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) supercon- ducting 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 e®ect in amorphous superconducting ¯lms 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 ¯lms of Nb0:15Si0:85 and InOx , a ¯nite Nernst co- e±cient can be detected in a wide range of temperature and magnetic ¯eld3. 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-¯eld 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 ¯nite magnetic ¯eld, the Nernst coe±cient is set by the size of superconducting fluctuations. The Nernst coe±cient emerges as a direct probe of the ghost critical ¯eld, the normal-state mirror of the upper critical ¯eld4. Moreover, upon leaving the normal state with fluctuating Cooper pairs, we show that the temperature evolution of the Nernst coe±cient is di®erent 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-¯eld-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 e®ect 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 e®ect as a probe of supercon- ducting fluctuations in disordered thin ¯lms. 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 vor- tex 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.