DotsenkoB,‡ Sergey KorshunovC§

ATheoretische Physik, ETH-Zurich, 8093 Zurich, Switzerland BLPTL, Universit´eParis VI, 75252 Paris, France CL.D. Landau Institute for Theoretical Physics, 119334 Moscow, Russia

We study various aspects of the 1+1-dimensional random directed polymer problem, i.e., an elastic string subject to a Gaussian random force/potential and conﬁned within a plane. Among others, this model serves to understand the weak correlated-pinning problem of vortices in disordered type II superconductors. We concentrate on three related and exactly solvable versions of this problem, the random force (or Larkin) problem, the shifted random force, and the harmonically correlated potential problem. These models approximate the behavior of the random polymer on short scales and hence are traditionally used to provide information within this restricted regime; the results (on the line wandering and on the free- energy distribution function) will be discussed in the light of the full short-range correlated disorder problem.

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] Eﬀect of zero-ﬁeld (annihilation) lines on ac response in superconductors

Leonid BurlachkovA,∗ Eli ShwartzA†

ADepartment of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel

We show that the presence of the zero-field lines (where B = 0) in a superconducting sample can dramatically affect the flux motion and the ac response of high-temperature superconductors. The same lines can be called “annihilation lines” since (in the slab geometry) these are the places where the Abrikosov vortices of different polarity annihilate each other. Such a annihilation line changes significantly the distribution of currents in the sample and has a retardation effect on the flux motion in the whole sample. The effect is mostly pronounced if the frequency ω of the external field is relatively high, such that 1/ω is of the same order of lower than the characteristic relaxation time of vortices in the sample in the flux-flow regime. In this limit the Bean model can no longer be used for the description of vortex motion in the sample1, and one has to solve numerically the equation of flux diffusion 2. One of the results of the retardation effect of the zero-field lines is the appearance of two maxima (per one half period of the external magnetic field) in the voltage associated with the flux motion. One (usual) maximum is in phase with the external field, the other one is out of phase with the field and its position is determined by the zero-field lines appearance in the sample. The presence of two maxima have been recently found out experimentally3, and our analysis forms a theoretical basis for understanding the effect. We study the dependence of the effect on the amplitude of the magnetic field, its frequency as well as on the value of the transport current flowing in the sample and get quite encouraging agreement between the experimental data and theoretical analysis.

[1] G.P. Mikitik and E.H. Brandt, Phys. Rev. B 64 (2001) 92502 [2] L. Burlachkov, D. Giller and R. Prozorov, Phys. Rev. B 58 (1998) 15067 [3] G. Lukovsky et al, IEEE Trans. on Appl. Supercond. 17 (2007) 3137

∗E-mail: [email protected] †E-mail: [email protected] Vortex molecules in thin Films of Layered Superconductors

A. BuzdinA,∗ A. Mel’nikovB,† A. SamokhvalovB‡and A. TonomuraC§

AInstitut Universitaire de France and Universite Bordeaux I, Talence, 33405, France BInstitute for Physics of Microstructures, Russian Academy of Sciences, 603950 Nizhny Novgorod, GSP-105, Russia CHitachi, Ltd., Hatoyama, Saitama, 350-0395, Japan

Both the equilibrium and transport properties of the vortex matter are essentially affected by the behavior of the intervortex interaction potential. In isotropic bulk superconductors this potential is well known to be repulsive and screened at intervortex distances R greater than the London penetration depth λ. As a result, in perfect crystals quantized Abrikosov vortices form a triangular lattice. In thin films of anisotropic superconductors this stan- dard interaction potential behavior appears to be strongly modified because of the interplay between the long-ranged repulsion predicted in the pioneering work1 by J. Pearl and the attraction caused by the tilt of the vortex lines with respect to the anisotropy axes2, 3. We demonstrate that it exists a crossover betweeen tilted vortex atraction and repulsion depending on the film thickness and tilting angle. The Lorentz microscopy experimental data for thin YBaCuO films provide some evidences for this crossover4. Our theoretical analysis shows that due to the long-ranged Pearl’s repulsion the formation of the infinite chains of the tilted vortices in thin films is impossible in contrast with the bulk samples. On the other hand it appears an intriguing possibility to observe vortex chains of finite length, i.e., vortex molecules. The vortices can form stable pairs, trimers etc. The energetically favorable number of vortices in such a ”molecule” grows as we increase the film thickness and/or the tilting angle. The formation of long vortex chains may be considered in some sense as a polymerization of the vortex molecules. The crossover from the vortex molecule state to the chain structure is strongly influenced by the increase in the vortex concentration governed by the component of the external magnetic field perpendicular to the film. We expect this crossover to occur when the mean intervortex spacing approaches the molecule size. [1] J. Pearl, Appl. Phys. Lett. 5 (1964) 65. [2] A. I. Buzdin and A. Yu. Simonov, JETP Lett. 51, 191 (1990). [3] A. M. Grishin, A. Yu. Martynovich, and S. V. Yampolskii, Sov. Phys. JETP 70, 1089 (1990). [4] A. I. Buzdin, A. S. Mel’nikov, A. V. Samokhvalov , T. Akashi, T. Masui, T. Matsuda, S. Tajima, H. Tadatomo, and A. Tonomura, Phys. Rev. B79 (2009) 094510.

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] Complex pinning behavior of REBCO coated conductors in oblique ﬁelds

Noriko Chikumoto,∗ Sergey Lee, Keiichi Tanabe

Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan

Recently remarkable progress has been made in the fabrication process of REBa2Cu3Oy (RE: Y and rare-earth elements, REBCO) coated conductors (CC). They are now produced 2 with the performance, Jc > 3 MA/cm (T = 77 K, self-field) for > 500 m in length. The study of the vortex matter in such practical CC is very important both from the fundamental and practical points of view. Microstructural studies indicate that CC have a large variety of defects, contrary to single crystals, that may provide rather complex pinning behaviors. In the present study, we have performed the angular Jc(B) measurement for GdBCO CC. The tapes were fabricated by a pulsed laser deposition technique. We utilized reel-to-reel deposition that is used to fabricate long-length practical CC. We also studied the effect of BaZrO3 (BZO) doping, which has been known to form self-assembled “nano-rods” along the film thickness. Figure 1 shows an example of Ic(Θ) curves for an undoped and a 5 mol %- ◦ BZO doped GdBCO measured at 77 K. We can see an absence of intrinsic Ic-peak at Θ = 90 and existence of broad peak at 180◦ that indicate the strong contribution of both isotropic and anisotropic pinning centers. Further discussion will be presented at the workshop. This work is supported by the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1: Ic(Θ) curve of undoped(close symbols) and BZO-doped(open symbols) GdBCO CC measured at T = 77K and at Ba = 1T and 3T.

∗E-mail: [email protected] Superconducting density of states and vortex cores of boron-doped Diamond: a STM/STS study

Thomas DubouchetA,∗ Benjamin Sac´ep´eA,B, Claude ChapelierA, Marc SanquerA, Philipp AchatzC, Etienne BustarretC

AInstitute for Nanosciences and cryogenics, CEA Grenoble, Grenoble, France BDépartement de Physique, Universitéde Genève,Geneva, Switzerland CInstitut Néel,CNRS Grenoble, Grenoble, France

Highly boron-doped diamond has been found to be superconducting in 2004 [1]. We present here two scanning tunneling spectroscopy studies performed below 100mK of high- quality single crystalline boron-doped diamond for which the local density of states (LDOS) displays a clear superconducting gap with a temperature suppresion of the order parameter well described by the BCS theory [2]. For the first sample studied, vortex imaging at low magnetic field reveals localized states inside the vortex core that are unexpected for such a dirty superconductor [3] with an apparent periodicity λF (fig. 1a). The more detailed study made on the second sample confirms that the DOS inside the vortex core remains gapped with localized resonances instead of a flat LDOS as expected for a dirty superconductor but questions the periodic occurence of these resonances. The larger amount of measurements obtained here either on a large scale or with a very high resolution gives us more information about these resonances such as their spatial extension and their energy dependence (fig. 1b). The complete analysis of these results still requires a better understanding of the origin of these resonances in the light of recent theories for superconductivity in heavily-doped semiconductors [4,5].

Figure 1: a, Line of spectra across a vortex core revealing resonances inside the gap. b, Nanometer-scale cartography showing spatial modulations of the LDOS in the vortex-core and resonances outside the core.

[1] E. A. Ekimov et al, Nature (London), 428 (2004) 542. [2] B. Sac´ep´e et al, Phys. Rev. Lett. 96 (2006) 097006. [3] Ch. Renner et al, Phys. Rev. Lett. 67 (1991) 1650. [4] Y. Yanase et al, J. Phys. Soc. Jpn. 78 (2009) 034715. [5] X. Blase et al, Nature Materials. 8 (2009) 375-382.

∗E-mail: [email protected] Scanning Tunneling Spectroscopy on Amorphous Indium Oxide: Spectral Signature of Incoherent Cooper-Pairs

Thomas DubouchetA,∗ Benjamin Sac´ep´eA,B, Claude ChapelierA, Marc SanquerA, Maoz OvadiaC, Dan ShaharC

AInstitute for Nanosciences and cryogenics, CEA Grenoble, Grenoble, France BDépartement de Physique, Universitéde Genève,Geneva, Switzerland CDepartment of condensed matter physics, Weizmann Institute of Science, Rehovot, Israel

The disorder-induced superconductor-insulator transition (SIT) has intrigued scientists for several decades both theoretically and experimentally. Indeed, transport measurements at low temperature on critically disordered films revealed very unusual phenomena on both side of the SIT whose origin is still under debate [1,2]. For the first time, Scanning Tunneling Spectroscopy (STS) at very low temperature has been done on amorphous Indium Oxide samples close to the SIT. It reveals spatial inhomogeneities of the superconducting gap ∆ in the local density of states (LDOS) as recently seen in Titanium Nitride thin films [3]. Meso- scopic fluctuations of disorder were predicted to be at the origin of such an inhomogeneous state which in turn could explain the peculiar transport properties mentioned above [4,5]. Besides, although these measurements have been made well below Tc, we have also observed strong spatial fluctuations of the coherence peak height in the spectra at eV = ±∆. Analyz- ing the temperature dependence of the LDOS has revealed that this BCS singularities height is actually a measure of macroscopic quantum coherence so that, in the extreme limit where coherence peaks are absent, the local gap probed by STS is due to incoherent Cooper-pairs confined in a localization volume.

[1] G. Sambandamurthy et al, Phys. Rev. Lett. 92, 107005 (2004) [2] T.I. Baturina et al, Phys. Rev. Lett. 98, 127003 (2007) [3] B. Sac´ep´e et al, Phys. Rev. Lett. 101, 157006 (2008) [4] A. Ghosal et al, Phys. Rev. Lett. 81, 3940 (1998); Phys. Rev. B 65, 014501 (2001) [5] Y. Dubi et al, Nature 449, 876 (2007)

∗E-mail: [email protected] Pauli Paramagnetic Eﬀects on the Flux Line Cores in CeCoIn5 Morten R. EskildsenA,∗ J. S. WhiteB, P. DasA, L. DeBeer-SchmittA, E. M. ForganB, A. D. BianchiC, M. KenzelmannD, M. ZollikerD, S. GerberE, J. L. GavilanoE, J. MesotE, R. MovshovichF, E. D. BauerF, J. L. SarraoF, C. PetrovicG

ADepartment of Physics, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A. BSchool of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK CDépartement de Physique et RQMP, Universitéde Montréal,Montréal,QC, H3C 3J7, Canada DLaboratory for Developments and Methods, Paul Scherrer Institute, CH-5232 Villigen, Switzerland EPaul Scherrer Institute, ETH Zürichand EPF Lausanne, CH-5232 Villigen, Switzerland FLos Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S.A. GBrookhaven National Laboratory, Upton, New York 11973, U.S.A.

The heavy-fermion superconductor CeCoIn5 continues to attract great interest, because it shows strong Pauli paramagnetic effects and also a close proximity of superconductivity to a quantum critical point. It has a superconducting Tc ∼ 2.3 K in zero field.At low temperature, the transition to the normal state is first-order, showing that the superconductivity is suppressed by coupling of the field to the anti-parallel spins of the singlet Cooper pair (the Pauli effect) rather than the more common coupling to the orbital motion of Cooper pairs in the mixed state (the orbital effect). Here we report on extensive small-angle neutron diffraction studies of the flux line lattice in CeCoIn5 with H k c. We obtain the field- and temperature-dependence of the form factor, which is a measure of the spatial modulation of the field in the mixed state. For temperatures up to 1250 mK, we observe effects of flux line core paramagnetism resulting 1 in an increase of the form factor with field as reported earlier . Near Hc2 this field contrast decreases again, with the fall-off extending outside the proposed FFLO region. The decrease is attributed to an expansion of the flux line cores due to paramagnetic suppression of Cooper pairing, which also provides a possible explanation for the unusual evolution of the flux line lattice symmetry in CeCoIn5. At higher temperatures, a gradual crossover towards more conventional mixed state behavior is observed. In addition to the results outlined above, we also report on measurements performed with the applied field perpendicular to the c-axis.

[1] A. D. Bianchi et al.,Science 319, 177 (2008).

∗E-mail: [email protected] Extended snapshots of glassy vortex phases

Y. Fasano,A,B∗A.P. Petrovi´c,B D. Salloum, B,C P. Gougeon, C M. Potel, C and Ø. FischerB, H. PastorizaA, M. KonczykowskiD, C. J. van der BeekD, M. LiE and P.H. KesE

AInstituto Balseiro and Low Temperatures Laboratory, Bariloche, Argentina BDépartement Physique Matière Condensée,Universitéde Genève,Geneva, Switzerland C Sciences Chimiques, CSM UMR CNRS 6226, Universitéde Rennes 1, Rennes, France D Laboratoire Solides Irradiés,CNRS-UMR 7642, Ecole´ Polytechnique, Palaiseau, France E Kamerlingh Onnes Laboratorium, Rijksuniversiteit Leiden, Leiden, The Netherlands

We studied the field-evolution of the topological properties of glassy vortex phases by means of extended scanning-tunnelling-spectroscopy (STS)1 and magnetic-decoration2 imaging of the vortex structure. This investigation was performed in two different systems: the low-Tc Chevrel phase superconductor SnMo6S8 and the electron-irradiated high-Tc Bi2Sr2CaCu2O8−δ compound. We obtained the first extended snapshots of the vortex glass revealing that this phase is not amorphous and posses intermediate-range positional order. For SnMo6S8 we have obtained maps of around 100 vortices from 2 to 9 Tesla by perform- ing STS at 400 mK. The orientational and positional orders at fields larger than 5 Tesla are depleted with respect to the 2 Tesla structure and dislocations proliferate, but no amorphous structure is observed at high fields.3 In the case of electron-irradiated Bi2Sr2CaCu2O8−δ, we have obtained maps of thousands of vortices in a sample displaying an order-disorder transition at 74 Oe4 by means of magnetic- decoration imaging at 4.2 K. We obtained extended images of both, the vortex and Bragg glass phases, varying the applied field between 30 and 85 Oe. For every field we compared the structural properties of vortex matter nucleated in irradiated and pristine twin-samples. The vortex glass phase of the irradiated samples is not amorphous although the positional order is suppressed compared with that of the Bragg glass.

[1] Ø. Fischer, M. Kugler, I. Maggio-Aprile, Ch. Berthod, and Ch. Renner, Rev. Mod. Phys. 79, 353 (2007). [2] Y. Fasano and M. Menghini, Supercond. Science and Tech. 21, 023001 (2008). [3] A.P. Petrovi´c,Y. Fasano, R. Lortz, C. Senatore, A. Demuer, A.B. Antunes, A. Par´e,D. Salloum, P. Gougeon, M. Potel, and Ø. Fischer, condmat:0903.2389 . [4] M. Konczykowski, C.J. van der Beek, A.E. Koshelev, V. Mosser, M. Li, and P. H. Kes, J. Phys.: Conf. Ser. 150, 052119 (2009).

∗E-mail: [email protected] Fermi Surface and Order Parameter Driven Vortex Lattice Structure Transitions in Detwinned YBa2Cu3O7

Ted ForganA,∗ Jon WhiteA,† Vladimir HinkovB, Bernhard KeimerB, Jo¨elMesotC, Andreas ErbD

ASchool of Physics and Astronomy, University of Birmingham, B15 2TT, UK. BMax Planck Institut f¨urFestk¨orperforschung, D-70569 Stuttgart, Germany. CPaul Scherrer Institut, Villigen PSI, CH-5232, Switzerland. DWalther Meissner Institut, BAdW, D-85748 Garching, Germany.

We have made small-angle neutron scattering (SANS) measurements of the magnetic vortex lattice (VL) structure in near perfectly de-twinned, fully-oxygenated YBa2Cu3O7 crystals. We believe that our observations are the first ones uncontaminated by twin plane pinning. Our results at base temperature show the influence of chain superconductivity, which appears to be suppressed by increasing field. In addition, we observe two first-order VL structure transitions which are driven by the magnetic field and which have little temperature- dependence. We argue that the first at 2.0(2) T arises because of Fermi surface effects, and the second at 6.5(5) T is driven by the anisotropy of the superconducting order parameter. The transition at 6.5 T is between distorted hexagonal and rhombic VL structures whose distortions are of opposite sign. This suggests that the second transition marks a cross-over from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the anisotropy of the superconducting order parameter. We have also measured the temperature-dependence of the SANS intensities, which reflect the “field contrast” in the mixed state. At low fields, these vary with temperature as expected from the temperature-dependence of the superfluid density of a d-wave superconductor in the London model. However, at high fields, we observe bizarre temperature-dependences, which remain unexplained. The first part of this work has recently been published1.

[1] J.S. White et al., Phys. Rev. Lett. 102, 097001 (2009)

∗E-mail: [email protected] †E-mail: [email protected] M¨unchhausen eﬀect: tunneling in an asymmetric SQUID

V. B. GeshkenbeinA,∗ A. U. ThomannA, G. BlatterA

AInstitute for Theoretical Physics, ETH Zurich, 8093 Zurich, Switzerland

A classical system cannot escape out of a metastable state at zero temperature. However, a composite system made from both classical and quantum degrees of freedom may drag itself out of the metastable state by a sequential process. The sequence starts with the tunneling of the quantum component which then triggers a distortion of the trapping potential holding the classical part. Provided this distortion is large enough to turn the metastable state into an unstable one, the classical component can escape. This process reminds of the famous baron Münchhausen who told the story of rescuing himself from sinking in a swamp by pulling himself up by his own hair—we thus term this decay the ‘Münchhausen effect’. We show that such a composite system can be conveniently studied and implemented in a dynamically asymmetric dc-SQUID with two Josephson junctions of equal critical current Ic but strongly different shunt capacities C and/or shunt resistances R. We determine the dynamical phase diagram of this SQUID for various choices of junction parameters.

∗E-mail: [email protected] Emergence of superconducting textures in two dimensions

Andreas GlatzA,∗ Igor AransonA,† Valerii VinokurA,‡ Nikolay ChtchelkatchevB,§ Tatyana BaturinaC,¶

AMaterials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA BL.D. Landau Institute for Theoretical Physics, Russian Academy of Sciences, 117940 Moscow, Russia and Department of Theoretical Physics, Moscow Institute of Physics and Technology, 141700 Moscow, Russia CInstitute of Semiconductor Physics, 13 Lavrentjev Ave., Novosibirsk, 630090 Russia

Self-organized patterns are ubiquitous in nature, and one of their most celebrated mani- festations is the Abrikosov vortex lattice: under an applied magnetic field, the homogeneous superconductivity becomes unstable and cast itself into a regular texture of the ”normal” filaments, called Abrikosov vortices, immersed into a superconducting matrix. Its prediction and the experimental discovery became a breakthrough in our understanding of superconductivity and founded a new direction in physics. Here we show that the interplay between the superconducting order parameter and elastic fields, which are intimately connected to the very existence of the superconductivity itself, can result in a novel superconducting state dual to the Abrikosov state: a regular texture of superconducting islands. The fact that both dual patterns emerge within the framework of the Ginzburg-Landau description of superconductivity, i.e., in the close vicinity of the superconducting phase transition, indicates that the formation of regular patterns may be an inherent feature of any phase transition. Importantly, the emergence of superconducting island structures is not specific to the effect of the elastic forces, but may result from any inherent mechanism that generates long-range non-local interactions in the Ginzburg-Landau functional. In particular, such non-local interactions can result from long-range Coulomb forces that appear in the critical region of the superconductor-insulator transition due to suppressed dynamic screening, suggesting that this transition can occur via the formation of a superconducting island textures.

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] ¶E-mail: [email protected] Observation of spontaneous vortex formation in a rapidly cooled superconductor

D. Golubchik,∗ E. Polturak , and G. Koren

Physics Department, Technion, Israel

According to a prediction by Kibble and Zurek [1, 2], under nonequilibrium conditions magnetic flux lines should spontaneously appear during a conductor-superconductor phase transition, even at zero external field. Individual flux lines should have a random sign. This prediction, known as the Kibble-Zurek model, is relevant both to cosmology and to nonequilibrium thermodynamics. Several conflicting predictions exist with respect to the spatial density of flux lines and the correlations within the flux array. In order to test this model we developed a new magneto-optical system capable of imaging relatively large areas (∼ 100 × 100 µm2) while at the same time resolving single flux lines of submicrometer dimensions. These capabilities are essential to measure low density vortex distribution and are almost impossible to achieve with other techniques. The Kerr effect in thin EuSe film is used to measure locally the magnetic field above the surface of superconductor. The imaging system was integrated within a setup capable of producing temperature cooling rates of up to 108 K/sec. In the experiment, we used thin film of niobium which were heated above the critical temperature and then cooled rapidly. We indeed found that flux lines of with random signs were created in the sample during the phase transition, even at zero external field (figure 1). We will present new measurements of the density and of the spatial correlations within spontaneously created flux line arrays.

Figure 1: Typical MO image of a spontaneously created flux lines in a superconductor. Brightness is proportional to local magnetic field. White and dark spots represent flux lines with opposite polarity. The background was subtracted to enhance contrast.

[1] Zurek W. H. , Nature 31 (1985) 505 [2] Kibble T. W. and Rajantie A. , Phys. Rev. B 68 (2003) 17451

∗E-mail: [email protected] Magnetization Studies in a Single Crystal of FeSe0.5Te0.5 Pradip Dasa,d, C. V. Tomya,b, G. Balakrishnanb, S. S. Banerjeec, S. Ramakrishnand, A. K. Groverd∗

aDepartment of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India bDepartment of Physics,University of Warwick, Coventry, UK cDepartment of Physics, Indian Institute of Technology Kanpur, Kanpur 208016, India dDepartment of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India

There is a large interest in the nature of superconducting order parameter in FeSe1−xTex system. We have carried out detailed magnetism studies in a single crystal at the stoi- chiometry FeSe0.5Te0.5 (Tc(0) ∼ 14.3 K). The chosen crystal piece is moderately pinned and displays Fishtail anomaly for H k c. The temperature dependences of the intrinsic physics and material parameters, like, the lower critical ﬁeld, the upper critical ﬁeld and the onset of the Fishtail anomaly show unusual behaviour. We are exploring the correspondence between these responses. Some of the novel features in magneto-transport data near Tc(0) are reminiscent of the behaviour documented for magnetic superconductors in 1980s. A vortex phase diagram based on our studies shall be presented.

∗E-mail: [email protected] Surface superconductivity, Paramagnetic Meissner Eﬀect, Peak Eﬀect and metastability in ultrapure crystals of Nb

Pradip Dasa, C. V. Tomya, S. S. Banerjeeb,∗ H. Takeyac, S. Ramakrishnand, A. K. Groverd

aDepartment of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India bDepartment of Physics, Indian Institute of Technology Kanpur, Kanpur 208016, India cNatonal Institute of Materials Science, Ibraki 305-0047 Japan d Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India

Investigations in the neighbourhood of the upper critical field (Tc2(H)) has evoked interest in recent times. Unusual effects reported in this region of field (H) - temperature (T) phase diagram have ranged from, surface superconductivity, Paramagnetic Meissner Effect (PME) and order - disorder phase transformations in the vortex state via the Peak effect (PE). Proposals relating to these effects have also varied from unconventional nature of the superconducting order parameter (as in HTSC materials) to flux compression scenario’s valid for a general class of superconductors. We have investigated the ac and dc magnetization (M) response of a very high purity, spherical single crystal of Nb. We report the simultaneous observation of signatures associated with surface superconductivity, PME and PE phenomenon in this crystal. In the H − T phase diagram we demarcate the regions associated with, (i) surface superconductivity via the Tc2(H), Tc3(H) curves, (ii) the PME regime on 1 by identifying the TPME(H) curve and (iii) the onset of PE by identifying the Tp curve . We also report the observation significant thermo-magnetic history dependence in the vicinity of TPME(H) in isofield M(T ) measurements. We propose that the temperature dependence of the superconducting coherence length is responsible for flux compression, thereby producing a significant PME signal between Tc2(H) and Tc3(H). We argue that the thermo-magnetic history effects associated with TPME(H) in the vicinity of Tc2(H) are associated with the nucleation of single quantum fluxons and their subsequent pinning.

[1] Pradip Das et al. Phys. Rev. B 78, 214504 (2008).

∗E-mail: satyajit@@iitk.ac.in Ultrafast dynamics of vortex penetration, and nonlinear surface resistance under strong rf ﬁelds

A. GurevichA,∗ G. CiovatiB,†

ANational High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA BThomas Jeﬀerson National Accelerator Facility, Newport News, Virginia 23606, USA

We consider dynamics of penetration and oscillation of a single vortex in a superconductor in a strong parallel rf magnetic field B0 sin ωt. Using the London theory, we calculate the dissipated power Q(B0, ω), and the transient time scales of nonlinear vortex oscillations for the Bardeen-Stephen viscous drag force, which results in supersound vortex velocities during vortex penetration through the oscillating surface barrier. It is shown that penetration of a single vortex through the ac surface barrier always involves penetration of an antivortex and the subsequent annihilation of the vortex antivortex pairs. Larkin-Ovchinnikov (LO) -type nonlinearities in the viscous drag force at higher vortex velocities v(t) result in a jump-wise vortex penetration through the surface barrier and a significant increase of the dissipated power. We calculate the effect of dissipation on the nonlinear vortex viscosity η(v) and the rf vortex dynamics and show that it can also result in the LO-type behavior, instabilities, and thermal localization of penetrating vortex channels. We propose a thermal feedback model of η(v), which not only results in the LO dependence of η(v) for a steady-state motion, but also takes into account retardation of temperature field around rapidly accelerating vortex, and a long-range interaction with the surface. We also address the effect of pinning on the nonlinear rf vortex dynamics and the effect of trapped magnetic flux on the surface resistance 1 Rs calculated as a function or rf frequency and field. .

[1] A. Gurevich and G. Ciovati, Phys. Rev. B 77 (2008) 104501

∗E-mail: [email protected] †[email protected] Unconventional Superconductivity in Transuranium Materials

Yoshinori HagaA,∗ Dai AokiB,C, Yoshiya HommaB, Tatsuma D. MatsudaA, Shugo IkedaA, Hironori SakaiA Naoyuki TateiwaA, Etsuji YamamotoA, Akio NakamuraA, Kunihisa NakajimaA, Yasuo AraiA, Rikio SettaiD, Yoshichika Onuki¯ A,D

AJapan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan BInstitute for Materials Science, Oarai, Ibaraki 319-1313, Japan CINAC/SPSMS, CEA-Grenoble, Grenoble 38054, France DGraduate School of Science, Osaka University, Osaka 560-0043, Japan

NpPd5Al2 crystallizing in the tetragonal ZrNi2Al5 structure is the first neptunium-based heavy fermion superconductor. The superconductivity is characterized by the conduction electrons with a large 5f contribution: large electronic specific heat above Tsc = 5 K and a large spin susceptibility obeying the Curie-Weiss law in the normal state1. The most remarkable feature in the superconductivity of this compounds is the first-order phase transition at the upper critical field Hc2. Magnetization shows a stepwise anomaly at Hc2. At the same time, large magnetic anisotropy suddenly appears above Hc2 corresponding to that of normal state. Furthermore, electronic specific heat shows a discontinuous jump at Hc2, indicating a recovery of the heavy fermion state in the normal state. These features and the NMR study2 are consistent with a singlet pairing state with a strong paramagnetic effect. After the discovery of NpPd5Al2, isostructural actinide compound UPd5Al2 was found It is interesting to note that the magnetic anisotropy of NpPd5Al2 (XY -type) contrasts with the Ising-type UPd5Al2.

[1] D. Aoki, Y. Haga, T.D. Matsuda, N. Tateiwa, S. Ikeda, Y. Homma, H. Sakai, Y. Sh- iokawa, E. Yamamoto, A. Nakamura, R. Settai and Y. Onuki,¯ J. Phys. Soc. Jpn. 76 (2007) 063701. [2] H. Chudo, H. Sakai, Y. Tokunaga, S. Kambe, D. Aoki, Y. Homma, Y. Shiokawa, Y. Haga, S. Ikeda, T.D. Matsuda, Y. Onuki¯ and H. Yasuoka, J. Phys. Soc. Jpn. 77 (2008) 083702-1-4.

∗E-mail: [email protected] Relationship between the Vortex ’Checkerboard’ and the Quasi-Particle Interference in Bi2Sr2CaCu2Oy

Tetsuo HanaguriA,∗ Yuhki KohsakaA,† Tsuyoshi TamegaiB,‡ Hidenori TakagiA,C§

AMagnetic Materials Laboratory, RIKEN, Wako 351-0198, Japan BDepartment of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan CDepartment of Advanced Materials, University of Tokyo, Kashiwa 277-8561,Japan

An electronic superstructure with a characteristic periodicity of about 4 Cu-Cu distance has been observed in a vortex core of cuprate superconductors and is argued in relation to the possible electronic orders nucleated in the vortex core1, 2. In order to elucidate the origin of this so-called vortex ’checkerboard’, we have investigated the relationship between the ’checkerboard’ and the quasi-particle interference effect3, 4 in optimally-doped Bi2Sr2CaCu2Oy (Tc = 92 K). Quasi-particle interference effect, which can be detected by spectroscopic-imaging STM, has been used as a measure of the Fermi surface geometry. By comparing Fourier-transform spectroscopic images taken with and without magnetic field, we found that the periodicities of the vortex ’checkerboard’ coincide with those of the quasiparticle interference patterns, suggesting that the vortex ’checkerboard’ is associated with the Fermi surface geometry. In addition, we found a sharp density-of-state peak at the center of the vortex core. This peak decays rapidly within a few lattice constants, followed by the oscillatory spatial variation, which is nothing but the ’checkerboard’ modulation. Based on these observations, we argue the Friedel oscillation around the quantum-limit vortex core as a possible origin of the vortex ’checkerboard’5, 6.

[1] J. E. Hoﬀman et al., Science 295 (2002) 466. [2] K. Matsuba et al., J. Phys. Soc. Jpn. 76 (2007) 063704. [3] J. E. Hoﬀman et al., Science 297 (2002) 1148. [4] K. McElroy et al., Nature 422 (2003) 592. [5] N. Hayashi et al., Phys. Rev. Lett., 80 (1998) 2921. [6] M. Kato and K. Maki, Prog. Theo. Phys. 107 (2002) 941.

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] Excess Resistance in Vortex Flow state of Mesoscopic Aluminum Disks

A. HaradaA,∗ K. EnomotoA, T. YakabeA, M. KimataA, H. SatsukawaA, K. HazamaA,B, K. KodamaA,B, T. TerashimaA, and S. UjiA,B

ANational Institute for Materials Science, Ibaraki 305-0003, Japan BGraduate School of Pure and Applied Sciences, University of Tsukuba, Ibaraki 305-8577, Japan

We have measured the electric resistance and the current-voltage (I-V ) characteristics in mesoscopic Al disks at low temperatures to investigate the vortex dynamics. As magnetic field increases perpendicular to the disk, the resistances show successive small peaks, which are ascribed to vortex state transitions with vorticity L = n → n + 1 (L is the number of the flux quanta penetrating the disks), as shown in Fig. 1.1, 2. It should be noted that some of the resistance peaks near the critical field are larger than the normal state resistance Rn. The result argues that the Al disk is superconducting even in excess resistance state R > Rn: anomalously large energy dissipation is induced by the vortex flow in the mesoscopic Al disk. Recently, we have observed periodic steps in the I-V characteristics near the critical field when the RF current is superimposed on the DC current. The step voltages are given by Vn = nhf/2e (n = 1, 2, 3,…), where f is the RF frequency. This indicates that n vortices penetrate and escape from the disk in each period of the RF current. We will show the RF current dependence of the steps and discuss the vortex dynamics in the confined geometry.

L = 0 1 2 3 4 5 6 1.3K 1.295K 1μm 1.29K 12 1.285K 1.28K 100nm 1.275K 10 1.27K 1.265K 1.26K 1.25K 8 1.24K 1.22K 1.2K 1.17K 6 1.14K 1.1K 1.05K 4 1K

Resistance (Ohm) 0.95K 0.9K 0.875K 2 R 0.85K n 0.8K 0.75K 0 0.7K 0 100 200 300 400 500 600 700 0.65K 0.6K Magnetic field (Oe) 0.562K

Figure 1: Magnetic ﬁeld dependence of electric resistance at various temperatures for an Al disk with 1.0 µm in diameter. The inset shows the schematic view of the Al sample.

[1] V. V. Moshchalkov et al., Nature (London) 373 (1995) 319. [2] K. Enomoto et al., Physica E 29 (2005) 584.

∗E-mail: [email protected] Vortex Matter in Nano-structured Pinning Arrays

Kazuto Hirata,∗ Ajay Thakur, Shuuichi Ooi, Takashi Mochiku

Superconducting Materials Center, National Institute for Materials Science, Tsukuba 305-0047, Japan

Recent developments of nano-structuring technology for superconductors into sub-micron size and various shapes of holes make us to study new physics in vortex matter. We have ? fabricated anti-dot lattice array into low-Tc superconductors Nb and NbN , and high–Tc superconductor Bi-2212? with the diameter of r and the spacing of a in the anti-dots, and have measured the flow-resistance of vortices perpendicular to the array and magnetic field. Depending also to the ratio of r/a, we can produce a variety of vortex-matching effect; the √ 2 usual matching at n×H0(n; integer, H0=φ0/( 3a /2) for triangular hole-array), and the fractional matching at H0/2, H0/3, 2H0/5, etc. In low-Tc superconductors, the usual matching effect shows ”dips” in the flow resistance at the matching field. However, it shows ”humps” at higher magnetic fields, which may be related to the formation of giant vortices and interstitial vortices, depending on the ratio r/a. The matching phenomena seem to occur just like the Bloch electrons in two-dimensional electron system with crystalline lattice under magnetic field. The anti-dot array acts as a crystalline lattice to the vortices. In this case, vortex flow-resistance corresponds to Tc in the linearlized GL equation. Generation and annihilation of the fractional matching effect might be well reproduced. In high-Tc superconductor, the matching effect is closely related to the first order vortex lattice melting of the pristine samples in the presence of the anti-dot arrays and with changing the potential energy of the vortex pinning. Furthermore, the anti-dot array in high-Tc superconductor causes a large nonlinearity in I-V characteristics, which can be applied to produce a large rectificated voltage with biharmonics applied currents. This can also be realized in the Josephson vortex flow-resistance with incorporating the pancake vortices.

[1] A. Thakur, S. Ooi, S. P. Chokalingam, J. Jesudasan, P. Raychaudhuri and K. Hirata, Appl. Phys. Lett. 94 (2009) 262501 [2] S. Ooi, T. Mochiku and K. Hirata, J. Phys.:Conf. Series 150 (2009) 052203

∗E-mail: [email protected] Theory on the π Kink State in Intrinsic Josephson Junctions and THz Electromagnetic Radiation Xiao Hu and Shizeng Lin

WPI Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan

It was known that a layered cuprate high-Tc superconductor, such as Bi2Sr2CaCu2O8+δ (BSCCO) behaves as a stack of intrinsic Josephson junctions (IJJs). Much effort has been devoted for developing IJJs as a source of strong THz electromagnetic (EM) radiation. Compared with artificial JJ arrays, the superconductivity gap of BSCCO is of tens of meV, which covers fully the THz band, and the homogeneity of the stack is guaranteed by the high quality of single crystal. A breakthrough was achieved very recent1 where a coherent THz EM radiation from a mesa of BSCCO single crystal was demonstrated. We start our theoretical analysis from the coupled sine-Gordon equations with a dc bias current and dissipations2. The following features particular to the recent experiment are taken into account in our study: (i) The inductive-coupling constant is huge in the equations, due to the large value of the penetration depth compared with the thickness of the superconducting CuO layers; (ii) The mesa is thin (∼ 1µm) in the c axis compared with the EM wave length (∼ 300µm), as such the tangential component of the magnetic field at the mesa edge should be small, thus turning the mesa into a cavity for the Josephson plasma. A new dynamic state has been found2 which is characterized as follows: (1) The phase difference rotates with the frequency given by the bias voltage according to the ac Josephson relation; (2) The phase differences exhibit ±π kinks, which are aligned alternatively along the c axis; (3) The π phase kinks pump a large amount of energy into the standing wave of Josephson plasma uniform in c axis which radiates coherent EM waves, when the bias voltage is tuned to the cavity value; (4) The alternative alignment of the ±π kinks provides a strong interlock between phase differences in adjacent junctions making use of the large inductive-coupling constant, which stabilizes the present dynamic state over several hundred junctions. The π kink state explains successfully the main features of experimental observations. To further confirm the π kink state we propose to use an annular mesa3 instead of the rectangular one used in the first experiment. This work was supported by WPI Initiative on Materials Nanoarchitectonics, MEXT of Japan and by CREST-JST, Japan.

[1] L. Ozyuzer et al., Science 318 (2007) 1291 [2] S. -Z. Lin and X. Hu, Phys. Rev. Lett. 100 (2008) 247006 [3] X. Hu and S. -Z. Lin, Phys. Rev. B 78 (2008) 134510 Depinning, creep and possible new phase transition in vortex glasses

Mengbo LuoA,B,∗ Xiao HuB†

AWPI Center for Materials Nanoarchitectonics, National Institute for Material Science, Tsukuba 305-0047, Japan BDepartment of Physics, Zhejiang University, Hangzhou, 310027, China

Dynamics of three-dimensional flux lines with point-like defects are studied by large- scale accurate molecular dynamical simulation based on Langevin dynamics. A continuous depinning transition is observed at zero temperature, where scaling function is described as 1/δ −1/βδ v(T,F ) = T S[T (1−Fc0/F )]. We obtain two universal classes with different exponents β and δ for Bragg glass (BrG) and vortex glass (VG), respectively. Especially, the product βδ = 3/2 in the BrG indicates a non-Arrhenius creep motion, in contrast to βδ = 1 in the 1 µ VG. The creep at F < Fc0/2 can be described by v ∝ exp[−U(Fc0/F ) /T ], in agreement with collective creep theory. The exponent is evaluated as µ = 0.5 ± 0.02 universal for the BrG and µ = 0.28±0.02 universal for the VG, with the latter a new estimate whose estimate was theoretically very difficult since the elastic restoration force is not available. In the BrG, ∼ we observe a new phase transition at Tg = Tm/2 where (1) nonlinear creep motion at low temperature is rendered to linear v − F behavior, and (2) at equilibrium, pinned flux lines become mobile via quick swapping of nearest neighbors.2

[1] M. B. Luo and X. Hu, Phys. Rev. Lett. 98 (2007) 267002. [2] M. B. Luo, X. Hu,Luo and V. Vinokur, arXiv:0902.0858.

∗E-mail: [email protected] †E-mail: [email protected] Inﬂuence of Magnetism on the Abrikosov Vortex Structure

Maria IavaroneA,∗ Goran KarapetrovA,† Wai K. Kwok A,‡ Milorad V. Milosevic B,§ Francois M. Peeters B,¶ Terukazu Nishizaki C,k Norio Kobayashi C∗∗

AMaterials Science Division, Argonne National Laboratory, Argonne IL 60439, USA BDepartement Fysica, Universiteit Antwerpen, Antwerpen, Belgium CInstitute for Materials Research, Tohoku University, Sendai 980-8577 Japan

We studied the interplay between magnetism and superconductivity on two different length scales: first, on atomic scale by intrinsically doping a superconductor single crystal with a minute amount of magnetic impurities and second, on macroscopic scale by fabricating magnetically coupled hybrid S/F systems. In order to characterize these systems, we use low temperature scanning tunneling microscopy (STM) which allows to measure the local electronic density of states and directly visualize Abrikosov vortex configurations. I will present the effect of magnetic atomic impurities (such as Co and Mn) in NbSe2 on the vortex structure. We observed a pronounced peak effect in the magnetization of CoxNbSe2 single crystals having critical temperatures Tc ranging between 7.1 K and 5.0 K, and MnxNbSe2 single crystals with critical temperatures down to 3.4 K. We correlate the peak effect in magnetization with the structure of the vortex lattice across the peak effect region using scanning tunneling microscopy1. In the case of hybrid S/F we studied the vortex structure in magnetically coupled superconductor-ferromagnet hybrid structures2. We observe strongly anisotropic vortex pinning effect due to the presence of the ordered magnetic domain structure. We show that the hybrid exhibits commensurability features that are related to the matching periodicities of the Abrikosov vortex lattice and the magnetic stripe domains. Using STM vortex imaging we show that the periodic magnetic induction in the superconductor creating a series of (anti)pinning channels for externally added magnetic flux quanta forcing confinement of the Abrikosov vortices and formation of quasi-1D vortex arrays3.

[1] M. Iavarone, R. Di Capua, G. Karapetrov, A. E. Koshelev, H. Claus, D. Rosenmann, C. Malliakas, M. Kanatzidis, T. Nishizaki, N. Kobayashi, Phys. Rev. B 78 (2008) 174518 [2] A. I. Buzdin, Rev. Mod. Phys. 77 (2005) 935 [3] G. Karapetrov, et al. submitted

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] ¶E-mail: [email protected] kE-mail: [email protected] ∗∗E-mail: [email protected] Flux Line Lattice Form Factor Studied by Eilenberger Theory

Masanori Ichioka ∗, Kazushige Machida

Department of Physics, Okayama University, Okayama 700-8530, Japan

In order to quantitatively estimate physical quantities in vortex states, we study the vortex states by microscopic calculations of quasiclassical Eilenberger theory. In our calculations, the spatial structure of the pair potential and the internal field are selfconsistently calculated in the vortex lattice state, so that we can appropriately evaluate the vortex core size and the contributions. From the vortex structures, we quantitatively estimate the temperature- and magnetic field- dependences of specific heat, magnetization, Knight shift and the flux line lattice (FLL) form factor. In this presentation, we mainly report our studies on the FLL form factor. The FLL form factors are experimentally studied for various superconductors by small angle neutron scattering (SANS). First, we report our theoretical study on anomalous field dependences of the FLL form factors in superconductors with strong Pauli-paramagnetic effect. There are two mechanisms for pair-breaking of superconductivity by magnetic fields. One is the diamagnetic pair breaking due to the screening current around vortices from the contribution of the vector potential. The other is the Pauli-paramagnetic pair breaking due to the mismatched Fermi surface of up and down spin electrons by the Zeeman shift. Based on the quasiclassical Eilenberger theory, we investigate the paramagnetic effect for the vortex state in addition to the diamagnetic contributions.1 In the presence of paramagnetic contribution, the paramagnetic moment is induced around the vortex core. The induced paramagnetic moment further enhances the internal field around the vortex core, and vortex core size is enlarged. We quantitatively evaluate magnetic field(H)-dependence of low temperature specific heat and magnetization, so that we show how the paramagnetic effect suppresses the superconductivity. We also evaluate the FLL form factor from the calculated internal field distribution. In conventional superconductors with weak paramagnetic effect, the FLL form factor exponentially decreases as a function of H. However, when the paramagnetic effect is strong, the intensity of the FLL form factor increases towards high fields. Based on our results, we discuss the anomalous 2 field-dependence of the FLL form factor observed by SANS in CeCoIn5. While the strong paramagnetic effect qualitatively explains the anomalous H-dependence, it is not enough to quantitatively reproduce the experimental data. This analysis suggests that in CeCoIn5 we have to consider also other contributions such as effects near a quantum critical point. We also discuss properties of higher order terms of the FLL form factors obtained by Eilenberger theory, comparing with those of London theory.

[1] M. Ichioka and K. Machida, Phys. Rev. B 76 (2007) 064502 [2] A.D. Bianchi et al., Science 319 (2008) 177

∗E-mail: [email protected] Exotic Vortex Lattices in FFLO and Noncentrosymmetric Superconductors

Ryusuke Ikeda,∗ Kazushi Aoyama,† Taro Saiki‡

Department of Physics, Kyoto University, Kyoto 606-8502, Japan

Inspired by recent observations of a high field - low temperature (HFLT) phase in heavy electron1 and organic superconductors2, we are now on a good occasion of research on superconducting states with a spatial modulation induced by the paramagnetic depairing. However, unlike the vortex-free Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state proposed originally, these states in bulk superconductors under a nonzero field inevitably include vortices and hence, need to be considered as vortex states influenced by a strong paramagnetic depairing3. Among such states, the HFLT phase discovered in the heavy fermion material CeCoIn5 is a candidate of the FFLO vortex lattice modulating along the magnetic field. This system is peculiar in the sence that strong effects of the paramagnetic depairing and an antiferromagnetic fluctuation, both of which are enhanced with increasing field, coexist. It will be stressed that, due to effects of antiferromagnetic quantum critical fluctuation, physical phenomena near Hc2(0) are highly deviated from those expected in the weak-coupling picture. Another candidate to show novel modulated vortex states is a Rashba noncentrosymmetric superconductor in a magnetic field parallel to the basal plane4. In these systems, an anisotropic Zeeman energy stemming from the anisotropic spin-orbit coupling suppresses a modulation parallel to the field but, consistently, superimposes an uniaxial modulation on the vortex lattice structure in the plane perpendicular to the field. In particular, the resulting vortex-lattice structure depends highly on the relative orientation between the applied field and nodal directions of the pairing gap and thus, becomes a fingerprint of the pairing symmetry. Details of the resulting lattice structure will also be discussed.

[1] A. Bianchi et al., Phys. Rev. Lett. 91, 187004 (2003); T. Watanabe et al., Phys. Rev. B 70, 020506(R) (2004). [2] R. Lortz et al., Phys. Rev. Lett. 99, 187002 (2008); K. Cho et al., Phys. Rev. B 79, 220507(R) (2009). [3] H. Adachi and R. Ikeda, Phys. Rev. B (2003). [4] N. Hiasa, T. Saiki, and R. Ikeda, Phys. Rev. B 80, 014501 (2009).

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] Growth of thin ﬁlms FeSe1xTex with PbOtype structure by pulsed laser deposition method

Yoshinori ImaiA,C, Ryo TanakaA,C, Takanori AkiikeA,C Masafumi HanawaB,C Ichiro TsukadaB,C Atsutaka MaedaA,C

ADepartment of Basic Science, the University of Tokyo, Tokyo 153-8902, Japan BCentral Research Institute of Electrical Power Industry, Kanagawa 240-0916, Japan CTRIP, JST, Japan

FeSe withTc of 8 K has the tetragonal PbOtype structure, which is the simplest structure among the iron-based superconductors1. The study of superconductivity in FeSe system is expected to provide a clue of understanding the mechanism in these materials. The purpose of this research is to grow thin films, FeSe1xTex, where Tc raised up to 13 K by the 2 partial substitution of Te for Se in FeSe . FeSe1xTex thin films were deposited on MgO(100) and LaSrAlO4(001) single crystal substrates by pulsed laser deposition method using KrF laser (wavelength: 248 nm) from FeSe1xTex+d polycrystalline targets at several different substrate temperatures. The film deposited on the MgO substrate (film thickness 63 nm) shows superconductivity at 6.7 K (onset) and 5.5 K (zero resistivity). On the other hand, the film deposited on the LaSrAlO4 substrate (film thickness 52 nm) exhibits only the onset of superconducting transition at 2.6 K, but does not show zero resistivity. This indicates superconducting properties of FeSe1xTex thin film is strongly influenced by the epitaxial strain3.

[1] F. C. Hsu et al., PNAS 105 (2008) 14262. [2] M. H. Fang et al., Phys. Rev. B 78 (2008) 224503. [3] Y. Imai et al., in preparation.

E-mail: [email protected] The Vortex-Core Contribution to the Magnetic Torque in Multi-Band Superconductors ∗

Daichi KubotaA,† Nobuhiko HayashiB,C, Takekazu IshidaA,C,D‡

ADepartment of Physics and Electronics, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan BNanoscience and Nanotechnology Research Center (N2RC), Osaka Prefecture University, 1-2 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan CCREST(JST), Sanbon-cho, Chiyoda-ku, Tokyo 102-0075, Japan DInstitute for Nanofabrication Research, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan

Thanks to the extensive studies on the multi-band superconductivity in MgB2, the torque has been recognized as a powerful tool for investigating anisotropic superconductors. Ko- gan1, 2 introduced the two-independent anisotropy parameters for describing the multiband superconductivity, i.e., the anisotropy in coherence length γξ = ξa/ξc and the anisotropy in penetration depth γλ = λc/λa. However, the contribution of vortex core has not been taken into account properly in the theory while it is supposed that the role of core becomes effec- tual in analyzing γξ in multiband superconductors such as MgB2. In this work, we extended the Kogan torque theory with the aid of the prescription of Yaouanc et al.3 in expressing the magnetic field in the mixed state, and obtained the modified torque formula successfully as ∂F θ HV ∂ B b4 v K v τ θ V ( c) 0(1 − ) pq 1( pq), ( c) = − = − 2 2 ∂θc π X ∂θc p pq q 8 (p,q)6=(0,0) − + where θc is an angle between the direction of applied field H and the c-axis, V is sample vol- ǫ θ 2 θ γ2 2 θ p q ume, ξ,λ( c) = qsin c + ξ,λ cos c, lattice indices and in the reciprocal space are inte- √ 2 2 1/3 gers, K1(vpq) is a modified Bessel function, Φ0 is the flux quantum, B0 = 3Φ0ǫλ(θc)/8π λ γλ , ⊥c 2 4 2 2 2 b = Hǫξ(θc)/H , v = 4πb(1 + b )[1 − 2b(1 − b) ][βξλ(θc)(q − p/2) + p /βξλ(θc)], and c2 √pq βξλ(θc) = 2γξǫλ(θc)/ 3γλǫξ(θc). The Kubota-Hayashi-Ishida (KHI) formula developed here is a natural extension of the Kogan theory, and should be very useful in analyzing the torque curves of various sorts of anisotropic superconductors. The nature of the multiband features is of special interest in utilizing the KHI torque formula for understanding anisotropic superconductors. [1] V. G. Kogan, Phys. Rev. B 38 (1988) 7049. [2] V.G. Kogan, Phys. Rev. Lett. 89 (2002) 237005. [3] A. Yaouanc, P. Dalmas de Réotier,and E. H. Brandt, Phys. Rev. B 55 (1997) 11107.

∗This work was partly supported by a Grant-in-Aid for Scientiﬁc Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No. 19206104), a grant from JST-Sentan Program, and a special grant from Osaka Prefecture University. †E-mail: [email protected] ‡Corresponding author: [email protected] Iron Pnictide Superconductors in Very Strong Magnetic Field

Jan Jaroszynski∗

National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA

We present detailed measurements of the longitudinal resistivity and the upper critical field Hc2 of different iron pnictide superconductors in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. In particular, we studied 1111 compounds [1-3] as ReFeAsO (Re=La, Sm, Nd), 122 compounds [4] as BaFe2As2 and 11 compounds as Fe(Se,Te). We found that the field scale of Hc2 is over 100 T for 1111, and 50-70 T for 122 and 11 compounds, respectively. Hc2(T ) measured parallel to the c-axis exhibits a pronounced upward curvature similar to what was extracted from measurements on polycrystalline samples. Thus this behavior is indeed an intrinsic feature of iron pnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of Hc2 shows strong deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as T decreases. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of Hc2, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on Hc2(T ), which may significantly reduce Hc2(0) as compared to Hc2(0) ∼ 200 − 300 T based on extrapolations of Hc2(T ) near Tc down to low temperatures. Moreover, pnictide superconductors with lower Tc behave as an intermediate- Tc superconductor like MgB2 in which thermal fluctuations of vortices do not significantly affect the H–T diagram to the extent that they do in the layered cuprates. However, the situation is different for the higher Tc oxypnictides, which exhibit a larger mass anisotropy and enhanced thermal fluctuations. Thus, the series of pnictide superconductors bridges a conceptual gap between conventional superconductors and the high-temperature cuprates.

[1] F. Hunte, J. Jaroszynski, A. Gurevich, D.C. Larbalestier, R. Jin, A.S. Sefat, M.A. McGuire, B. C. Sales, D.K. Christen, and D. Mandrus, Nature 453, (2008) 903. [2] J. Jaroszynski, S.C. Riggs, F. Hunte, A. Gurevich, D.C. Larbalestier, G.S. Boebinger F. F. Balakirev, A. Migliori, Z. A. Ren, W. Lu, J. Yang, X.L. Shen, X.L. Dong, Z. X. Zhao R. Jin, A. S. Sefat, M. A. McGuire, B.C. Sales, D.K. Christen, and D. Mandrus Phys. Rev. B78 (2008) 064511. [3] J. Jaroszynski, F. Hunte, L. Balicas, Youn-jung Jo, I. Raiˇcevi´c,A. Gurevich, D.C. Larbalestier, F.F. Balakirev, L. Fang, P. Cheng, Y. Jia, and H. H. Wen, Phys. Rev. B78 (2008) 174523. [4] A. Yamamoto, J. Jaroszynski, C. Tarantini, L. Balicas, J. Jiang, A. Gurevich, D.C. Larbalestier, R. Jin, A. S. Sefat, M.A. McGuire, B. C. Sales, D.K. Christen and D. Mandrus, Appl. Phys. Lett. 94 (2009) 062511.

∗E-mail: [email protected] Vortex Noise Anisotropy in YBa2 Cu3O7−δ Thin Films

E. ZilberA, I. BarboyA, C. CamerlingoB, and G. JungA

A Department of Physics, Ben Gurion University of the Negev, 84105 Beer Sheva, Israel B Consiglio Nazionale del le Ricerche, Istituto di Cibernetica E. Caianiel lo, 80078 Pozzuoli (NA), Italy

The dependence of voltage noise on the direction of current flow with respect to c-axis of high-TC YBa2Cu3O7−δ (YBCO) thin films has been investigated. The experiments were performed using vicinal (103)/(013) oriented YBCO films with c-axis tilted 45◦ from the direction normal to the substrate. Patterning of several strips along various angles to the substrate edges enabled us to force current flow in various directions with respect to the c-axis. It has been determined that noise in the normal state is isotropic while the noise intensity and its spectral form in the superconducting state depend on the direction of current flow. The difference steams from different origins of noise in both states. The noise in normal state is due to randomness in the motion of charge carriers while in the superconducting state it arises from fluctuations of density and/or velocity of moving magnetic flux vortices. Possible Strain and Charge-Induced Superconductivity in Yttrium-doped Sr-122 FeAs System

Tomoki Goya,∗ Shen V. Chong, Kazuo Kadowaki†

Institute of Materials Science and Graduate School of Pure & Applied Sciences, University of Tsukuba, Ibaraki 305-8573, Japan

The SrFe2As2 iron-arsenide system poises as an interesting jig-saw piece in recently discovered iron-based superconductors. In many ways, it behaves similarly to the other 122- FeAs family members where chemical doping1 or applying hydrostatic pressures2 on the non-doped parent compound can induced superconductivity with a complete suppression of the spin-density wave (SDW) anomaly at optimum doping or applied pressures. SrFe2As2 is dissimilar to the other 122 Fe-arsenides where two additional mechanisms can also induce superconductivity in the parent compound. Superconductivity has been reported in 3 SrFe2As2 single crystals without chemical doping and in thin-films of SrFe2As2 exposed to 4 water vapor at ambient pressure with Tc registering at around 21 and 25 K, respectively, without suppressing or shifting of the SDW anomaly. In these later two cases, it has been proposed that residual strain on the single crystals created during the synthesis process or in the case of SrFe2As2 thin-film some sort of charge-doping and a c-axis contraction caused by the adsorption of water molecules gave rise to superconductivity. In our study on the doping of yttrium into SrFe2As2, we have observed Tc as high as 26 K on polycrystalline samples prepared via conventional solid state reaction. Similar to the strain and water induced superconductivity mentioned above the SDW anomaly in temperature dependent resistivity (R-T) measurements was not suppressed. We have recently substantiated this finding in a separate experiment by melting a nominal Sr0.6Y0.4Fe2As2 polycrystalline mixture in an inductive furnace subsequently cooled very slowly to room temperature. A Tc slightly higher than 25 K was achieved also without suppressing the SDW anomaly in R-T. Hall effect measurements on the polycrystalline sample with Tc ca. 26 K indicates the injection of extra electron carriers into the Sr-122 system, while powder x-ray diffraction did not indicate any significant lattice parameters alteration. Based on these, we have proposed a combination of strain and charge-induced superconductivity in Sr1−xYxFe2As2.

[1] K. Sasmal et al., Phys. Rev. Lett. 101 (2008) 107007. [2] P. L. Alireza et al., J. Phys.: Condens. Matter 21 (2009) 012208; T. Terashima et al., J. Phys. Soc. Jpn. 78 (2009) 083701. [3] S. R. Saha et al., unpublished (cond-mat/0811.3940). [4] H. Hiramatsu et al., Phys. Rev. B 80 (2009) 052501.

∗E-mail: [email protected] †E-mail: [email protected] Macroscopic quantum tunneling in ultra-thin mesa structures of highly-doped BSCCO crystals

I. KakeyaA,∗ T. TachikiA, K. HamadaA, T. WatanabeB, M. SuzukiA,C†

ADepartment of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan BDepartment of Advanced Physics, Hirosaki University, Aomori 036-8561, Japan CPhotonics and Electronics Science and Engineering Center, Kyoto University, Kyoto 615-8510, Japan

Macroscopic quantum tunneling (MQT) is one of the most remarkable phenomena in a Josephson junction, where a freedom of the quantum mechanics is found in a macroscopic quantity such as voltage or current. In an intrinsic Josephson junction (IJJ) of a high-Tc superconductor, the crossover temperature Tcr between MQT and thermal activation (TA) regions is expected as a few Kelvins because of its high Josephson critical current in contrast to an artificially fabricated Josephson junction. Recently, it was reported that MQT with 1 Tcr ' 1 K was found in a switching dynamics of a Bi2Sr2CaCu2O8+δ (Bi2212) IJJ. So far, we obtained Tcr ' 0.4 K in a Bi2212 mesa structure with a dimension of less than 2 µm × 2 µm in the ab-plane and a few IJJ superlattices along the c-axis.2 The mesa structure has advantages for controlling number of IJJs and equilibrium between the sample and the thermal bath, whereas the normal electrode reduces the critical current Ic of the topmost IJJ because of the proximity effect. In order to increase Ic of the topmost IJJ, we prepared mesa structures from crystals of Bi1.9Pb0.1Sr2.1Ca0.8Cu2.0O8+δ (PbBi2212) and Bi2.1Sr1.9Ca2Cu3O10+δ (Bi2223). In the current-voltage characteristics, Ic of the superconducting (R = 0) branch was found to be comparable with other branches, indicating that the reduction in Ic of the topmost IJJ was suppressed. With decreasing temperature, switching probability distributions (SPDs) of the superconducting branch start to deviate from the TA model around 10 K and their widths tend to saturate at 5 K. These are considered as symptoms of MQT because Tcr’s are estimated via the fluctuation-free critical current yielded by SPDs in the TA region as 2.2 and 1.4 K for PbBi2212 and Bi2223, respectively. In the conference, results of SPD measurements up to 0.4 K will be presented.

[1] K. Inomata, et al., Phys. Rev. Lett. 95 (2005) 107005. [2] K. Ota, et al., Phys. Rev. B 79 (2009) 1234505.

∗E-mail: [email protected] †E-mail: [email protected] Manipulation of vortex states in mesoscopic superconductors by local supercurrent injection

A. KandaA,∗ M. V. Miloˇsevi´cB,† S. HatsumiA, Y. KurodaA, Y. OotukaA, F. M. PeetersB

AInstitute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan BDepartement Fysica, Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium

In mesoscopic superconductors with sizes comparable to the superconducting coherence length and the magnetic penetration depth, the vortex arrangement is strongly influenced by the sample boundary, leading to novel vortex states such giant vortex states and multivortex states. Up to date, changing the magnetic field is the only way to control the quantum vortex states in mesoscopic superconductors. In this paper, we report strategic current injection in a small mesoscopic superconductor and control of the (non)equilibrium quantum states in an applied homogeneous magnetic field. In doing so, we realize a current-driven splitting of multi-quanta vortices, current-induced transitions between states with different angular momenta, and current-controlled switching between otherwise degenerate quantum states. These fundamental phenomena form the basis for electronic and logic applications, and are confirmed in both theoretical simulations and delicate transport measurements1.

[1] M. V. Miloˇsevi´c,A. Kanda, S. Hatsumi, F. M. Peeters, Y. Ootuka, submitted.

∗E-mail: [email protected] †E-mail: [email protected] Switching dynamics of Bi2Sr1.6La0.4CuO6+δ intrinsic Josephson junctions

Hiromi KashiwayaA,∗ Tetsuro MatsumotoA, Hajime ShibataA, Hiroshi EisakiA, Yoshiyuki YoshidaA, Shiro KawabataA, Yukio TanakaB, Satoshi KashiwayaA

ANational Institute of Advanced Industrial Science and Technology (AIST), Ibaraki 305-8568, Japan BFaculty of Science and Technology, Nagoya University, Nagoya 464-8603, Japan

The switching dynamics of Bi2Sr1.6La0.4CuO6+δ (Bi2201) intrinsic Josephson junctions (IJJs) is studied by focusing on the effects of the strong coupling between junctions in the stack. We succeeded in fabricating high quality Bi2201 IJJs using a FIB process. The fabricated junctions showed peculiar switching properties, i.e., the number of junctions switching simultaneously to the finite voltage states could be tuned by changing the load resistance of a bias circuit. Thus we can compare the dynamics of the single junction switching (SJS) and the multiple junction switching (MJS) on the same junction. We believe that the appearance of the MJS is the manifestation of the rather enhanced coupling effect due to the single layered structure of Bi2201. The switching probability distributions and their microwave responses of Bi2201 IJJ were measured as the functions of the temperature and the number of simultaneously switching junctions. The distribution width showed saturation behavior below about 0.4K suggesting the transition of the switching mechanism from the thermal activation to the macroscopic quantum tunneling (MQT). The measured switching properties of MJS at MQT region were inconsistent with either those of Bi2212 SJS1 or those of Bi2212 uniform switching cases2. The effects of strong coupling effect on the MQT are discussed.

[1] H. Kashiwaya, T. Matsumoto, H. Shibata, S. Kashiwaya, H. Eisaki, Y. Yoshida, S. Kawabata, and Y. Tanaka, J. Phys. Soc. Jpn. 77 (2008) 104708 [2] X. Y. Jin, J. Lisenfeld, Y. Koval, A. Lukashenko, A. V. Ustinov, and P. Muller, Phys. Rev. Lett. 96 (2006) 177003

∗E-mail: [email protected] Vortex structures and dynamics in ﬁnite and asymmetric superconducting networks

Masaru KatoA,B,∗ Yoshiteru IwamotoA, Osamu SatoC†

ADepartment of Mathematical Sciences, Osaka Prefecture University, 1-1, Gakuencho, Nakaku, Sakai, Osaka 599-8531, Japan BCREST, JST,5 Sanban-cho, Chiyoda-ku, Tokyo 102-0075, Japan CDepartment of Liberal Arts, Osaka Prefectural College of Technology,26-12 Saiwai-cho, Neyagawa, Osaka 572-8572, Japan

Superconducting networks are composed of superconducting wires, which are connected to other wires. The simplest network is a ring of superconducting wire. It shows periodic change of the transition temperature with increasing applied magnetic field perpendicular to the ring. This oscillation comes from the entrance of the quantized magnetic flux Φ0 = hc/2e, and is well known as the Little-Parks oscillation. Previously we studied finite superconducting networks satokato1,katosato1. Their magnetic field dependence of the transition temperature and magnetic structures are much different from the periodic networks. There appear the giant or doubly quantized vortices and anti-vortices and their structures have the symmetry of the original network. But in the experiment, observed the magnetic structures are somewhat different from the theoretically expected structures, because of the disorders in the networks. About the vortex dynamics, rectified motions of vortices in superconducting films with asymmetric antidots were observed. Such motion is also expected for the asymmetric networks. Therefore in this work, we investigate the vortex structures in the disordered networks and vortex dynamics in the asymmetric networks. We use the phenomenological Ginzburg-Landau (GL) Theory for both networks. For disordered networks, we investigate the transition temperature and the vortex structures. For this purpose, we use the linearized GL equation or de Gennes-Alexander (dGA) equation. The dGA equation is given as,

iγ ′ cos θij ′ e ij i − j . ∆ X X ∆ = 0 (1) j ξij sin θij j ξij sin θij

The disorders are introduced by changing the coherence length at zero temperature and T transition temperature in, ξij = ξ0ij/ 1 − . We also studied the dynamics of the vortices q Tcij in the asymmetric networks, solving the time-dependent Ginzburg-Landau equation (TDGL) with the Free energy, with the Maxwell equation.

[1] O. Sato and M. Kato, Phys. Rev. B 68(2003) 094509. [2] M. Kato, Y. Iwamoto and O. Sato, Phys. Rev. B 80 (2009) 024510.

∗E-mail:[email protected] †E-mail: [email protected] Coherence eﬀect in chiral p-wave vortex

Yusuke KatoA,∗ Nobuhiko HayashiB,C†

ADepartment of Basic Science, University of Tokyo, Tokyo 153-8902, Japan BNanoscience and Nanotechnology Research Center (N2RC), Osaka Prefecture University, 1-2 Gakuen-cho, Naka-ku, Sakai 599-8570 Osaka, Japan CCREST (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

We show that non-magnetic impurities affect, in a nontrivial way, quasi-particles within a chiral p-wave vortex core. There are two types of singular vortices in two-dimensional chiral p-wave superconductors; In a chiral p-wave vortex with vorticity opposite to chirality in sign, the impurity scattering rate Γ by Born scatterers vanishes at low energy limit. In the case where vortex and chirality have a same sign, on the other hand, Γ is the same order as that in normal state. These results have been obtained1, 2, 3, 4, 5 with the use of quasiclassical theory of superconductivity. The vanishing impurity scattering rates are attributed to the coherence effect of Andreev bound states within vortex core1, 3. Recently, however, it was reported6 that the impurity effects in chiral p-wave vortex do not depend so much on the relative sign between vorticity and chirality within the T-matrix approximation for impurity self-energy. We attribute the different consequences between [1-5] and [6] to different types of scatterers.

[1] Y. Kato, J. Phys. Soc. Jpn. 69 (2000) 3378. [2] Y. Kato and N. Hayashi, J. Phys. Soc. Jpn. 71 (2002) 1721. [3] Y. Kato and N. Hayashi, Physica C 388-389 (2003) 519 [4] N. Hayashi and Y. Kato, J. Low Temp. Phys. 139 (2005) 79. [5] Y. Tanuma, N. Hayashi, Y. Tanaka, and A. A. Golubov, Phys. Rev. Lett. 102 (2009) 117003. [6] J. Sauls and M. Eschrig, New J. Phys. 11 (2009) 075008.

∗E-mail: [email protected] †E-mail: [email protected] Magneto-optical imaging of the micowave induced DC ﬂux penetration Heating phenomena

J.KermorvantAB, C.J van Der BeekA, J.C MageB, B. MarcilhacB, Y.LemaitreB, J. BriaticoB, R. BernardB, J.VillegasB

A Laboratoire des solides irradie´s Ecole Polytechnique - CEA/DSM/IRAMIS - CNRS (UMR 7642) BUnite´ Mixte de Recherche en Physique CNRS/THALES

In order to study the effect of microwave magnetic fields on vortex penetration, we have used magneto-optical setup that allows for the visualisation of flux penetration in YBCO thin films during surface impedance measurments. The microwave measurements were performed by using the dielectric resonator method, with a TiO2 (rutile) resonator, operating at 9.9 GHz. Both the magneto-optical imaging and the use of the TiO2 dielectric show that usually observed nonlinearities in the surface impedance of superconductors are due to microwave heating. Magneto-Optical study of grain boundary and critical current in MgB2 and iron-pnictides

Kohji KishioA,∗ Jun-ichi ShimoyamaA,† Akiyasu YamamotoB‡ Anatolii PolyanskiiB§, David LarbalestierB¶

ADepartment of Applied Chemistry, School of Engineering, University of Tokyo Tokyo 113-8656, Japan BApplied Superconductivity Center, National High Magnetic Field Laboratory Tallahassee, FL 32310, USA

Grain boundary properties are an important issue for application of all high-Tc superconductors because practical forms of superconductors are polycrystalline. The recent discovery 1 of the new-class of high-Tc superconductivity in iron-pnictides raised the question“Are grain boundary weak-links inherent to high-Tc superconductors ? ”Recent studies of the inter- and intra-granular current flow in polycrystalline forms of both 11112,3 and 1224 show distinct evidence for weak link behavior which in bicrystal thin film form is now explicit. On the other hand, MgB2 with twice the Tc of 122 pnictides was found to be weak-link-free owing to its rather metallic superconductivity5. Even though there is a weak-link-free supercurrent flow in polycrystal samples, recent studies show that polycrystalline MgB2 samples have anomalously high resistivity values and relatively low transport critical current density, in- 6 dicating that the electrical connectivity of polycrystalline MgB2 is somehow suppressed . In the present study, we performed Magneto-Optical Imaging (MOI) and remanent magnetization analysis using SQUID magnetometry to evaluate the intergranular current of MgB2 and good quality polycrystalline iron pnictides. The relationship between their critical current properties, microstructures and current obstacles will be compared.

[1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008) 3296 [2] A. Yamamoto et al., Supercond. Sci. Technol. 21 (2008) 095008 [3] F. Kametani et al., ArXiv Condmat/0907.4454 (2009) [4] S. Lee et al., ArXiv Condmat/0907.3741 (2009) [5] D. C. Larbalestier et al., Nature 410 (2001) 186 [6] J. M. Rowell, Supercond. Sci. Technol. 16 (2003) R17

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] ¶E-mail: [email protected] Direct observations of vortex states in amorphous MoGe disks by scanning SQUID microscopy

N. KokuboA,∗ S. OkayasuB, A. KandaC, B. ShinozakiD

ACenter for Research and Advancement in Higher Education, Kyushu University, Fukuoka 819-0395, Japan BJAEA, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan CInstitute of Physics, University of Tsukuba, Ibaraki, 305-8571, Japan DDepartment of Physics, Kyushu University, Fukuoka 812-8581, Japan

Vortices confined into mesoscopic superconductors can display unique spatial configurations imposed by the sample geometry, via the influence of the screening current flowing along the sample edge. In the mesoscopic disk, for instance, vortices may form symmetric polygons along a concentric ring(s) characterized by radical and rotational order with respect to the disk center, which were shown in intensive numerical studies in pin-free ”ideal disks”. In experiments, the influence of the bulk pinning and/or the size of the superconductors can be relevant In this study, we report spatial configurations of vortices in mesoscopic ”large” superconducting disks of weak pinning amorphous MoGe film by a scanning SQUID microscope technique. This technique allows us to acquire directly magnetic images over the disks and also to perform how the vortex configuration evolves with magnetic field in a specific sample. We made the disks in diameter ranged from 20-56 µm, which is 30-100 (4000-10000) times larger than the penetration depth (coherence length) of the amorphous MoGe film. In field cool measurements, we observed vortex polygons with quasi-radial, and orientational symmetry with respect to the disk center when the pinning properties are reasonably weak and homogeneous over the disk. These include the concentric vortex ring formations, called ”vortex shell”, for vorticity more than 6. When a relatively strong pinning center is present, this influences significantly on the vortex configurations, showing an interesting evolution of vortex patterns with magnetic field.

∗E-mail: [email protected] Pointlike disorder and vortex matter of Bi2Sr2CaCu2O8 in oblique ﬁelds

M KonczykowskiA,∗ C J van der BeekA,† A E KoshelevB‡V MosserC M LiD and P H KesD

ALaboratoire des Solides Irradi´es,Ecole Polytechnique, CNRS-UMR 7642 & CEA/DSM/IRAMIS, 91128 Palaiseau cedex, France BMaterials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA CITRON, 76 Avenue Pierre Brossolette, 92240 Malakoﬀ, France DKamerlingh Onnes Laboratorium, Rijksuniversiteit Leiden, P.O. Box 9506, 2300 RA Leiden, the Netherlands

Two main first order phase transition lines define diagram of the vortex matter of 1 Bi2Sr2CaCu2O8 in oblique magnetic fields : the melting (or decoupling) transition from solid to liquid with increasing at c-axis field and from crossed lattice to tilted lattice at high in-plane field. The analysis of angular dependence of the solid to liquid transition (FOT) in the regime of tilted lattice allows determination of fundamental parameters of Bi2Sr2CaCu2O8. One can extract the anisotropy factor and the relative importance of the Josephson and magnetic couplings between ”pancake” vortices. Since the magnitude of magnetic coupling is directly related to the in-plane penetration depth λ, its determination provides indirect measurement of this parameter. We use this novel understanding to explore the effect of the point-like disorder on the on the phase diagram of Bi2Sr2CaCu2O8. Disorder was introduced by 2.3 MeV electron beam impinging samples cooled to 20K to prevent defect migration. We have explored the regime of high irradiation doses leading to strong depression of the critical temperature Tc , down to 1/3 of the initial value of 87K. In our previous experiments, limited to low doses, depression of Tc was detectable. At low temperatures the second peak (SMP) in the irreversible magnetization marks an order-to- disorder transition, while at higher temperatures jump in reversible magnetization indicates FOT. Surprisingly, the signatures of FOT remains robust, in the low- as well as in the high temperature region. The strong reduction of the position of SMP is proportional neither to the slope of ∂HFOT /∂T , nor to Tc. An increase of the anisotropy factor after electron irradiation can be inferred from the measurements of the angular dependence of the FOT above the composite-to-tilted vortex lattice transition in oblique magnetic fields. An increase of the penetration depth λ with increasing point disorder can be deduced from the variation of the magnetic coupling contribution between ”pancake” vortices.

[1] M. Konczykowski, C.J. van der Beek, A.E. Koshelev, V. Mosser, M. Dodgson and P.H. Kes, Phys. Rev. Lett.97 (2006) 1237005.

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] Vortex Core States in Superconducting Graphene

N.B. KopninA,B,∗ I.M. KhaymovichC, A. S. Mel’nikovC,† I. A. ShereshevskiiC

ALow Temperature Laboratory, Helsinki University of Technology, Finland B L. D. Landau Institute for Theoretical Physics, 117940 Moscow, Russia CInstitute for Physics of Microstructures, 603950 Nizhny Novgorod, Russia

We consider the vortex core subgap electronic spectrum and the problem of zero energy modes in superconducting graphene1. We demonstrate that, with an increase in the doping level µ, the distance between the energy levels as functions of the angular momentum ν decreases, and the set of energy levels for vortex vorticity n gradually transforms into a set (i) 2 of n “spectrum branches” En (ν) of the Caroli–de Gennes–Matricon type where i assumes n integer values. These n energy branches cross the Fermi level3 as functions of ν. The detailed behavior of the spectrum for various µ crucially depends on the vorticity. For odd (0) n there exists one branch, En , which intersects zero energy at ν = 0. Its crossing point belongs to the spectrum, thus resulting in an exact zero energy mode. The crossing points of other n − 1 branches do not generally belong to the spectrum for ﬁnite µ, thus these zero modes do not exist, as distinct to the the case of zero doping4. For an even winding number the crossing points of all n branches do not generally belong to the spectrum if µ =6 0. In this case the exact zero modes are absent.

1 1 (a) (b) 0 0 ∆ ∆ E/ E/

0 0

−1 −1 −30 −20 −10 0 10 20 ν 30 −30 −20 −10 0 10 20 ν 30

Figure 1: The subgap spectrum vs the angular momentum ν for a singly (a) and doubly (b) quantized vortex for two doping levels: µ/∆0 = 1 (circles) and µ/∆0 = 10 (squares).

[1] I. M. Khaymovich, N. B. Kopnin, A. S. Mel’nikov, and I. A. Shereshevskii, Phys. Rev. B, 79, 224506 (2009). [2] C. Caroli, P. G. de Gennes, J. Matricon, Phys. Lett. 9, 307 (1964). [3] G. E. Volovik, The Universe in a Helium Droplet (Oxford University Press, 2003). [4] R. Jackiw and P. Rossi, Nucl. Phys. B 190, 681 (1981).

∗E-mail: [email protected].ﬁ †E-mail: [email protected] New dynamic kink states in intrinsic Josephson junctions generated by cavity resonance in magnetic ﬁeld

Alexei Koshelev ∗

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439

Intrinsic Josephson-junction stacks are realized in mesas fabricated out of high-temperature superconductors, such as Bi2Sr2CaCu2O8+x. Phase oscillations in different junctions can be synchronized via coupling to the internal cavity mode leading to powerful electromagnetic radiation in terahertz frequency range1. As homogeneous oscillations do not couple directly to the cavity modes, the mechanism of mode excitations is a nontrivial issue. New dynamic state providing a very efficient coupling has been demonstrated recently2. A key feature of this state is the alternating phase kinks and antikinks statically located near the center and coexisting with the oscillating phases. We studied evolution of this state with increasing magnetic field at fixed current and found several dynamic regimes. The initial kink state remains stable up to a certain critical field, similar to the lower critical field. Above this field the kink configuration starts to restructure and we observe formation of different static kink lattices existing inside dynamic states. These lattices represent completely new states stabilized by the resonance mode and they differ considerably from well-known static and moving Josephson vortex lattices. The voltage monotonically increases with the magnetic field. Be- havior dramatically changes when the voltage exceeds the resonance voltage. Above this voltage the kink states become more disordered and the voltage-field dependence becomes noisy. Nevertheless these states are still coherent and support strong resonance oscillations. Finally, above certain field, the coherence is lost completely and chaotic state emerges. Be- sides the kink lattices, we found that in the field range close to half flux quantum per junction a simple homogeneous state is generated. We will also discuss (i) stability analysis of the coherent states and (ii) synchronization regimes in inhomogeneous mesas. This work was supported by the U. S. DOE, Office of Science, under contract # DE-AC02-06CH11357.

[1] L. Ozyuzer et al. , Science 318, 1291 (2007); A. E. Koshelev and L. N. Bulaevskii, Phys. Rev. B 77, 014530 (2008). [2] Sh. Lin and X. Hu, Phys.Rev. Lett., 100, 247006 (2008); A. E. Koshelev, Phys. Rev., B 78, 174509 (2008).

∗E-mail: [email protected] Magnetic Quantum Critical Fluctuations Strongly Couple to the Superconductivity with High Tc.

L. Krusin-ElbaumA , T. ShibauchiB , Y. KasaharaC, R. OkazakiB, Y. MatsudaB , R. D. McDonaldD , C. H. MielkeD, & M.HasegawaE

AIBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA BDept. of Physics, Kyoto University, Kyoto 606-8502, Japan CInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan DNHMFL, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA EDept. of Materials Science and Engineering, Nagoya University, Nagoya 464-8603, Japan

There are two general explanations for high-transition temperature (high-Tc) superconductivity: the superconducting pairing might be mediated through lattice vibrations, or through magnetic (spin-singlet antiferromagnetic, or orbital) fluctuations. We have previously discovered that ‘strange’ non-Fermi-liquid normal state transforms into a conventional metal at high magnetic fields, which was taken as evidence for its magnetic origin, but left unresolved the critical issue of the connection of the superconductivity to the anomalous normal-state pseudogap, with its peculiar sensitivity to the number of charge carriers (doping). Our most recent transport study of heavily doped cuprates Tl2Ba2CuO6+x uncovers a unique doping point pc in the phase diagram where the pseudogap state transforms directly into a Fermi-liquid. The pseudogap boundary T ⋆ shows the hallmarks of a phase transition — it scales with field and with doping, and it develops a thermodynamic divergence (∂T ⋆/∂H → ∞) in the quantum limit corresponding to a zero entropy jump. The distinct pseudogap order is destroyed by its quantum critical fluctuations, and pc shifts under magnetic field toward lower doping in unison with changes in Tc. Our data lead us to conclude that the strange metal state is controlled by the magnetic quantum critical fluctuations, which strongly couple to the superconductivity with high Tc. NMR study of magnetic and superconducting order of CeCoIn5 in a high field and low temperature phase K. KumagaiA,∗ N. KondohA, H. ShishidoB and Y. MatsudaB

ADepartment of Physics, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan B Department of Physics, Kyoto University, Kyoto 606-8502, Japan

A heavy-fermion superconductor, CeCoIn5, is believed to host a Fulde-Ferrell-Larkin- Ovchinnkov (FFLO)1 state at a restricted region at high field and at very low temperature 2. This compound is also located near at the field induced quantum critical point. Re- cent NMR3 and neutron4 experiments discover an anomalous magnetic ordering within the novel inhomogeneous superconducting state. We direct our attention to a puzzling interplay between novel superconducting state and modulated magnetism of CeCoIn5. NMR of the In(1), In(2) and Co site have been measured in a wide frequency range and in the low temperature range down to 50 mK. Upon entering the novel SC state, NMR spectra change dramatically below T (Hc) in all the sites. The most striking future is that a well-separated peak structure and characteristic broadening of the spectra of the In(2)k site are observed only in the novel superconducting phase. This experimental fact indicates an appearance of finite hyperfine fields coupled to magnetically-ordered Ce moments. The magnetic anomaly is never observed in the normal state and also in the BCS superconducting phase. Based on our detail magnetic and temperature dependences of the NMR results, we will discuss a possible magnetic structure coupled inhomogeneous superconducting order parameter in the FFLO phase. We also discuss strong Pauli paramagnetic effects on the temperature and field dependences of the Knight shift in the vicinity of the first order transition at Hc2.

[1] P. Fulde, R.A. Ferrell, Phys. Rev. 135 (1964) A550, and A. I. Larkin and Y. N. Ovchin- nikov, Sov. Phys. JETP 20 (1965) 762. [2] A. Bianchi et al., Phys. Rev. Lett. 91 (2003) 187004. [3] B.L.Young et al., Phys. Rev. Lett. 98, 036402 (2007). [4] M. Kenzelmann et al., Science 321, 1161818 (2008).

∗E-mail: [email protected] Eﬀects of Compound Pinning in YBa2Cu3O7−d

W. K. KwokA, J. HuaAB, U. WelpA, J. SchleuterA, Z. L. XiaoAB, and A. KayaniC,

AMaterials Science Division, Argonne National Laboratory, Argonne, IL 60439 BDept. of Physic, Northern Illinois University, DeKalb, IL 60115 CDept. of Physics, Western Michigan University, Kalamazoo, MI 49008

The enhancement of vortex pinning by compound correlated and point defects induced by particle irradiation is investigated in a pristine untwined YBa2Cu3O7−d single crystal. It is well established that columnar defects created by high-energy heavy ions offer the strongest pinning sites in cuprate superconductors due to their ability to pin the vortex along its entire length and consequently enhance the irreversibility line of the superconductor to higher fields and temperatures. In contrast, point defects created by proton irradiation tend to lower the irreversibility line, although the critical current below this line is enhanced. Furthermore, point defects do not generally alter the inherent superconducting anisotropy whereas columnar defects, owing to its correlated nature can strongly alter the pinning anisotropy. We explored the combined effect of correlated and point defects on the vortex pinning response in a YBa2Cu3O7−d single crystal. We performed 1.4 GeV Pb ion irradiation on a pristine untwined YBCO crystal to a dose matching field of BΦ = 2.0 T. Subsequently, we irradiated the same crystal with 9 Mev and 6 MeV protons. We found that proton irradiation can further enhance the critical current Jc(H) through vortex creep mitigation by localizing the vortices near the columnar defects and furthermore, reduce the pinning anisotropy. Hence compound defects may open new avenues for overcoming the large pinning anisotropies that hinder the applications of cuprate superconductors.

This work was supported between the UChicago, Argonne, LLC and the US Department of Energy – Basic Energy Science and Oﬃce of Nuclear Physics – under Contract No. DE- AC02-06CH11357. Synchronization of intrinsic Josephson junctions in Bi-2212 by weak magnetic ﬁeld oriented perpendicular to the layers

Yu.I. LatyshevA, V.N. PavlenkoA, J. ChenB

AInstitute of Radio-Engineering and Electronics RAS, 125009 Moscow, Russia BNanjing University RISE, 210093 Nanjing, China

We studied a response of Bi-2212 mesa type structures to 100 GHz microwave radiation. We found that applying magnetic field of about 0.1 T across the layers enables to observe collective Shapiro step response corresponding to a synchronization of all 50 intrinsic Joseph- son junctions (IJJ) of the mesa. At high microwave power we observed up to 10th harmonics of the fundamental Shapiro step. Besides, we found microwave induced flux-flow step which position is proportional to the square root of microwave power and that can exceed at high enough powers 1 THz operating frequency of IJJ oscillations [1]. A remarkable feature is that Shapiro steps appear only at voltages below the voltage position of microwave induced JFF step. To explain these results we consider the following qualitative model. As known, at low temperatures perpendicular magnetic field of about 0.1 T drives Bi-2212 system close to the transition into the vortex glass state [2]. This state is characterized by a disappearance of the hysteresis of the IV characteristics [3]. In that state pancake vortex lines become highly entangled. That leads to the appearance in a system of Josephson vortices (JVs) and antivortices. Combined DC and microwave currents, applied across the layers, directly affect only JVs. Therefore, due to the weak attractive interaction between JVs and pancake vortex lines [4], the motion of JVs may be considered as a motion in a quasiperiodic washboard potential with a periodicity of pancake vortex lines [5]. The pancake vortex lines piercing the whole thickness of the mesa synchronize periodic potential in different IJJs and thus synchronize the motion of Josephson vortices in the whole mesa. For H⊥ = 0.1 T the 1/2 spacing between PV lines Λ ∼ (Φ0/H⊥) ∼ 0.15 µm. That provides terahertz washboard frequency for easily achievable velocity of JVs 106 − 107 cm/s. Two types of characteristic modes appear in this model. First one corresponds to the washboard frequency and equals to the n times of the external frequency where n is the number of periods which JV travels for one cycle of the microwave current. That is proportional to the amplitude of microwave current or P 1/2. In a presence of DC current the motion of JV can be extended. Then mode locking appears when for one cycle of AC current JV displaced in one direction on integer number m of periods. Those modes correspond to Shapiro steps of the mth order. To observe Shapiro steps of higher order at the given amplitude of microwave current one needs to apply more high DC current. That is consistent with the experiment. Also from this consideration it is clear that n always exceeds m, i.e. frequency of Shapiro step harmonics is always less then washboard frequency. That is also consistent with our experiment. The work has been supported by Presidium of RAS Program#27“ Base researches of nan- otechnologies and and nanomaterials ”and RFBR grant # 06-02-72551-CNRS a. [1] V.N. Pavlenko, Yu.I. Latyshev, J. Chen, M.B. Gaifullin, A. Irzhak, S.-J. Kim, P.H. Wu, JETP Lett, 89, 291 (2009). [2] see e.g. M. B. Gaifullin, Yuji Matsuda, N. Chikumoto, J. Shimoyama, and K. Kishio, Phys. Rev. Lett., 84, 2945 (2000). [3] M.B. Gaifullin, K. Hirata, S.Ooi, S. Savel ’ev , Yu. I. Latyshev, T. Mochiku, Physica C, 468, 1896 (2008). [4] A.E. Koshelev, Phys. Rev. Lett.83, 187 (1999). [5] Y. Yamada, Koji Nakajima and Kensuke Nakajima, J. Kor. Phys. Soc., 48, 1053 (2006). Dynamics of Josephson vortex chains probed by microwave excitation

Gil-Ho Lee, Yong-Duk Jin, Hu-Jong Lee∗

Department of Physics, Pohang University of Science and Technology, Pohang, Korea

Atomically tailored intrinsic Josephson junctions (IJJs) are formed in Bi2Sr2CaCu2O8+x single crystals, which consist of stacks of superconducting CuO2 layers weakly coupled with each other. We fabricated long stack of IJJs with a lateral size of 1 µm×10 µm with double- side-cleaving technique. Applied with the in-plane magnetic field (4.5 T) penetrating into the long side of the stack, current-voltage characteristic curve shows multiple Josephson vortex ?ow branches at low bias regime, of which switching corresponds to a depinning process of Josephson vortex chains out of the pinning potential. We investigated the switching current distribution of the last branch with lowering temperature from 8 K to 0.3 K. Down to the crossover temperature, Tcr ∼ 3.8 K , the current distribution width gets smaller, which is in good agreement with theoretical fit to a thermal activation (TA) process out of the microresistor-type pinning potential. Below Tcr the distribution width is almost insensitive to the bath temperature, which is presumably due to a macroscopic quantum tunneling (MQT) process as a depinning mechanism. At base temperature of 0.3 K we examined the possibility of the energy level quantization in the pinning potential by applying microwave to the stack. With the appropriate conditions of microwave frequency and power, we could observe evolving feature of the depinning current distribution, which consists of the primary and the resonance peaks. The resonance peak position (Ir) data with the microwave 1/4 frequency (f) fit well to a theoretical relation of f = f0/n(1 − (Ir/Ic)) , where n is the number of photons involved and f0 is a small-oscillation frequency (∼ 81 GHz) and Ic is the critical current in the absence of any fluctuations. Although genuine MQT behavior of multi-vortex system is not clear yet, the double peak feature of the depinning current distribution under microwave excitation indicates the quantized energy levels in the pinning potential.

∗E-mail: [email protected] Non-collective Josephson-vortex dynamics caused by pancake-vortex pinning in naturally stacked Josephson junctions

Yong-Duk JinA, Gil-Ho LeeA, Hu-Jong LeeA,∗ Myung-Ho BaeB, and and A. E. KoshelevC,

APohang University of Science and Technology, Pohang 790-784, Korea BUniversity of Illinois, Urbana-Champaign, Illinois, USA CArgonne National Laboratory, Argonne, Illinois, USA

Highly anisotropic cuprate superconductors such as Bi2Sr2CaCu2O8+x (Bi-222) have both Josephson vortices (JVs) and pancake vortices (PVs) in an in-plane and a c-axis magnetic field, respectively. The interaction between Josephson and pancake vortices attracts high interest of current studies. It has been demonstrated that the motion of PVs can be manipulated by the JVs via attractive interaction between them, which suggests that the JVs can also be controlled by the PVs. A JV behaves as a quantum object, for which the PVs can provide an intrinsic potential wells to implement the vortex-based quantum bits in highly anisotropic superconductors. In this study, we report the influence of PVs on the motion of JVs in Bi-2212 single crystals in a dynamic regime. We observe that the JVs forced by the tunneling bias current along the c axis, in the presence of low-density PVs, induce a group of new sub-branches in the JV-flow current-voltage characteristics. Analysis of Josephson- vortex-flow tunneling conductance characteristics in different field strength and slight tilt angles from the in-plane position confirms that the multiple JVF branches arise from the non-collective depinning JVs in each junction. Intrinsic pinning by the PVs therefore provides a means for the quantum control of JVs for the future quantum electronics based on the high-critical-temperature superconductors.

∗E-mail: [email protected] Fluctuation, vortex glass phase and gap symmetry for single crystals of the BaFe1.8Co0.2As2 superconductor

Sung-Ik LeeA,∗ Ki-Young ChoiA, Soo Hyun KimA, Changho ChoiA, Myung Hwa JungA, X.F.WangB, and X.H.ChenB,

ANational Creative Research Initiative Center for Superconductivity, Department of Physics, Sogang University, Seoul 121-742, Republic of Korea BHefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

To understand the superconductivity of iron-pnictide superconductors, we synthesized optimally doped single crystals of BaFe1.8Co0.2As2, which had a critical temperature, Tc, of 23.6 K. We observed that the fluctuation magnetization and fluctuation conductivity follow the three dimensional scaling form in the critical fluctuation region above 0.5 Tesla which indicates that this material belongs to the three dimensional superconductors. The I-V characteristics in magnetic field followed the 3D scaling behavior and identified the existence of vortex glass phase. The full range of the temperature dependence of Hc1 was explained by using a multi-gap structure, such as two s-wave gap symmetry. We estimated the magnitude of the two gap as ∆1(0) = 1.64 ± 0.2 meV for the small gap and ∆2(0) = 6.20 ± 0.2 meV for the large gap. The super-fluid density deviated from the Uemura relation.

∗E-mail:[email protected] Excitation of Josephson Plasma by π Phase Kink and Terahertz Radiation in Layered High-Tc Superconductors

Shizeng Lin∗and Xiao Hu†

WPI Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japan

Intrinsic Josephson junctions in high-Tc superconductors are very important for generation of electromagnetic waves in terahertz band, which have many applications ranging from sensing, imaging to spectroscopy. In 2007, a great breakthrough has been achieved by the joint group from Argonne National Lab and University of Tsukuba, in which continuous and 1 coherent terahertz radiation from Bi2Sr2CaCu2O8+δ was observed clearly . However it is not clear how to pump dc input energy into plasma oscillation in the absence of external magnetic ﬁeld. We proposed a novel dynamic state in a stack of intrinsic Josephson junctions to explain the experiment2. In this state, there is static phase kink piled up periodically along the c axis, through which large amount of dc energy is pumped into the plasma oscillation. Furthermore, the oscillation is coherent in all junctions. As a result, we have superradia- tion, which means that the radiation power is proportional to N squared, with N being the number of junctions. The maximum radiation power is about 10mW at 1THz according to our theoretical calculations.

[1] L. Ozyuzer et al., Science 318, 1291 (2007). [2] S. -Z. Lin and X. Hu, Phys. Rev. Lett. 100, 247006 (2008).

∗E-mail: [email protected] †E-mail: [email protected] Theory of Josephson Vortices and Collective Modes in Multi-bands Superconductors

Masahiko MachidaA,C,D ,∗ Yukihiro OtaA,C, Tomio KoyamaB,C, Hideki MatsumotoB,C

ACCSE, Japan Atomic Energy Agency, Tokyo 110-0015, Japan BInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan CCREST(JST), Saitama 332-0012, Japan DJST, TRIP, Tokyo 102-0075, Japan

1 The discovery of iron-based high-Tc superconductor has triggered a numerous number of studies on its superconducting mechanism and properties. Consequently, it is now well- known through various experiments that multi-bands contribute to the superconductivity and multi superconducting full(s-wave)-gaps open below the transition temperature. On the other hand, many theories have suggested that a sign change occurs between the multiple s-wave gaps by regarding the magnetic fluctuation between the disconnected Fermi surfaces as a pairing channel. Such a sign change has been called ±s-wave and its peculiar features have been intensively explored. In this presentation, we theoretically study typical features of Josephson vortex structure in Josephson junctions having multiple tunneling channels mainly caused by multiple superconducting gaps. The target junction systems are a heterotic junction composed of multi-gap-superconductor insulator single-gap-superconductor (SIS)2, and a grain-boundary junction formed by two identical multi-gap superconductors3. On the two junction systems, we find that the magnetic field distribution of the Josephson vortex unexpectedly enlarges for ±s-wave through “± coupled sine-Gordon equation” derived from actions considering multiple tunneling channels. The reason is theoretically ascribed to effective enlargement of the penetration depth due to cancellation of multiple Josephson currents. We display such an anomalous vortex image on the two cases. Furthermore, we predict the presence of in-phase and out-of-phase collective oscillation modes of superconducting phases in the above SIS junctions2. The former corresponds to the Josephson plasma mode whose frequency is drastically reduced for ±s-wave symmetry, and the latter is a counterpart of Leggett’s mode in Josephson junctions. The mode should be called “Josephson-Leggett’s” mode. We also reveal that the critical current and the Fraunhofer pattern strongly depend on the symmetry type of the two-gap superconductor.

[1] Y. Kamihara, et al., J. Am. Chem. Soc. 130 (2008) 3296 [2] Y. Ota, M. Machida, T. Koyama, and H. Matsumoto, Phys. Rev. Lett. 102 (2009) 237003 [3] Y. Ota, M. Machida, and T. Koyama, arXiv: 0905.3592

∗E-mail: [email protected] Intrinsic nature of the 2nd switching in BSCCO intrinsic junctions

A. MaedaA,∗ K. OtaA, K. HamadaB, R. TakemuraB, M. OhmakiB, T. MachiC, K. TanabeC, M. SuzukiB,D, and H. KitanoE

ADepartment of Basic Science, University of Tokyo, Tokyo 153-8902, Japan BDepartment of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan CSuperconductivity Research Laboratory, ISTEC, Tokyo 135-0062, Japan DPhotonics and Electronics Science and Engineering Center, Kyoto University, Kyoto 615-8510, Japan EDepartment of Physics and Mathematics, Aoyama Gakuin University, Kanagawa 229-8558, Japan

We investigated macroscopic quantum tunneling MQT of Bi2Sr2CaCu2Oy (BSCCO) intrinsic Josephson junctions (IJJs) for two device structures1. One is a small mesa, which is a few nanometers thick with only two or three IJJs, and the other is a stack of a few hundred IJJs in a narrow bridge structure. The experimental results regarding the switching-current distribution for the first switch from the zero-voltage state were in good agreement with the conventional theory for a single Josephson junction, indicating that the crossover temperature from thermal activation to the MQT regime for the former device structure was similar to that for the latter device structure. Together with the observation of multiphoton transitions between quantized energy levels in the MQT regime, these results strongly suggest that the observed MQT behavior is intrinsic to a single IJJ in high-Tc cuprates and is independent of the device structure. The switching-current distribution for the second switch from the first resistive state, which was carefully distinguished from the first switch, was also compared with respect to the two device structures. In spite of the differences between the heat transfer environments, the second switch exhibited a similar temperature-independent behavior for both devices up to a much higher temperature than the crossover temperature for the first switch. We argue that this cannot be explained in terms of self-heating caused by dissipative currents after the first switch. As possible candidates for this phenomenon, the MQT process for the second switch and the effective increase in the electronic temperature due to the quasiparticle injection are discussed.

[1] K. Ota et al.: Phys. Rev. B79, 134505 (2009).

∗E-mail: [email protected] Vrotex dynamics close to the threshold driving force at low temperatures

Atsutaka MaedaA,∗ and Daisuke NakamuraA

ADepartment of Basic Science, University of Tokyo, Tokyo 153-8902, Japan

Dynamics of driven vortices in high-Tc superconductors was investigated in terms of the elementary process of the vortex motion. The I − V characteristics of driven vortices were measured in La2−xSrxCuO4 thin ﬁlms. With an aid of a recently proposed scaling relation between the driving force and the velocity1, we found that the non-Arrhenius process did exist for interacting vortices at low temperatures. With increasing magnetic ﬁeld, Arrhenius process revived suddenly at the vortex-glass vs Bragg-glass transition, which shows that this approach is suitable as a new method to investigate the equilibrium phase diagram of vortices.

[1] B. M. Luo and X. Hu: Phys. Rev. Lett. 98 (2007) 267002.

∗E-mail: [email protected] Unconventional Vortex Dynamics in Mesoscopic Superconducting Corbino Disks

Vyacheslav R. Misko ,∗ Nansheng Lin ,† Fran¸coisM. Peeters ‡

Department of Physics, University of Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium

In contrast to large (macroscopic) superconducting disks where magnetic flux patterns appear in form of a vortex lattice deformed near the boundaries, in mesoscopic disks vortices are arranged in shells, due to strong shape effect. The discrete shell structure strongly influences the flux dynamics in disks which can be accessed using a Corbino setup when a current is injected in the center of the disk and removed at its perimeter thus resulting in tangential Lorentz force decreasing with radius as 1/ρ. While the vortex dynamics, in particular, the onset of plastic motion, is well understood in large1 and in very small mesoscopic disks (i.e., containing only few circular shells)2, the dynamical behavior in the intermediate-size regime occurs to be much more complex and unusual, due to (in)commensurability between the vortex shells and appearance of intra- and inter-shell defects. We found that the interplay between the (in)commensurability effects, the vortex-vortex interaction and the gradient Lorentz force in a mesoscopic Corbino disk can result in a very unusual and counter-intuitive dynamics of vortex shells3. Thus we demonstrated: (i) unconventional angular melting when, with increasing driving current, the melting first occurs near the boundary where the shear stress is minimum and propagates towards the center, in an opposite way to the “classical” scenario of conventional angular melting in a Corbino disk; (ii) unconventional shell dynamics when a shell that experiences a weaker Lorentz force moves faster than the adjacent shell driven by a stronger Lorentz force. These examples of strongly non-linear dynamical behavior can be useful for understanding and prediction of dynamics of other complex interacting systems. We acknowledge useful discussions with Franco Nori and Fabio Marchesoni. This work was supported by the “Odysseus” program of the Flemish Government, FWO-Vl, and IAP (Belgium). V.R.M. acknowledges support by the EU Marie Curie program, Contract No. MIF1-CT-2006-040816.

[1] D. Lopez, W.K. Kwok, H. Safar, R.J. Olsson, A.M. Petrean, L.M. Paulius, and G.W. Crabtree, Phys. Rev. Lett. 82, 1277 (1999). [2] V.R. Misko and F.M. Peeters, Phys. Rev. B 74, 174507 (2006). [3] N.S. Lin, V.R. Misko, and F.M. Peeters, Phys. Rev. Lett. 102, 197003 (2009).

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] Magnetic Field Response of Superconducting CeCoIn5 Roman MovshovichA,∗ Yoshifumi TokiwaB,† Nobuyuki KuritaA,‡ F. RonnningA,§ E. D. BauerA,¶ J. D. ThompsonAk

ALos Alamos National Laboratory, Los Alamos, NM USA BI Physikalisches Institut, Georg-August Universitaet Goettingen, Goettingen 37077, Germany

Magnetic field stabilizes a novel phase in the High-Field-Low -Temperature (HFLT) cor- ner of the superconducting phase in CeCoIn5, originally suggested as a possible realization of the spatially inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconducting phase, predicted in early 1960’s. Recent NMR measurements revealed presence of the long range antiferromagnetic (AFM) order1. Neutron diffraction measurements recently established the nature of the magnetic order with magnetic field along [110]2. We investigated the effect of Cd, Hg3, and Sn doping on the superconducting (SC) transition temperature Tc, the superconducting critical field Hc2, and the HFLT phase. Tc decreases monotonically for both dopants, whereas Hc2 rises initially with Hg & Cd substitution, while dropping for Sn doping. In spite of this opposite trends, THF LT scales with Tc, for a given field, for both Sn and Hg doping. We interpret these results as supporting the superconducting origin of the HFLT phase, and suggest a possible scenario of the interplay between AFM and FFLO orders.

[1] B. L. Young et al., Phys. Rev. Lett. 98, 036402 (2007). [2] M. Kenzelmann et al., Science 321, 1652 (2008). [3] Y. Tokiwa et al., Phys. Rev. Lett. 101, 037001 (2008).

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] ¶E-mail: [email protected] kE-mail: [email protected] Sign-reversing s-wave superconductors: Analysis of Fe-based superconductors

Yuki NagaiA,B,∗ Nobuhiko HayashiC,D, Noriyuki NakaiD,E , Hiroki NakamuraB,D,E ,Masahiko OkumuraD,E and Masahiko MachidaB,D,E

ADepartment of Physics, University of Tokyo, Tokyo 113-0033, Japan BJST, TRIP, Chiyoda, Tokyo, 102-0075, Japan CNanoscience and Nanotechnology Research Center (N2RC), Osaka Prefecture University, 1-2 Gakuen-cho, Naka-ku, Sakai, 599-8570 Osaka, Japan DCREST(JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ECCSE, Japan Atomic Energy Agency, 6-9-3 Higashi-Ueno, Tokyo 110-0015, Japan

We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al.1 in Fe-based superconductors2. We show that a fully gapped anisotropic ±s-wave superconductivity consistently explains experimental observations. With the use of the five-band model, we calculated the nuclear magnetic relaxation rate 1/T1 and the superfluid density ρxx and showed that the anisotropic ±s-wave pairing function can explain the seemingly contradictory experimental results on Fe-pnictide superconductors. That is, the anisotropic ±s-wave pairing function reproduces consistently 3 1/T1 ∼ T and the T -independence of ρxx at low T . We also discuss the tunneling spectroscopy at a surface in multi-band systems such as Fe-based superconductors with the use of the quasiclassical approach. We extend the single- band method by Matsumoto and Shiba3 into n-band systems (n > 1)4. We show that the appearance condition of the zero-bias conductance peak does not depend on details of the pair-potential anisotropy, but it depends on details of the normal state properties in the case of fully-gapped superconductors.

[1] K. Kuroki, S. Onari, R. Arita, H. Usui, Y. Tanaka, H. Kontani and H. Aoki, Phys. Rev. Lett. 101 (2008) 087004. [2] Y. Nagai, N. Hayashi, N. Nakai, H. Nakamura, M. Okumura, and M. Machida, New J. Phys. 10 (2008) 103026. [3] M. Matsumoto and H. Shiba, J. Phys. Soc. Jpn. 64 (1995) 1703. [4] Y. Nagai and N. Hayashi, Phys. Rev. B 79 (2009) 224508.

∗E-mail: [email protected] Transient dynamics of driven vortices and superconducting ﬂuctuation of La2−xSrxCuO4 thin ﬁlms

Daisuke NakamuraA,∗ Yuki ShibuyaA, Yoshinori ImaiA, Atsutaka MaedaA and Ichiro TsukadaB

ADepartment of Basic Science, the University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo, 153-8902, Japan BCentral Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan

We discuss two different aspects of vortex dynamics in high-temperature superconductors (HTSCs). (1) Dynamics of driven vortices in HTSCs is strongly affected by the thermal fluctuation, which causes the relaxation phenomena. We have investigated the waiting-time dependence 1 of the critical current in La2−xSrxCuO4 (LSCO) thin films . Different kinds of the waiting- time dependence was observed depending on the positions in the B − T phase diagram. Together with the results in samples with columnar defects and with bridge-type structures, we found that both the pinning strength and the system size are crucial to the relaxation phenomena of driven vortices. We will discuss these healing effects in terms of physics of friction, and will provide a simple criteria for the validity of Amontons-Coulomb’s law. (2) In HTSC, large thermal fluctuation causes the realization of the wide critical region in the B − T phase diagram. The existence of the vortex-like precursor of the superconducting transition is asserted by the measurement of the Nernst signal, which starts to show up from 2 about 100 K above Tc in the underdoped region . On the other hand, the investigation of the superconducting fluctuation by measuring the microwave complex conductivity of LSCO thin films suggested that the superconducting fluctuation exists up to T ∼ 2Tc at most even at ω → ∞3. The reason why such a large difference exists between the result of the microwave conductivity and that of the Nernst signal has not been understood yet. We measured the complex conductivity on LSCO thin films by the time-domain THz spectroscopy (0.3-2.0 THz). In underdoped (x = 0.07, 0.12) LSCO, we found that the imaginary part of the complex conductivity, σ2(T ), increased considerably from T ∼ 2Tc, which is consistent with the suggestion based on the microwave conductivity results. Therefore, we believe that the fluctuation of superfluid density is observable up T ∼ 2Tc at most. In the presentation, we will discuss also the data of optimally doped and overdoped samples.

[1] D. Nakamura et al., Phys. Rev. Lett. submitted. [2] Z. A. Xu et al., Nature 406 (2000) 486. [3] T. Ohashi et al., Phys. Rev. B79 (2009) 184507.

∗E-mail: [email protected] Pinning by planar defects and the planar glass phase

Thomas Nattermann,∗ Aleksandra Petkovic ,† Thorsten Emig ‡

Institut fürTheoretische Physik, Universitätzu Köln,Germany

The influence of randomly distributed point impurities and planar defects on order and transport in type-II superconductors and related systems is studied. It is shown that the Bragg glass phase is unstable with respect to planar defects. Even a single weak defect plane oriented parallel to the magnetic field as well as to one of the main axes of the Abrikosov flux-line lattice is a relevant perturbation in the Bragg glass. A defect that is aligned with the magnetic field restores the flux density oscillations, which decay algebraically with the distance from the defect. The theory exhibits striking similarities to the physics of a Luttinger liquid with a frozen impurity. The exponent for the flux-line creep in the direction perpendicular to a relevant defect is derived. We find that the flux-line lattice exhibits in the presence of many randomly distributed parallel planar defects aligned to the magnetic field a glassy phase which we call planar glass. The planar glass is characterized by diverging shear and tilt moduli, a transverse Meissner effect, and resistance against shear deformations. We also obtain sample-to-sample fluctuations of the longitudinal magnetic susceptibility and an exponential decay of translational long-range order in the direction perpendicular to the defects. The flux creep perpendicular to the defects leads to a nonlinear resistivity (J /J)3/2 ρ(J → 0) ∼ e− D . Strong planar defects enforce arrays of dislocations that are located at the defects with a Burgers vector parallel to the defects in order to relax shear strain.1.

[1] A. Petkovic, T. Emig, and T. Nattermann, Phys. Rev. B 79 (2009) 224512

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] Dislocation-Mediated Collective Vortex Motion in YNi2B2C Observed by STM in Real Time and Space Nobuhiko NishidaA,∗ Kazuharu UchiyamaA, Akira KuwaharaA, Satoshi SuzukiA, Shin-ichi KanekoA, Hiroyuki TakeyaB, Kazuto HirataB

ADepartment of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan BInstitute for Materials Research, Tsukuba, Ibaragi 305-0057, Japan

The collective vortex motion in a square vortex lattice of single crystal YNi2B2C has been successfully observed in real time and space by scanning tunneling microscopy at 0.45 K and 4.2K in magnetic ﬁelds up to 4T. Edge dislocations have been found to play important roles on the vortex creep above 1 T: The vortex lattice is divided into rectangular-shaped vortex bundles bordered by the glide planes of dislocation and the motions of bundles are mediated by motions of dislocations interacting with each other. The bulk measurement of a hysteresis of magnetization against applied magnetic ﬁeld will be discussed in relation with the STM observation of dislocations.

∗E-mail: [email protected] Phase Diagram and Anomalous Vortex Motion in Underdoped YBCO in Parallel Magnetic Fields

A A A Terukazu Nishizaki ∗, Yoshifumi Tokita , Yuki Takahashi , A Norio Kobayashi †

AInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan

In high-Tc superconductors, the vortex lattice structure is modified by the layered crystal structure and novel vortex phases and anomalous vortex dynamics appear when the magnetic field H is applied parallel to the ab-plane (H ab). In order to study the vortex phase k diagram and dynamics in YBa2Cu3Oy with large anisotropy parameter γ,wehaveprepared untwinned single crystals of underdoped YBa2Cu3Oy (Tc = 30, 50, 60 K) and measured the c- axis resistivity ρc(T,H)inhigh-H up to 15 T. In YBa2Cu3Oy (Tc =30K,γ 50), the vortex liquid freezes into the Josephson vortex solid through two-stage processes∼ with decreasing temperature T in the field region between 4.5 T and 9.5 T. The two phase transition lines consist of the first-order melting transition at Tm(H) and the broad transition at Ts(H). The resistivity ρc(T,H) in the intermediate phase [Tm(H)

∗E-mail: [email protected] †E-mail: [email protected] Phase Transitions and a Supercooling Phenomenon of the Vortex Matter in MgB2 Single Crystals

Tsutomu NojimaA,∗ Minemi ChotokuA, Kazuma TakahashiA, Hye-Gyong LeeB, Sung-Ik LeeB,

AInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan, Japan BDepartment of Physics, Sogang University, Seoul 121-742, Republic of Korea

The magnetic field and temperature (H-T ) phase diagram of MgB2 single crystals, containing the vortex matter phase transitions and the properties of vortices related to a two-gap nature, was determined from torque magnetometry. By combining the vortex shaking technique, we successfully measured both the reversible and irreversible magnetizations, Mrev and Mirr, as a function H and T at various H directions. At high T above 26 K, a step like decrease in both Mrev(H) and Mirr(H), which is ascribed to the first order Bragg glass melting transition, is observed at Hm. On the other hand, at low T below 25 K, Mrev(H) ∗ shows the step and kink anomalies, which occurs successively at H and Hg with increasing H around the upper critical field. Since these characteristic fields correspond to the peak effect and irreversibility fields in Mirr(H), respectively, the observed anomalies denote the first order disordering transition and the second order vortex glass melting transition. The ∗ existence of three transition lines, Hm(T ), H (T ) and Hg(T ), which appear to merge into a critical point, suggests that the H-T diagram of MgB2 is described with well-defined thermodynamic phases of the Bragg glass, vortex glass and vortex liquid, as seen in the cuprate superconductors. In addition to the phase transitions, we succeeded in observing a supercooled state of the vortex glass phase in Mirr(T ) of the cooling process. This supercooled state extends far below H∗(T ) line in the H-T diagram and transfer to the Bragg glass phase trough a flux ∗ jump at a characteristic temperature Td (¿ T (H )), which is almost independent of cooling rate and sample. Based on the analyses of Mrev(H) curves using the GL model, we found that the Td(H) is close to a border line below which the additional contribution of the π band superconductivity to the magnetization becomes remarkable. This implies that the appearance of the two gap nature can trigger the transition of the unstable vortex state to a more stable one.

∗E-mail: [email protected] Fluxtronics: nanodevices manipulating ﬂux quanta

Franco NoriA,B ∗

AAdvanced Science Institute, RIKEN, Saitama,Japan BPhysics Department, University of Michigan, Ann Arbor, MI, USA

It would be very desirable to have superconductors with an infinite number of matching fields. This would certainly increase the critical current Jc (which is one of the “Holy Grails” of this field). We have shown [1] that a quasiperiodic array of pinning sites (APS) increases the critical current Jc in a wide range of magnetic fields. Our proposal has motivated several experiments. We have found [2] that negative differential conductivity occurs in superconductors with artificial APS. We predict new voltage-current (V -I) characteristics combining both N- shaped and S-shaped parts in the same V -I curve. These unique characteristics offer bistable flux devices, e.g. fluxtronic memories. There is now considerable interest in vortex ratchets [3,4,5], which are capable of controlling vortex motion. Ratchets work as diodes that rectify ac vortex motion induced by, e.g., ac electrical currents in asymmetric pinning potentials. We have also described [4] another concept of ratchets employing time-asymmetric drives instead of spatially-asymmetric pinning. Recently a vortex lens of this type has been realized [4]. Remarkably, no spatial symmetry of any kind can provide remarkable forms of control over the motion of vortices [5], as seen in recent experiments, and predicted by our group in 2005. We derive [6] a quantum field theory of Josephson plasma waves (JPWs) in layered superconductors, which describes two types of interacting JPW bosonic quanta. We propose a mechanism of enhancement of macroscopic quantum tunneling (MQT) in stacks of intrinsic Josephson junctions. This allows us to quantitatively describe striking recent experiments in Bi2212 stacks. [7]

[1] V.R. Misko, S. Savel’ev, F. Nori, Critical Currents in Quasiperiodic Pinning Arrays: Chains and Penrose Lattices, Phys. Rev. Lett. 95, 177007 (2005); Phys. Rev. B 74, 024522 (2006). [2] V. R. Misko, S. Savel’ev, A. L. Rakhmanov, F. Nori, Non-uniform Self-Organized Dy- namical States in Superconductors with Periodic Pinning, Phys. Rev. Lett. 96, 127004 (2006); Phys. Rev. B 75, 024509 (2007). [3] F. Nori, Reversible diodes for moving quanta, Nature Physics 2, 227 (2006); J.E. Villegas et al., Science 302, 1188 (2003); Y. Togawa et al, PRL 95, 087002 (2005). [4] S. Savel’ev, F. Nori, Nature Materials 1, 179 (2002); D. Cole et al., ibid 5, 305 (2006). [5] S. Ooi, S. Savel’ev, M.B. Gaifullin, T. Mochiku, K. Hirata, F. Nori, Nonlinear Nan- odevices Using Magnetic Flux Quanta, Phys. Rev. Lett. 99, 207003 (2007). [6] S. Savel’ev, A.L. Rakhmanov, F. Nori, Quantum Terahertz Electrodynamics and Macroscopic Quantum Tunneling in Layered Superconductors, Phys. Rev. Lett. 98, 077002 (2007). ∗E-mail: [email protected] Designing superconducting qubit circuits that exhibit atomic-physics-like phenomena on a chip

Franco NoriA,B ∗

AAdvanced Science Institute, RIKEN, Saitama,Japan BPhysics Department, University of Michigan, Ann Arbor, MI, USA

Superconducting (SC) circuits can behave like atoms making transitions between a few energy levels. Such circuits can test quantum mechanics at macroscopic scales and be used to conduct atomic-physics experiments on a silicon chip [1].

This presentation overviews a few of our theoretical studies [2] on SC circuits and quantum information processing (QIP) including:

SC qubits for photon generation and for lasing; 2-1 photon coexistence; cooling qubits and their environment; using SC qubits to probe nearby defects; hybrid circuits involving both charge and flux qubits; quantum tomography in SC circuits; preparation of macroscopic quantum superposition states of a cavity field via coupling to a SC qubit; generation of nonclassical photon states using a SC qubit in a microcavity; and controllable couplings among qubits. These controllable couplings between qubits can be achieved either directly or indirectly [for instance, with or without coupler circuits; as well as with or without “data-buses” like resonators (e.g., electromagnetic fields in cavities, LC circuits, or transmission line resonators)] [3].

[1] For brief overviews, see: J.Q. You and F. Nori, Physics Today 58, No. 11, 42 (2005). F. Nori, “Atomic physics with a circuit”, Nature Physics 4, 589 (2008).

[2] The PDF ﬁles of our publications are available online at: http://dml.riken.jp and also at: http://www.umich.edu/˜nori

[3] Work supported in part by JSPS, LPS, NSA, ARO, and NSF.

∗E-mail: [email protected] Electronic states of vortex core in beta-pyrochlore materials

Kentaro ObaA,∗ Yoshinori ImaiA,† Junichi YamauraB,‡ Atsutaka MaedaA,§ Zenji HiroiB¶

ADepartment of Basic Science, University of Tokyo, Tokyo 153-8902, Japan BThe Institute for Solid State Physics, University of Tokyo, 277-8581, Japan

β-pyrochlore oxcide AOs2O6(A = Cs, Rb, K) show superconductivity below Tc = 3.3, 6.3, 9.6K for A = Cs, Rb, and K, respectively. These alkaline ions are thought to be able to oscillate in an anharmonical manner in the caged structure formed by Os and O atoms (rattling)1. It has been considered that the rattling motion plays a key role for superconductivity in these materials2. However the detailed relation between rattling motion and superconductivity has not been clarified yet. Therefore, investigation of the thorough excitation spectrum of the rattling motion is particularly important to understand the SC in these materials. Since rattling energy is thought to be 0.4∼1.4 THz, it is expected that frequency dependent conductivity can be observed in milimeter freqeuncy-wave region, which might give crucial information on the above mentioned subjects. Thus, we performed systematic measurements of microwave and milimeter-wave surface impedances on these materials by cavity perturbation technique, and discuss the effect of rattling motion on electrical conductivity. At present, the data in Cs compound show a prominent frequency dependence of conductivity in millimeter-wave frequency regions[3]. We will discuss the results of Cs, Rb, and K salts in a comparative manner, and will try to clarify the effect of the rattling motion on SC.

[1] J. Yamaura et al,, J. Solid State Chem 179 (2006) 336-340 [2] Z. Hiroi et al., Physica C 460-462 (2007) 20-27

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] ¶E-mail: [email protected] Lower Critical Fields of Fe-based and Heavy Fermion Superconductors

Ryuji Okazaki∗

Department of Physics, Kyoto University, Kyoto 606-8502, Japan

We report the lower critical fields Hc1 of the Fe-pnictide PrFeAsO1−y (Tc ∼ 34 K) and the heavy fermion URu2Si2 (Tc ∼ 1.5 K) superconductors. To avoid the ambiguous determination of Hc1 in bulk magnetization measurements which average the inhomogeneous field distribution due to the pinning, we developed the precise Hc1 measurement using a miniature Hall-sensor array which clearly resolves the first flux penetration from the edge of the crystal [1]. The temperature dependence of Hc1 in PrFeAsO1−y single crystals for H∥ c is well scaled by the in-plane superfluid density determined by the penetration depth measurements. The anisotropy of Hc1 at low temperatures is estimated to be ≅ 3, which is smaller than that of coherence lengths. This indicates the multiband superconductivity, in which the active bands for the superconductivity are more two-dimensional. In the ultraclean URu2Si2 single crystals, Hc1(T ) exhibit striking deviation from conventional BCS curve. Hc1(T ) increase without saturation at low temperatures, indicating the existence of line nodes in the gap structure. The discrepancy in the temperature dependence of the anisotropies of Hc1 and Hc2 provides the strong evidence for the multiband superconductivity in URu2Si2. The uniqueness is highlighted by the distinct kink behavior at ∗ T ∼ 1.2 K in Hc1(T ) for H∥ c, implying a possible phase transition in the superconducting phase similar to the multi-component superconductor UPt3. We also found the vortex lattice melting transition in the ultraclean URu2Si2 single crystals [2]. In magnetic fields, the resistivity decreases very sharply at the melting temperature Tm well below the mean-field transition temperature Tc. The vortex lattice melting at sub- Kelvin temperatures can be attributed to the thermal fluctuations which are highly enhanced by the small number of carriers with heavy mass of this compound, leading to a sizable region of the vortex liquid phase. This work has been done in collaboration with M. Shimozawa, M. Konczykowski, C.J. van der Beek, M. Ishikado, H. Kito, A. Iyo, H. Eisaki, S. Shamoto, Y. Kasahara, Y. Haga, T.D. Matsuda, Y. Onuki, K. Behnia, T. Shibauchi and Y. Matsuda.

[1] R. Okazaki et al., Phys. Rev. B 79, 064520 (2009). [2] R. Okazaki et al., Phys. Rev. Lett. 100, 037004 (2008).

∗E-mail: [email protected] Quantum Melting of a Driven Vortex Lattice in Amorphous Superconducting Films

Satoshi OkumaA,∗ Hiroki ImaizumiA, Nobuhito KokuboB

ARes. Ctr. for Low Temp. Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan BCtr. for Res. and Adv. in Higher Education, Kyushu Univ., Fukuoka 819-0395, Japan

Melting of solids is one of the most fundamental physical phenomena that occur in nature. Of particular interest is a melting transition driven solely by quantum fluctuations (QFs)1. This zero-temperature (T = 0) transition (i.e., quantum phase transition) has long been an active research area in condensed matter physics and studied in various physical systems, which include the vortex system in type-II superconductors. At high T properties of vortex lines are dominated by thermal fluctuations, while at sufficiently low T they are subject to QFs. If the QFs are large enough, the vortex solid may melt into a quantum vortex liquid1, 2, as reported in several superconductors1, 3, 4. The experimental observation of quantum melting requires very low temperatures close to T = 0 and high fields close to the upper critical field Bc2(0). In the relevant T and B region, however, the influence of the quenched disorder (pinning) inevitably contained in actual superconductors becomes nontrivial and it results in a structural transition from an ordered lattice into a disordered solid5. This accompanies a peak behavior in the critical current, known as a peak effect, and it masks the observation of “intrinsic” quantum melting. In order to overcome this fact, the preparation of a sample with zero pinning is desirable, but it is actually difficult. The influence of pinning is effectively reduced by driving vortices analogously to dynamic friction at a solid-solid interface. To detect melting of the driven vortex lattice (i.e., dynamic melting), we have conducted an experiment of a mode-locking resonance for amor- 6 phous MoxGe1−x films . We observe dynamic melting down to low enough T (≥0.8 K) and ∞ determined the dynamic melting line Bc,dyn(T ) in nearly the whole B − T phase diagram. ∞ At low T , Bc,dyn(T ) shows a weak T dependence, deviating downward from the mean field line Bc2(T ) or the static “melting” (depinning) line, which is determined by ordinary transport measurements. The results indicate that intrinsic quantum melting of the vortex lattice decoupled from the underlying pinning potential occurs at 0.8Bc2, which is well below Bc2(0).

[1] G. Blatter et al., PRB 50, 13013 (1994). [2] M.P.A. Fisher, PRL 65, 923 (1990): R. Ikeda, IJMPB 10, 601 (1996). [3] T. Shibauchi et al., PRB 67, 064514 (2003). [4] S. Okuma et al., PRL 86, 3136 (2001): 91, 067001 (2003): 94, 047003 (2005). [5] Y. Paltiel et al., PRL 85, 3712 (2000): S. Okuma et al., PRB 77, 212505 (2008). [6] S. Okuma et al., PRB 76, 172503 (2007): J. Phys.: Conf. Ser. 150, 052200 (2009).

∗E-mail: [email protected] Vortex phase diagram of Bi2Sr2CaCu2O8+y with antidot lattice

Shuuichi Ooi,∗ Takashi Mochiku, Kazuto Hirata

National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Toward control of vortex states and its dynamics in superconductors, a number of works have been performed to study influence of artificially introduced pinning centers on properties of vortex, i.e., vortex-flow resistance or magnetization. Since these experiments require high quality of samples and enough thinness to get observable effect by fabrications on the surface, thin films of conventional superconductors, Nb, Al, Pb, etc., have been mainly used. Normally, matching effect, which is expected to occur when the density of vortices coincides with that of pinning centers, has been observed in these high quality films with regular array of pinning centers. 1 On the other hand, experiments on high-Tc superconductors were rare because of the difficulties to obtain the thin films whose quality is as high as that of single crystals, although it is interesting to study the changes of vortex phases including the well-known first order melting transition of Bragg glass. Bi2Sr2CaCu2O8+y (Bi2212) is a good candidate for this purpose, because there are some vortex phases where the residual pinnings are quite weak, like vortex liquid and the high-temperature region of Bragg glass, and single-crystal thin films can be easily prepared from bulk single crystals by cleaving. To study the vortex behavior in 2D periodic pinning potentials in high-Tc superconductors, we measured the vortex-flow resistance Rab as a function of magnetic field or temperature in single-crystal Bi2212 films with various array of submicron antidots. Fractional matching effects were found in the cases of triangular and square arrays. Many fine structures in Rab(H) are observed in 1/4, 2/7, 1/3, 2/5, 1/2, 3/5, 2/3, 5/7, 3/4, etc. of the first matching field H1 in the sample with the triangular lattice of antidots. Furthermore, Rab(T ) show a sharp jump indicating first order melting transition of a pinned vortex solid at the same temperature range where the fractional matching effect appears.

[1] S. Ooi, T. Mochiku, S. Yu, E. Sadki and K. Hirata, Physica C 426-431 (2005) 113.

∗E-mail: [email protected] Direct determination of the Larkin radius in Bi2Sr2CaCu2O8 Hernan PastorizaA,∗ Moira DolzA,† Alejandro KoltonB‡

ALaboratorio de Bajas Temperaturas, Centro Atomico Bariloche, R8402AGP Bariloche, Argentina BTeoria del Solido, Centro Atomico Bariloche, R8402AGP Bariloche, Argentina

We study finite-size effects at the onset of the irreversible magnetic behaviour of micron sized Bi2Sr2CaCu2O8+δ single crystals by using silicon micro-oscillators. We find that the onset of irreversibility appears well below the thermodynamic Bragg-glass melting line and decreases with decreasing the sample transverse size. By analyzing the size and temperature dependence we show this onset can be identified with the finite-size crossover between the Larkin and random manifold roughness regimes, thus allowing to determine the transverse Larkin length of the weak collective pinning regime in a direct way.

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] Study of metastable states in NbSe2 using small angle neutron scattering

Alain PautratA,∗ Muhamad Aburas, Charles SimonA, Patrice MathieuB, Annie BruletC, Charles DewurstD, Shobo BhattacharyaE

ACRISMAT laboratory, Caen, France BLPA, Ecole Normale Superieure, Paris, France CLLB, Saclay, France DILL, Grenoble, France ETata Institude, Bombay, India

We have studied the structure of the flux line lattice (FLL) in Iron doped NbSe2 using small angle neutron scattering. Structures which are dependant of the magnetic field history are observed. In particular, the field cooling states stabilized when crossing the peak effect are shown to be very peculiar. They present Bragg peaks which can be rotated by overcritical current, evidencing an annealing of some complicated current distribution. The zero field cooling FLL structures do not present signs of disordering inside the peak effect1, 2, and quantitative analysis of scattered intensity is close to what is expected for a regular lattice. The implication of these results will be presented, in particular the important role of the current distribution for the peak effect.

[1] A. Pautrat, M. Aburas, Ch. Simon, P. Mathieu, A. Brulet, C. D. Dewhurst, S. Bhat- tacharya, and M. J. Higgins, Phys. Rev. B 79, 184511 (2009). [2] A. Pautrat, J. Scola, Ch. Simon, P. Mathieu, A. Brulet, C. Goupil, M. J. Higgins, and S. Bhattacharya, Phys. Rev. B 71, 064517 (2005).

∗E-mail: [email protected]

1 Synchronized lattice vortex motion in amorphous superconductors with array of nanodefects

D. Perez de LaraA∗

ADepartamento Fisica Materiales, Universidad Complutense de Madrid, Madrid, 28040, Spain

We have fabricated arrays of nanodefects embedded in amorphous MoSi films by electron beam lithography, sputtering and etching techniques. In the mixed state the vortex lattice dynamics could be modified by the competition between the random intrinsic pinning potentials (superconducting film) and the artificial periodic pinning potentials (array of nanodefects) which leads to interesting commensurability effects. Vortex lattice channeling effects1, synchronized vortex lattices motion2, vortex lattice rectification effects3, and so on could be achieved in these hybrids. In this poster we will explore some of these phenomena in hybrids of a-MoSi superconducting films on arrays of Ni nanodots.

[1] J. E. Villegas, E. M. Gonzalez, M. I. Montero, I. K. Schuller, J. L. Vicent, Physical Review B72, 064507 (2005). [2] D. Perez de Lara, A. Alija, E. M. Gonzalez, J. I. Martin, M. Velez, J. M. Colino, J. V. Anguita, and J. L. Vicent., Applied Physics Letters 94, 122506 (2009). [3] J. E. Villegas, S. Savelev, F. Nori, E. M. Gonzalez, J. V. Anguita, R. Garcia, J. L. Vicent, Science.

∗E-mail: dperezla@ﬁs.ucm.es Pseudogapped vortex cores: a novel feature of multi-band superconductivity?

Alexander Petrovi´cA,B,∗ Rolf LortzC, Yanina FasanoA,D, C´edricDuboisA, Gilles SantiA, Diala SalloumE, Patrick GougeonE, Michel PotelE, Øystein FischerA,

ADPMC-MaNEP, Universitéde Genève,Quai Ernest-Ansermet 24, 1211 Genève4, Switzerland BPhase Control in Smart Materials Laboratory, School of Physical and Mathematical Sciences, Nanyang Technological University, SPMS-04-01, 21 Nanyang Link, Singapore 637371 CDepartment of Physics, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong DInstituto Balseiro and Centro Atómico Bariloche, Avenida Bustillo 9500, Bariloche, Argentina ESciences Chimiques, CSM UMR CNRS 6226, Universitéde Rennes 1, Avenue du Général Leclerc, 35042 Rennes Cedex, France

We have performed scanning tunnelling spectroscopy in the vortex cores of two Chevrel phase superconductors, at a temperature of 400 mK and fields from 1-9 T. Core spectra in SnMo6S8 display a shallow pseudogap, whose depth decreases with increasing magnetic field. A much wider and deeper gap is visible in PbMo6S8, with spectra scarcely changing as we enter the vortex core from the superconducting matrix. The depth of the pseudogap also appears to scale with Hc2 (∼ 40 T in SnMo6S8 and 70-80 T in PbMo6S8). It must be stressed that this phenomenon is unrelated to the gap seen in high-Tc cuprate vortex cores, since there is no pseudogap present in zero-field spectra above Tc in the Chevrel phases. We postulate that our pseudogap is a novel consequence of the significant inter-band scattering present in these materials.

∗E-mail: [email protected] Magnetic Properties of Pnictide Superconductors in the Mixed State: Comparison with the Cuprates Ruslan Prozorov ∗

Ames Laboratory and Department of Physics & Astronomy, Iowa State University, Ames, IA 50011, U.S.A.

Anisotropic magnetic properties of single crystals of both 11111, 2 and 1223, 4, 5, 6 pnictide superconducting systems were studied by several techniques, including dynamic susceptibility, magnetization, transport and magneto-optics. A particularly detailed study of Ba(Fe1-xCox)2As2 (FeCo-122) single crystals in a range of cobalt concentrations covering from underdoped through overdoped regime allowed for a comparison with the cuprates. Whereas normal transport and superfluid density anisotropy of the pnictides is low2, 5, 6 vortex behavior shows a pronounced difference when probed with a magnetic field perpendicular or parallel to the FeAs planes3, 4. In fact, the irreversible vortex behavior does resemble high- Tc cuprates, such as BSCCO1, 3, 4. In particular, doping-dependent fishtail magnetization, colossal magnetic relaxation even at the lowest temperatures and an apparent crossover from elastic to plastic creep are clearly observed . All this, together with the relatively weak pinning strength imply applicability of the collective pinning scenario for the pnictides1, 3, 4. Nonetheless, there are features of the irreversible vortex behavior that are unique to the pnictides. In particular, in slightly underdoped regime vortices interact the magnetic/structural domains and twin boundaries7 and our results suggest that this region may be important for practical applications of at least the 122 pnictide system.

[1] R. Prozorov et al., New J. Phys. 11, 035004 (2009) [2] C. Martin et al., Phys. Rev. Lett. 102, 247002 (2009) [3] R. Prozorov et al., Phys. Rev. B 78, 224506 (2008) [4] R. Prozorov et al., Physica C 469, 667 (2009) [5] R. T. Gordon et al., Phys. Rev. Lett. 102, 127004 (2009) [6] R. T. Gordon et al., Phys. Rev. B 79, 100506(R) (2009) [7] M. A. Tanatar et al.,Phys. Rev. B 79, 180508(R) (2009)

∗E-mail: [email protected]

1 Vortex pinning and critical current density in iron-based pnictide superconductors

R. PuzniakA,∗ S. WeyenethB, N. D. ZhigadloC, Z. BukowskiC, J. KarpinskiC, E. PomjakushinaD, K. ConderD, A. WisniewskiA, M. BerkowskiA, and H. KellerB

AInstitute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, PL-02-668 Warsaw, Poland BPhysik-Institut der UniversitätZürich,Winterthurerstrasse 190, CH-8057 Zürich, Switzerland CLaboratory for Solid State Physics, ETH Zurich, CH-8093, Zurich, Switzerland DLaboratory for Developments and Methods, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

Iron-based pnictide superconductors with Tc’s ranging up to 56 K and with the general formula LnFeAsO1−xFy (Ln=La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, ...) have been discovered recently. The undoped parent compound LnFeAsO has a layered crystal structure, very similar to that one of cuprate superconductors, where CuO2 planes are separated by doped, nearly insulating sheets that serve as charge reservoir. More recently, the discovery of superconductivity at 38 K in Ba1−xKxFe2As2 with the ThCr2Si2-type structure has been reported. Together with the materials as e.g. LiFeAs, FeSe1−x, and Sr3Sc2O5Fe2As2 superconductivity has been found to exist in a whole variety of materials with an iron-pnictide layered structure. Temperature dependence of the upper critical field, irreversibility line, and critical current density have been determined for representatives of different classes of iron-based pnictides and compared with those for the cuprates. For the synthesis of single crystals of SmFeAsO1−xFy, the cubic anvil high-pressure technique was applied. Single crystals of Rb- substituted BaFe2As2 were grown using a Sn flux method. Single crystals of FeSe1−xTex were grown by a Bridgeman method. It was found that the irreversibility line for the studied iron- n based pnictides is very well described by a power-law temperature dependence (1 − T /Tc) with n very close to 3/2, the value typical for high-Tc superconductors, characterized by an unusually small coherence length and an exceptionally high thermal activation of vortices at high temperatures. The irreversibility line for Rb-substituted BaFe2As2 is located in significantly higher magnetic fields and temperatures than those found for La1.86Sr0.14CuO4, Bi2Sr2CaCu2O8+δ, and HgBa2Ca2Cu3O8+δ. Its position at the phase diagram (H, T /Tc) is comparable only with that one for YBa2Cu3O7−δ, i.e., with the position of the irreversibility line for the high-Tc superconductor characterized by the lowest anisotropy value among the compounds compared here. Furthermore, the power-law exponents describing the irreversibility lines for both Rb-substituted BaFe2As2 and YBa2Cu3O7−δ are essentially identical. The critical current density at 5 K exceeds 106 A/cm2 for all of the reviewed pnictides, which together with the high upper critical fields, is very promising for applications.

∗E-mail: [email protected] Tunneling studies in a strongly disordered s-wave superconductor: NbN

Pratap Raychaudhuri∗

Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Rd., Colaba, Mumbai 400005.

I will present the evolution of superconducting properties with increase in disorder in homogeneously disordered epitaxial NbN thin films grown on (100) MgO substrate using d.c. magnetron sputtering. Tunneling measurements reveal that for films with large disorder the superconducting transition temperature is not associated with a vanishing of the superconducting energy gap but with a large broadening of the superconducting density of states. This is further substantiated from the measurement of penetration depth in as a function of disorder. Our results provide strong evidence of the loss of superconductivity via phase fluctuations in a homogeneously disordered s-wave superconductor.

[1] S. P. Chockalingam, Madhavi Chand, Anand Kamlapure, John Jesudasan, Archana Mishra, Vikram Tripathi and Pratap Raychaudhuri , Phys. Rev. B79 (2009) 094509 [2] S. P. Chockalingam, Madhavi Chand, John Jesudasan, Vikram Tripathi and Pratap Raychaudhuri, Phys. Rev. B77 (2008) 214503

∗E-mail:[email protected] Induced magnetic ﬁeld memory eﬀect into the JV systems in high anisotropy superconductors Bi2212 and Bi2223. Observation of a low temperature glassy state phase diagram in Bi2212.

D. ShaltielA,∗ H-A. Krug von NiddaB, B. Ya. ShapiroC , B. RosensteinD, M. GolosovskyA, B. BogoslavskyA, T. FujiiE, T.Watanabe andF, T. TamegaiE.

AThe Hebrew University, Israel BUniversity of Augsburgh, Germany CBar Ilan University, Israel DNational Chia Tung University, Taiwan EThe University of Tokyo, Japan FHirosaki University, Japan.

A memory effect in JV systems, created by magnetic field in the high anisotropy Bi2212 and Bi2223 superconductors is demonstrated using microwave absorption technique1, indicating a glass transition. This surprising effect is observed by field cooling the samples in a DC magnetic field Hm oriented parallel to the CuO planes from above Tc down to 4K, with subsequent reduction the field to zero and then increasing the field above Hm. A microwave absorption signal is observed at a magnetic field just above the cooling field, clearly indicating a memory effect (fig. 1). The signal intensity and shape was almost the same, for the wide range of cooling fields 0.15T to 1.7T, indicating that the microwave absorption peak obeys a universal scaling that appears above Hm at a field just above the sample was equilibrated in this wide field range. Phase diagram in Bi2212 of the glass state obtained from the persistence of the signal memory intensity at higher cooling temperatures will be presented (fig.2).

[1] D. Shaltiel et al. Phys. Rev. B 77, 014508 (2008).

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Fig.2 Phase diagram of the JV system in Bi2212. The process

Fig. 1. Signal intensity as a function of DC magnetic field obtained by field

for obtaining the memory at 4K is indicated by errows 1 to 4. cooling at H from above Tc to 4K. The signal maxima are slightly above H

m m

∗[email protected] Non-universal Gap Structure in Iron-Pnictide High Temperature Superconductors

T. ShibauchiA,∗ K. HashimotoA,B, M. YamashitaA, S. KasaharaC, Y. SenshuA, N. NakataA, S. TonegawaA, K. IkadaA, A. SeraﬁnB, A. CarringtonB, T. TerashimaC, H. IkedaA, Y. MatsudaA

ADepartment of Physics, Kyoto University, Kyoto 606-8502, Japan BH. H. Wills Physics Laboratory, University of Bristol, Bristol, UK CResearch Center for Low Temperature and Materials Sciences, Kyoto University, Japan

The discovery of Fe-pnictide superconductors with transition temperature Tc exceeding 55 K raises fundamental questions about origin of high-Tc superconductivity. The microscopic pairing interactions with give rise to superconductivity are intimately related to the structure of the superconducting energy gap. Recent single-crystal studies in several Fe- 1, 2 arsenide compounds , with perhaps the cleanest example being (Ba,K)Fe2As2 (Tc ≃ 30 K), have suggested that the energy gap is fully formed in all directions. This is in sharp contrast to the high-Tc cuprate superconductors where zeros (nodes) in the energy gap are found at some points on the Fermi surface. Here we report bulk measurements of the magnetic penetration depth and heat transport 3 in high-quality single crystals of BaFe2(As0.67P0.33)2 (Tc = 30 K) down to 100 mK (∼ Tc/300), which clearly show that this material has an energy gap with line nodes. A set of results, the linear-in-temperature superfluid density, finite residual normal fluid in the zero-temperature limit and the striking enhancement of the quasiparticle excitations out of the condensed electrons by magnetic fields, consistently provide unambiguous evidence for line nodes in the order parameter, where the gap changes its sign at some lines on the Fermi surface. This is distinctly different from the nodeless gap found for (Ba,K)Fe2As2 which has similar Tc, electronic structure, and phase diagram. Our results indicate that repulsive electronic interactions play an essential role for Fe-based high-Tc superconductivity but that uniquely there are quasi degenerate pairing states, with and without nodes, which have similar Tc.

[1] K. Hashimoto, T. Shibauchi, T. Kato, K. Ikada, R. Okazaki, H. Shishido, M. Ishikado, H. Kito, A. Iyo, H. Eisaki, S. Shamoto, and Y. Matsuda, Phys. Rev. Lett. 102, 017002 (2009). [2] K. Hashimoto, T. Shibauchi, S. Kasahara, K. Ikada, S. Tonegawa, T. Kato, R. Okazaki, C. J. van der Beek, M. Konczykowski, H. Takeya, K. Hirata, T. Terashima, and Y. Matsuda, Phys. Rev. Lett. 102, 207001 (2009). [3] S. Kasahara, T. Shibauchi, K. Hashimoto, K. Ikada, S. Tonegawa, H. Ikeda, H. Takeya, K. Hirata, T. Terashima, and Y. Matsuda, arXiv:0905.4427 (2009). ∗E-mail: [email protected] Temperature and ﬁeld-dependent terahertz transmission of superconducting NbN layer

2, L. Skrbek1, 1,2 and R.Tesaˇr2

AInstitute of Physics of the AS CR, Prague, Czech Republic BDepartment of Low-Temperature Physics, Charles University in Prague, Prague, Czech Republic

The terahertz transmission of a thin NbN superconducting layer deposited on the sapphire substrate at several frequencies within the (0.40 - 2.52) THz range for several magnetic fields up to 10 T perpendicular to the layer, spanning the temperature range (3-20) K has been measured. Both temperature scans in a fixed magnetic field and magnetic field scans at fixed temperature have been used. For frequencies not far below the optical gap, distinct peaks in the transmission have been observed. While the zero magnetic field data are in relatively good agreement with the theoretical model predictions based on the BCS theory1 that correctly describes also the characteristic peaks below the transition temperature, with increasing magnetic field these peaks decrease and in high magnetic field gradually vanish. The likely reason for this disagreement is the additional absorption caused by vortex motion that cannot be taken into account by the BCS-based model.

[1] Zimmermann W., Brandt E.H., Bauer M., Seider E., Genzel L., Physica C 183(1991) 99-104 Ferromagnetic Josephson Resonance: Interplay between Josephson Eﬀect and Spin Dynamics

S. TakahashiAB,∗ S. HikinoA,† M. MoriAB,‡ S. MaekawaAB§

AInstitute for Materials Research, Tohoku University, Sendai 980-8577, Japan BCREST, Japan Science and Technology Agency, Tokyo 102-0075, Japan

There has been an increased interest in the Josephson effect in ferromagnetic Joseph- son junctions, which consist of a ferromagnet between two superconductors. An interesting phenomenon is the oscillation of the Josephson critical current, which originates from the exchange splitting of up and down spins of Cooper pairs in a ferromagnet. When the thickness of ferromagnet is about half the oscillation period, the current-phase relationship is shifted by π from the conventional Josephson current-phase relation. In addition to the exchange splitting, ferromagnets have low-lying excitation of spin wave as an inherent property. Of particular interest is the coupling of the Josephson current and a precessional motion of magnetization (uniform-mode spin wave), which is excited by using ferromagnetic resonance (FMR). In this study, we focus on the interplay between the spin dynamics of ferromagnetic order and the phase dynamics of superconducting condensate in ferromagnetic Josephson junctions. We consider a ferromagnetic Josephson junction in which a ferromagnet is subject to microwave and the magnetization is precessing in the ferromagnetic layer. The motion of magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation. The precessional magnetization induces the time-dependent magnetic flux in FM and affects the Josephson current across the junction, which is incorporated in a RSJ model of a ferromagnetic Joseph- son junction. The resulting equation of the extended RSJ model and the LLG equation are solved to calculate the I-V characteristics. When microwave frequency Ω is tuned to FMR frequency Ω0 (on resonance), a dc component of Josephson current appears as Shapiro-like steps in the I-V curve, which originates from resonance between the ac Josephson current and the magnetization precession by FMR. This “ferromagnetic Josephson resonance” (FJR) gives rise to the step structure at voltages (2nΩ0)/(2e) with n being integers, indicating that ferromagnetic Josephson resonance occurs at the frequencies of spin wave frequency Ω0 mul- tiplied by even integer 2n.1 The result provides a new method to probe the spin dynamics of ferromagnetic materials using ferromagnetic Josephson resonance in ferromagnetic Josephson junctions.

[1] S. Hikino, M. Mori, S. Takahashi, and S. Maekawa, J. Phys. Soc. Jpn. 77, 053707 (2008).

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] Vortex dynamics in heavy-ion irradiated Ba(Fe1−xCox)2As2 and FeTe1−xSex

T. TamegaiAB,∗ H. TaenA,† Y. TsuchiyaA,‡ Y. NakajimaAB,§ S. OkayasuC,¶ M. SasaseD∥

ADep. of Applied Physics, Univ. of Tokyo, Hong, Bunkyo-ku, Tokyo 113-8656, Japan BJST, TRIP, Hong, Bunkyo-ku, Tokyo 113-8656, Japan CAdvanced Science Research Center, Japan Atomic Energy Agency, Tokai, Japan DThe Wakasa-wan Energy Research Center, Research and Development Group, 64-52-1 Nagatani, Tsuruga, Fukui 914-0192, Japan

The discovery of superconductivity in LaFeAsO1−xFx has stimulated the research activity on new high temperature superconductors1. Rare-earth element substitutions for La have 2 boosted Tc up to 55 K . To date, four kinds of iron-containing layered superconductors have 1, 3, 4, 5 been identified . We will report vortex dynamics and critical current densities, Jc’s, of 6 these Fe-containing superconductors. High-quality single crystals of Ba(Fe1−xCox)2As2 and 7 FeTe1−xSex have been obtained by self-flux and slow-cooling method, respectively. Jc’s at 5 2 5 K under zero field for pristine Ba(Fe1−xCox)2As2 and FeTe1−xSex are 6x10 A/cm and 1x105 A/cm2, respectively. Both crystals show fish-tail effect in the intermediate field range similar to high temperature superconductors. Heavy-ion irradiation has been successfully 6 2 applied to boost Jc up to 4x10 A/cm in Ba(Fe1−xCox)2As2. The effect of columnar defects on the field and temperature dependences of the magnetic relaxation will be discussed in detail. Similarities and differences of the effect of columnar defects on the vortex dynamics in Ba(Fe1−xCox)2As2 and high temperature superconductors will also be reviewed.

[1] Y. Kamihara et al., J. Am. Chem. Soc. 130, 3296 (2008). [2] Z.-A. Ren et al., Europhys. Lett. 82, 57002 (2008). [3] M. Rotter, M. Tegel, and D. Johrendt, Phys. Rev. Lett. 101, 107006 (2008). [4] X. C. Wang et al., Solid State Commun. 148, 538 (2008). [5] F. C. Hsu et al., Proc. Natl. Acad. Sci. 105, 14262 (2008). [6] Y. Nakajima, T. Taen, and T. Tamegai, J. Phys Soc. Jpn. 78, 023702 (2009). [7] T. Taen et al., arXiv0906.1951. [8] Y. Nakajima et al., to be published in Phys. Rev. B (2009). ∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] ¶E-mail: [email protected] ∥E-mail: [email protected] Broadband ac conductivity measurement in the mixed state of LSCO: from vortex picture to ﬂuctuation picture

Ryo Tanaka,∗ Yoshinori Imai,† Ichiro TsukadaA,‡ Atsutaka Maeda§

Department of Basic Science, University of Tokyo, Tokyo 153-8902,Japan ACentral Research Institute of Electoric Power Industry, Tokyo 201-8511, Japan

Cuprate superconductors under magnetic field, B, show vast vortex-liquid state, which is identical to the normal state in the thermodynamic sense1. A first-order vortex melting transition (FOT) exists at rather lower fields than the mean-field Bc2. However, just above the FOT, vortex picture still works. On the other hand, with decreasing temperature from high temperature side, superconductivity fluctuation shows up. For small B, the fluctuation can be well described by the conventional theories2. Therefore, it is challenging to understand quantitatively how these two different pictures cross over from each other in a single thermodynamic phase. We investigated microwave complex conductivity by broadband technique 3, 4, 2, and analyzed the result from the both viewpoints. Even just above the FOT, vortex picture alone failed to explain the data5. We will try to present quantitative expressions to explain all of the data in a unified picture.

[1] M. Tinkham: “ Introduction to superconductivity ”(2nd ed.) (McGraw Hill, 1996). [2] T. Ohashi et al., Phys. Rev. B 79 (2009) 184507. [3] H. Kitano et al., Rev. Sci. Instrum. 79 (2008) 074701. [4] H. Kitano et al., Phys. Rev. B 73 (2006) 092504. [5] A. Maeda et al., J. Phys. Conf. Ser. 150 (2009) 052146.

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] Transverse Vortex ratchet eﬀect : Experiments and Simulations

D. Perez de LaraA,∗ E. M. GonzalezA,† L. DinisB,‡ J. M. R. ParrondoB,§ J. L. VicentA¶

ADepartamento F´ısica de Materiales, Facultad de Ciencias F´ısicas, Universidad Complutense, 28040 Madrid, Spain BDepartamento F´ısica Atomica, Nuclear y Molecular, Facultad de Ciencias F´ısicas, Universidad Complutense, 28040 Madrid, Spain

A transverse ratchet effect has been measured in magnetic/superconducting hybrid films fabricated by electron beam lithography and magnetron sputtering techniques. The samples are Nb films grown on top of an array of Ni nanotriangles. Injecting an ac current parallel to the triangle reflection symmetry axis yields an output dc voltage perpendicular to the current, due to a net motion of flux vortices in the superconductor. The effect is reproduced by numerical simulations of vortices as Langevin particles with realistic parameters. Simulations provide an intuitive picture of the ratchet mechanism, revealing the fundamental role played by the random intrinsic pinning of the superconductor.

∗E-mail: dperezla@fis.ucm.es †E-mail: cygnus@fis.ucm.es ‡E-mail: dinis@fis.ucm.es §E-mail: parrondo@fis.ucm.es ¶E-mail: jlvicent@fis.ucm.es Vortex Matter in Iron Pnictide Supercondutors affected by Disorder

Cornelis J van der BeekA,∗ Giancarlo RizzaA, Marcin KonczykowskiA, Pierre FerteyB, Isabelle MonnetC, Luc FruchterD, Thierry KleinE,† Christophe MarcenatF, Zuzana PribulovaG, Jozef KacmarcikG, Serguey L. BudkoH, Matthew E. TillmanH, Ni NiH, Paul C. CanﬁeldH, Ryuji OkazakiI,M IshikadoJ, Hiroshi EisakiJ, Shin-Ichi ShamotoK, Takasada ShibauchiI, Yuji MatsudaI

ALaboratoire des Solides Irradiés,Ecole Polytechnique, 91128 Palaiseau, France BSynchrotron SOLEIL, Saint-Aubin - BP 48, F-91192 Gif-sur-Yvette cedex, France CCIMAP, CNRS UMR 6252, CEA-DSM-IRAMIS & ENSICAEN, Caen, France DLaboratoire de Physique des Solides, Bâtiment510, UniversitéParis-Sud, Orsay, France EInstitut Néel,CNRS - UPR 5031, Grenoble, France FCEA, Institut Nanosciences et Cryogénie,SPSMS-LATEQS, Grenoble, France GCentre! of Low Temperature Physics, IEP SAS & FS Kosice, Slovakia !HDepartment of Physics & Astronomy and Ames Laboratory, Iowa State University, U.S.A. IDepartment of Physics, Kyoto University, Kyoto 606-8502, Japan JQuantum Beam Science Directorate, JAEA, Tokai, Naka, Ibaraki 319-1195, Japan KNanoelectronics Research Institute (NeRI), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Central 2, Tsukuba,Ibaraki 305-8568, Japan

I shall briefly review recent results on the influence of strong pinning on the thermodynamic properties of three-dimensional high Tc superconductors, such as the specific heat, magnetic torque, and the vortex phase diagram?. Next, I shall discuss disorder relevant to flux pinning in iron-pnictide superconductors, such as the density of dopant atoms and spatial variations therein, dislocations, extended point defects, and warping of the FeAs layers. I hope to conclude the talk with a brief comparison of vortex matter in the iron pnictide 2 3 compounds PrFeAsO1−y , NdFeAs(O,F) , Ba1−xKxFe2As2, and BaFe2As2−xPx, and the high Tc compounds YBa2Cu3O7−δ and Ba1−xKxBiO3.

[1] C. J. van der Beek, M. Konczykowski, L. Fruchter, R. Brusetti, T. Klein, J. Marcus, and C. Marcenat, Phys. Rev. B ! 72, 214504 (2005). [2] R. Okazaki, M. Koncz! ykowski, C. J. van der Beek, T. Kato, K. Hashimoto, M. Shi- mozawa, H. Shishido, M. Yamashita, M. Ishikado, H. Kito, A. Iyo, H. Eisaki, S. Shamoto, T. Shibauchi, and Y. Matsuda, Phys. Rev. B 79, 064520 (2009); arXiv:0811.3669. [3] J. Kacmarcik, C. Marcenat, T. Klein, Z. Pribulova, C.J. van der Beek, M. Konczykowski, S.L. Budko, M. Tillman, N. Ni, and P. C. Canﬁeld, Phys. Rev. B 80, 014515 (2009).

∗E-mail: [email protected] †E-mail:Thierry.Klein! @grenoble.cnrs.fr Enhanced Field of First Flux Penetration Due to Andreev Bound States in Single Crystalline Bi2Sr2CaCu2O8+δ Cornelis J van der Beek,∗ Anna B¨ohmer,Marcin Konczykowski

Laboratoire des Solides Irradi´es,CNRS UMR-7642 & CEA/DSM/IRAMIS, Ecole Polytec! hnique, F 91128 Palaiseau cedex, France

The field of first flux penetration Hp of single crystalline Bi2Sr2CaCu2O8+δ is studied as function of the superconducting sample’s aspect ratio and orientation with respect to the principal crystalline axes. Since the presence of macroscopic inhomogeneities severely 2 influences the value of Hp, it is essential that the measurements be carried out on a crystal of the highest homogeneity. The crystal is then successively cut into ever smaller crystals of square shape. Flux penetration is measured using direct and differential magneto-optical imaging, and microscopic Hall array magneto! metry. Finally, one of the intermediate squares is cut at 45!◦ with respect to the crystal axes. We find that the dependence of the field of first flux penetration on aspect (thickness/width, or d/w) ratio is, at all temperatures, well described by the results of E.H. Brandt:3 βd H H∞ s . p = p tanh w (1) ∞ The prefactor Hp is found to correspond, above 50 K, to the first critical field of the d-wave superconductor in the limit d/w → ∞. The factor β decreases as function of temperature, from β(40K) = 6 to β(80K) = 2. The fact that it is much larger that the values of 0.36 (0.67) found by Brandt3 for strips (disks) is attributed to the square shape of the (top and bottom) sample surfaces, and to the effect of the Bean-Livingston barrier. After cutting one of the intermediate crystals to a square with d/w = 0.12, and the ◦ edges oriented at 45 with respect! to the principal crystal a and b axes, we find that Hp is enhanced, over entire investigated temperature range, by a factor 1.2 with respect to Hp of a square crystal of the same dimensions, but with the edges aligned with a, b. We attribute this effect to the presence of Andreev bound states at the edges oriented at 45◦.4

[1] A.E. B¨ohmer,M. Konczykowski, and C.J. van der Beek, submitted to Phys. Rev. B (2009). [2] Nurit Avraham, E. H. Brandt, G. P. Mikitik, Y. Myasoedov, M. Rappaport, E. Zeldov, C. J. van der Beek, M. Konczykowski, and T. Tamegai, Phys. Rev. B 77, 214525 (2008). [3] E.H. Brandt, Phys. Rev. B 60, 11939 (1999). [4] C. Iniotakis, T. Dahm, and N. Schopohl, Phys. Rev. Lett. 100, 037002 (2008).

∗E-mail: [email protected] Unsual transport in simple Superconducting/Ferromagnetic nanohybrids: geometry and magnetic-state induced eﬀects

Javier E. VillegasA∗

AUnit´eMixte de Physique CNRS/Thales, 91767 Palaiseau, France, and Universit´eParis Sud 11, 91405 Orsay, France

Superconductivity (S) and ferromagnetism (F) are antagonistic phenomena at the nanoscale. Due to proximity and confinement, their interplay is enhanced in artificial hybrid structures, which gives rise to novel, sometimes unexpected physical properties. For example, dramatic changes in the magnetotransport behavior can be induced by judicious choice of the geometry, size and magnetic state of the F nanostructures. We will show a few examples of this, realized in a simple S/F hybrid structure: an array of magnetic nanodots covered with a superconducting thin film. We will discuss effects induced by changes in the nanodot array geometry; for example, the observation of fractal behavior in the magnetoresistance1. More- over we find effects which are induced solely by changes in the nanodots magnetic state. In particular, this provides a novel system in which the interaction between flux quanta and “magnetic vortices” in the nanodots, produces “switchable” flux pinning potentials2. Finally, we will show that the magnetic reversal characteristics of the nanodot magnetic vortices are “imprinted” into the transport properties of the superconducting thin film3, 4]. This yields a very unusual hysteretic magnetotransport in superconductors. This magnetic history dependent effect is induced by the stray magnetic fields from the nanodots, which drive the superconducting-to-normal transition of the hybrid.

Work supported by US-NSF and US-AFOSR. * In collaboration with C.-P. Li, K.D. Smith, M.I. Montero, R. Morales, L. Huang, Y. Zhu and Ivan K. Schuller.

[1] J.E. Villegas et al., Phys. Rev. Lett 97, 027002 (2006). [2] J.E. Villegas et al., Phys. Rev. B. 77, 134510 (2008). [3] J.E. Villegas et al., Phys. Rev. Lett 99, 227001 (2007). [4] J.E. Villegas et al., Appl. Phys. Lett. 94, 252507 (2009).

∗E-mail: [email protected] Transport in mesoscopic superconductors with nonequilibrium environment

Valerii VinokurA,∗ Nikolay ChtchelkatchevA,B,† Andreas GlatzA,‡ Tatyana BaturinaA,C§

ADMaterials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA BL. D. Landau Institute for Theoretical Physics, Russian Academy of Sciences, 117940 Moscow, Russia CInstitute of Semiconductor Physics, 13 Lavrentjev Ave., Novosibirsk, 630090 Russia

Transport current in mesoscopic superconducting tunneling structures requires the energy exchange between the charge carriers (Cooper pairs and/or quasiparticles) and the environment comprised of the bosonic excitations, such as phonons, photons, many-body excitations (electron-hole pairs), and the electromagnetic modes in the leads. We investigate highly non-equilibrium tunneling transport controlled by the Coulomb blockade effects and show that at low temperatures where the distribution functions of the bosonic modes deviate considerably from their equilibrium form and have temperature different from the temperature of the electron reservoirs, the energy relaxation occurs in two stages. Namely, the carriers exchange the energy with the intermediate bosonic agent, either electron-hole pairs or electromagnetic excitations, which further relax the energy to the phonon thermostat. We refer to the mediating bosonic subsitem as to environment and the phonon thermostat call the bath. We show that in highly nonequilibrium situation the form of the I − V curves is governed by the energy exchange between the charge carriers and the environment. We determine the distribution functions of the environment modes from the kinetic equation with the scattering integral describing the energy exchange of the modes with the tunneling electrons. Considering a single tunnel junction we find that at low voltages V , the current I through the junction embedded into a nonequilibrium environment exhibits I ∼ (V/(RT)) ln(Λ/V ) rather 2/g than the “Luttinger liquid”-like behavior I(V ) = (V/(RT))(V/Λ) found in the case of the equilibrium environment case [here Λ is the ultraviolet scale of the electron-environment interaction, ρ, and 1/g = ρ(ω = 0)]. We find that at some critical temperature T ∗ the Coulomb interactions give rise to opening of a finite gap in the environment excitations spectrum and estimate T ∗ from the Finkelshtein’s charge fluctuation propagators (this effect is sometimes referred to as “many-body localization”). We consider a current across the single-electron circuit controlled by the co-tunneling processes and find that the co-tunneling is blocked (superinsulating regime) at temperatures/voltages below T ∗.

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] Strong coupling of Abrikosov vortices and magnetic domains in superconducting/ferromagnetic hybrids

Vitalii Vlasko-VlasovA,∗ Ulrich WelpA,† Alexander BuzdinB,‡ Wai KwokA§

AMaterials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA BUniversity of Bordeaux 1, UMR 5798, CNRS, F-33405 Talence Cedex, France

We will discuss effects of the orbital coupling between superconducting vortices and magnetic domains in hybrids of superconducting (SC) and ferromagnetic (FM) materials. Direct magneto-optical observations in different SC/FM bilayers1, 2 show that this coupling is strong and determines the dynamics of vortices through the enhanced magnetic pinning by ferromagnetic domain boundaries as predicted in3, 4. A possibility to manipulate the domain structure of ferromagnets with moderate magnetic fields allows the creation of tunable pinning potentials in SC/FM hybrids. In fact, the alignment of stripe domains in the FM film introduces a pronounced uniaxial anisotropy of the vortex motion and thus anisotropy of critical currents (Jc) in the SC layer. This anisotropy can be introduces in any chosen direction by realigning the stripe domains using in-plane magnetic fields. Transport measurements confirm that a considerable anisotropy of Jc remains up to temperatures close to Tc. The strong interactions of SC vortices and magnetic moments in SC domains results in the formation of combined domain structures that do not exist in separate SC and FM layers as proposed in5. Their static and dynamic properties are defined by the vortex/domain coupling and can be described by accounting this coupling in the thermodynamic balance.

[1] V. Vlasko-Vlasov, U. Welp, G. Karapetrov, V. Novosad, D. Rosenmann, M. Iavarone, A. Belkin, and W.-K. Kwok, Phys. Rev. B 77 (2008) 134518 [2] V. Vlasko-Vlasov, U. Welp, A. Imre, D. Rosenmann, J. Pearson, and W. K. Kwok, Phys. Rev. B 78 (2008) 214511 [3] L. N. Bulaevskii, E. M. Chudnovsky, and M. P. Maley, Appl. Phys. Lett. 76 (2000) 2594; L. N. Bulaevskii, E. M. Chudnovsky, and M. Daumens, Phys. Rev. B 66 (2002) 136502 [4] E. B. Sonin, Phys. Rev. B 66 (2002) 136501; R. Laiho, E. Lahderanta, E. B. Sonin, and K. B. Traito, Phys.Rev. B 67 ( 2003) 144522 [5] I. F. Lyuksyutov and V. L. Pokrovsky, Adv. Phys. 54 (2005) 67

∗E-mail: [email protected] †E-mail: [email protected] ‡E-mail: [email protected] §E-mail: [email protected] Fast Josephson vortex ratchet made of intrinsic Josephson junctions

H. B. WangA,∗ B. Y. ZhuB, C. G¨urlichC, M. RuoﬀC, S. KimA, T. HatanoA, B. R. ZhaoB, Z. X. ZhaoB, E. GoldobinC, D. KoelleC, R. KleinerC

ANational Institute for Materials Science, Tsukuba 3050047, Japan BNational Laboratory for Superconductivity, Institute of Physics, and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, China CPhysikalisches Institut – Experimentalphysik II and Center for Collective Quantum Phenomena, UniversitätTübingen,Auf der Morgenstelle 14, D-72076 Tübingen,Germany

We demonstrate the operation of a deterministic ﬂuxon ratchet made of a stack of 30 intrinsic Bi2Sr2CaCu2O8 Josephson junctions. The ratchet has the shape of a gear with 20 asymmetric teeth (periods). It produces a rectiﬁed voltage of about 100 µV at a 12 GHz drive frequency, extrapolating to more than 2 mV for a single tooth device. We analyze the device in the framework of the coupled 2D sine-Gordon equations and argue that intrinsic junction ratchets can rectify an input ac current with a frequency up to about 200 GHz.

∗E-mail: [email protected] Phase Diagram and Vortex Pinning of Iron-Arsenide Superconductors

Ulrich WelpA,∗ A. E. KoshelevA, J. SchlueterA, J. HuaA, W. K. KwokA, G. MuB, Q. LuoB, Z. S. WangB, P. ChengB, L. FangB, C. RenB, H.-H. WenB, A. KayaniC

AMaterials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA BInstitute of Physics, Chinese Academy of Science, Beijing, China CPhysics Department, Western Michigan University, Kalamazoo, USA

We present specific heat and magnetization measurements on single crystals of the nearly 1, 2 optimally doped pnictide superconductors NdF eAsO1−xFx and Ba1−xKxF e2As2 . We determine the phase diagrams from the specific heat measurements using an entropy conserving construction, which does not require the choice of a resistivity criterion to define the transition temperature. Both materials are characterized by a low superconducting anisotropy near Tc of Γ ∼ 4 for NdF eAsO1−xFx and 2.6 for Ba1−xKxF e2As2, which is promising for potential c applications. The upper critical field slopes are extraordinarily high, ∂Hc2/∂T = −6.5T/K ab and ∂Hc2 /∂T = −17.4T/K for Ba1−xKxF e2As2, which points to the emergence of paramagnetic limiting effects at low temperatures. A thermodynamic analysis reveals that this material is extreme type-II with κc ∼ 100 and κab ∼ 260. Measurements of the magnetization hysteresis reveal strong vortex pinning and critical cur- 2 rent densities that for Ba1−xKxF e2As2 at low temperatures reach 1MA/cm . At temperatures close to Tc the critical current is characterized by a pronounced peak effect. Upon irradiation with 6 MeV protons at a dose of 2x1015p/cm2 the critical currents at low temperatures increase by roughly a factor of two. At high temperatures enhancements by up to a factor of ten occur at fields below the peak in such a way that the peak is totally overcome. There is no change in Tc nor of Hc2 due to the irradiation. This work was supported by the US Department of Energy Basic Energy Science under contract DE-AC02-06CH11357, by the Natural Science Foundation of China, the Min- istry of Science and Technology of China (973 project No. 2006CB60100, 2006CB921802, 2006CB921107) and the Chinese Academy of Sciences (Project ITSNEM).

[1] U. Welp, R. Xie, A. E. Koshelev, W. K. Kwok, P. Cheng, L. Fang, H. H. Wen, Phys. Rev. B 78 (2008) 140510 [2] U. Welp, R. Xie, A. E. Koshelev, W. K. Kwok, H. Q. Luo, Z. S. Wang, G. Mu, H. H. Wen, Phys. Rev. B 79 (2009) 094505

∗E-mail: [email protected] Flux dynamics and vortex phase diagram revealed by magnetization relaxation measurements

Hai-Hu WenA,∗ Bing Shen, Peng cheng

Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

The vortex dynamics has been measured by magnetization relaxation in Co-doped BaFe2As2 single crystals in wide doping regime. It is found that all samples exhibit a strong second peak effect.1 Dynamical magnetization relaxation reveal that the formation of the second peak effect is due to the vortex dynamics: the wider magnetization hysteresis loops correspond to small relaxation rate. A sharp and huge magnetization peak was observed near zero field, which was accompanied by a dip of relaxation rate. This was interpreted as due to the edge effect of the sample. The analysis on the vortex dynamics indicate that the collective pinning model can be applied to describe the vortex dynamics.? Finally the vortex phase diagram was obtained for samples in very wide doping regime.

[1] Yang H, Luo HQ, Wang ZS, et al.APL 93 2008142506. bibitemYangHPRB Huan Yang, Cong Ren, Lei Shan, Hai-Hu Wen, Phys. Rev. B78 (2008) 092504.

∗E-mail: [email protected] Flux penetration and vortex avalanches in micro-patterned thin ﬁlm superconductors at small T/Tc ratio.

Rinke J. WijngaardenA,∗ Diana G. GheorgheA,† Werner GillijnsB, Alejandro V. SilhanekB, Mariela MenghiniB, and Victor V. MoshchalkovB

ADepartment of Physics and Astronomy, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands BINPAC-Institute for Nanoscale Physics and Chemistry, K.U. Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium

It was shown before by magnetization and transport measurements that the properties of superconducting thin films can be modified close to Tc by patterning with anti-dots (holes of a few micron diameter or less) or with magnetic dots. Here, we investigate by magneto- optics a region in the phase diagram that extends to temperatures far from Tc, which is the temperature domain relevant for most applications of superconducting thin film devices. Ex- periments were made on samples patterned with holes, blind holes or magnetic dots. These dots are arranged in rectangular, square or Penrose patterns. In the case of magnetic dots, we show that the critical current can be modified by a factor 2.5 by changing the magnetization state of the dots. This means that the pinning in such samples can be reversibly modified by a magnetization procedure (in much higher fields than the typical self-fields of the superconductor), thus creating the possibility for post-manufacture tuning and even for devices that make use of the switching between low and high jc states. We investigate the influence of (1) size of the dots (2) spacing of the dots (3) shape of the dots (circular, rectangular) and (4) pattern of the dots (rectangular, Penrose, square) on the critical current and its anisotropy, but also on the irregular flux penetration that may be due to thermo-magnetic instabilities.

∗E-mail: [email protected] †E-mail: [email protected] Spontaneous vortices and its magnetic properties in superconductor/ferromagnet Pb/Co nanocomposites

Y. T. XingA,∗ H. MicklitzA, T. G. RappoportB, and E. Baggio-SaitovitchA

A Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro 22290-180, Brazil B Instituto de Fisica, Universidade Federal do Rio de Janeiro, Cx. P. 68528, 219s41-972 , Rio de Janeiro, Brazil

Lead (Pb) films containing homogeneously distributed cobalt (Co) nanoparticles (mean size 4.5 nm, Co volume fraction of a few percent) have been prepared with the help of a so-called cluster source. The direction of the magnetic moments of the Co nanoparticles (µCo) embedded in the Pb matrix are completely random, however they can be aligned by an external magnetic field. Magnetic measurements with SQUID show that the sample has very unusual behavior compared with other superconductor (SC)/ferromagnet (FM) hybrids. The field-cooled (FC) magnetization as a function of temperature (M-T) shows that the sample displays a paramagnetic Meissner effect (PME) when µCo have been aligned parallel to the external field. Surprisingly, the PME disappears if the direction of µCo is opposite (anti- parallel) to the direction of the external magnetic field. We explain this novel phenomenon by the interaction between the Co nanoparticles and the Pb matrix creating spontaneous vortices, which have the direction given by the direction of µCo within the vortex. The total magnetization of the sample will be the summary of the moments induced by the external magnetic field and the spontaneous vortices. Furthermore, the PME also has been observed and manipulated in the zero-field-cooled M-T measurements.

This work was partially supported by a CAPES/DAAD cooperation program and the Brazilian agencies CNPq, FAPERJ (Cientistas do Nosso Estado and PRONEX) and L’Oreal Brazil. H. Micklitz acknowledges CAPES/DAAD and PCI/CBPF for ﬁnancial support.

∗E-mail: [email protected] NanoSQUID-on-tip for scanning probe microscopy

Amit FinklerA, Yehonathan SegevA, Yuri MyasoedovA, Michael L. RappaportA, Martin E. HuberB,C, Jens MartinD, Amir YacobyD, Eli ZeldovA∗

ADepartment of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel BDepartment of Physics, University of Colorado, Denver, CO 80217, USA CDepartment of Electrical Engineering, University of Colorado, Denver, CO 80217, USA DDepartment of Physics, Harvard University, Cambridge, MA 02138, USA

A new method for self aligned fabrication of nanoSQUIDs on sharp tips is presented. Hollow quartz tubes are pulled to an apex of 100 to 400 nm diameter as shown in Fig. 1. Three aluminum evaporation steps from different angles form self-aligned leads and a SQUID loop containing two weak links, without any need for lithographic processing. The nanoSQUIDs on tip operate over a wide range of fields up to 0.6 T. A nanoSQUID with effec- 6 1/2 tive radius of 208 nm displays flux sensitivity of 1.8 × 10 Φ0/Hz at 300 mK and expected 1/2 spin sensitivity of 65 µB/Hz [1]. A scanning SQUID microscope has been constructed based on these devices with the aim of investigation of vortex dynamics in superconductors with single vortex resolution.

Figure 1: (left) SEM image of a hollow quartz tube pulled to a sharp tip. Aluminum is evaporated on two opposite sides of the quartz tube forming two superconducting (SC) leads that are visible as bright regions separated by bare quartz gap of darker color in the middle. In a third evaporation step Al is evaporated onto the apex ring that forms the nano-SQUID loop. The Al ring and leads become superconducting below 1.2 K. The parts of the loop that are in contact with the leads form strong superconducting regions. The two regions of the loop between the leads (marked by arrows) form weak links acting as two Josephson junctions in the SQUID loop. The schematic electrical circuit of the SQUID is shown in the inset. (right) Measurement of quantum interference patterns of a nano-SQUID at T = 300 mK, fabricated on a tip similar to the one on left, over a wide field range. The color reflects the measured current through the SQUID as a function of applied bias voltage and magnetic field. The oscillation period corresponds to an effective diameter of the SQUID loop of 208 nm.

∗E-mail: [email protected] Periodic magnetoresistance oscillations in high-Tc superconducting La1.895Ce0.105CuO4 and Pr0.9LaCe0.1CuO4 narrow wires B. X. WuA, J. YuanB, H. B. WangB, S. ArisawaB, A.IshiiB, T. HatanoB, I. IguchiB, X. H. ChaoA, B. R. ZhaoA, A. V. SilhanekC and V. V. MoshchalkovC, B. Y. ZhuA∗

ANational Laboratory for Superconductivity, Institute of Physics, and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, China BNational Institute for Materials Science, Tsukuba 305-0047, Japan CINPAC - Institute for Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium

We study the transport properties of epitaxially grown high-Tc superconducting La1.895- Ce0.105CuO4 and Pr0.9LaCe0.1CuO4 narrow wires in perpendicular magnetic field. Pro- nounced periodic magnetoresistance oscillations at temperatures very close to Tc are observed for both increasing and decreasing fields. These oscillations are rather insensitive to the sample width although their period strongly depends on the field step used during the field sweep. Based on molecular dynamics simulations, we demonstrate that the magnetoresistance oscillations result from a purely dynamic effect of consecutive configuration transitions between ordered and disordered moving vortex arrays.

∗E-mail: [email protected]