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Tt 40.1–40.18 Thursday TT 40: Superconductivity: Ferropnictides 1 Time: Thursday 14:00–19:00 Location: HSZ 03 TT 40.1 Thu 14:00 HSZ 03 sruhe, Institut f¨urFestk¨orperphysik, 76021 Karlsruhe, Germany — Antiferromagnetic correlations in the normal state of 2Physikalisches Institut, Universit¨atKarlsruhe, 76128 Karlsruhe, Ger- LaFeAsO1−xFx with 0 ≤ x ≤ 0.125 — •Rudiger¨ Klingeler, Nor- many man Leps, Liran Wang, Christian Hess, Gunter¨ Behr, Vladislav Since the discovery of the new high-Tc iron pnictides, many scenar- Kataev, and Bernd Buchner¨ — IFW Dresden, P.O. Box 270116, ios were put forward to describe the symmetry of the order parameter 01171 Dresden, Germany including d-wave and unconventional s-wave. Early heat capacity mea- We have studied the interplay of magnetism and superconductivity in surements suggested that the electron-doped 1111 compounds show a LaFeAsO1−xFx with 0 ≤ x ≤ 0.125. For low doping with x ≤ 0.04, our nodal gap, while K-hole-doped 122 materials are fully gapped. Here we data confirm a moderate suppression of both the structural transition present a critical analysis of our own specific-heat data on Co-doped and the antiferromagnetic spin density wave formation. For x ≥ 0.05, 122 single crystals, in which we pay particular attention to the details both anomalies are completely suppressed and superconductivity is ob- of the phonon background subtraction as well as to the contribution served. Remarkably, the temperature dependence of the normal state of impurity phases. We also discuss an interesting field-dependence of susceptibility well above TC is almost independent of doping, i.e. both the thermal expansivity below Tc(H). the absolute value and the slope are nearly unchanged compared to the undoped case [1]. This implies at least local antiferromagnetic in- TT 40.5 Thu 15:00 HSZ 03 teractions which barely depend on hole doping although the ground Strong coupling of superconductivity to c/a in Ba(Fe,Co)2As2 state changes entirely from an orthorhombic antiferromagnetic poor — •Christoph Meingast1, Frederic Hardy1, Peter Adelmann1, metal (x ≤ 0.04) to a tetragonal superconductor (x ≥ 0.05). These Peter Schweiss1, Doris Ernst1, Hilbert v. Lohneysen¨ 1,2, surprising results are discussed in terms of (i) - pseudogap formation, and Thomas Wolf1 — 1Forschungszentrum Karlsruhe, Institut f¨ur (ii) - antiferromagnetic correlations, and (iii) - preformed bipolarons Festk¨orperphysik, — 2Physikalisches Institut, Universit¨at Karlsruhe, which might be relevant to the pairing mechanism. D-76128 Karlsruhe, Germany. [1] R. Klingeler et al., Preprint at http://arxiv.org/abs/0808.0708 Just as in the cuprates, magnetism and superconductivity occur in close proximity to each other in the newly discovered FeAs-based ma- TT 40.2 Thu 14:15 HSZ 03 terials. Here, using high-resolution thermal expansion and specific heat Electronic phase diagram of the LaO1−xFxFeAs supercon- measurements, we study the thermodynamic response of the lattice pa- 1 ductor: A muon spin relaxation study — •H. Luetkens , rameters to superconducting and magnetic order in Ba(Fe,Co)2As2 sin- 2 3 3 4 H.-H. Klauss , F.J. Litterst , T. Dellmann , R. Klingeler , gle crystals. We show that there is a strong coupling of the c/a ratio to 4 1 1 1 5 C. Hess , R. Khasanov , A. Amato , C. Baines , M. Kosmala , both the superconducting and magnetic/structural phase transitions. 5 5 4 4 O.J. Schumann , M. Braden , J. Hamann-Borrero , N. Leps , A. According to the Ehrenfest relationship, the ordering temperatures of 4 4 4 3 4 Kondrat , G. Behr , J. Werner , M. Kraken , and B. Buchner¨ both ordered states are expected to increase with increasing c/a. This 1 — Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institut, suggests that the occurrence of superconductivity is strongly linked to 2 Villigen, Switzerland — Institut f¨urFestk¨orperphysik, TU Dresden — the magnetic/structural transition. 3Institut f¨urPhysik der Kondensierten Materie, TU Braunschweig — 4Leibniz-Institut f¨ur Festk¨orper- und Werkstoffforschung (IFW) Dres- TT 40.6 Thu 15:15 HSZ 03 den — 5II. Physikalisches Institut, U K¨oln Electronic phase separation in the slightly underdoped iron 1 The structural and electronic phase diagram of LaO1−xFxFeAs and, pnictide superconductor Ba1−xKxFe2As2 — •Ji Tae Park , in particular, the exact nature of the change from the magnetically or- D. S. Inosov1, Ch. Niedermayer2, G. L. Sun1, D. Haug1, N. dered to the superconducting state that was determined by means of B. Christensen2, R Dinnebier1, A. V. Boris1, A. J. Drew3, L. x-ray scattering, muSR and M¨ossbauer spectroscopy will be presented Schulz3, T. Shapoval4, U. Wolff4, V. Neu4, X. Yang1, C. T. Lin1, [1-3]. A discontinuous first-order-like change of the N´eeltemperature, B. Keimer1, and V. Hinkov1 — 1Max-Planck-Institut f¨ur Festk¨orper- the superconducting transition temperature, the sublattice magnetisa- forschung, Heisenbergstraße 1, Stuttgart, Germany — 2ETHZ & PSI, tion and the superfluid density is found between x=0.04 and x=0.05. Villigen PSI, Switzerland — 3Department of Physics, University of Fri- While these results strongly question the relevance of quantum crit- bourg, Chemin du Mus´ee3, Fribourg, Switzerland — 4IFW Dresden, ical behaviour in iron pnictides they prove an important role of the Institute for Metallic Materials, P.O. Box 270116, D-01171 Dresden, structural orthorhombic distortion disappearing exactly at the SDW Germany magnetism and superconductivity phase boundary. We performed a combined study of the slightly underdoped novel iron [1] H. Luetkens et al., Phys. Rev. Lett. 101, 097009 (2008). pnictide superconductor Ba1−xKxFe2As2 by means of X-ray powder [2] H.-H. Klauss et al., Phys. Rev. Lett. 101, 077005 (2008). diffraction, neutron scattering, muon spin rotation (µSR), and mag- [3] H. Luetkens et al., arXiv:0806.3533 (2008). netic force microscopy (MFM). Commensurate static magnetic order sets in below Tm ∼ 70 K as inferred from the emergence of the mag- TT 40.3 Thu 14:30 HSZ 03 netic (1 0 3) reflection in the neutron scattering data and from the Functional renormalization group study of the iron pnictides observation of damped oscillations in the zero-field-µSR asymmetry. — •Christian Platt, Carsten Honerkamp, and Werner Hanke Transverse-field µSR below Tc shows a coexistence of magnetically or- — Institute for Theoretical Physics and Astrophysics, University of dered and non-magnetic states, which is also confirmed by MFM imag- W¨urzburg, Am Hubland, 97074 W¨urzburg, Germany ing. This coexistence could be explained by electronic phase separation Recently, a new class of superconductors (sc) - Fe-based sc - was dis- into antiferromagnetic and superconducting/normal state regions on a covered. These sc iron pnictides are most likely less correlated than scale of several tens of nanometers indicating that such mesoscopic the high-Tc cuprates but present again a challenging case of competing phase separation can be considered an intrinsic property of some iron magnetic and superconducting orders at low temperatures. Therefore, pnictide superconductors. perturbative functional renormalization group (fRG) methods appear adequate for the theoretical modelling of the phase diagram. Here, 15 min. break we apply the fRG to a four-band (Fe-d-orbital) model including intra- and interband couplings as well as interband pair hoppings. We com- TT 40.7 Thu 15:45 HSZ 03 pute the leading instabilities, i.e. spin-ordered phase in the ”under- The intrinsic electronic phase diagram of iron-pnictide su- doped”situation and the leading pairing instability as a function of the perconductors — •C. Hess, A. Kondrat, A. Narduzzo, J. E. electron density and interaction parameters. Hamann-Borrero, R. Klingeler, H. Grafe, G. Lang, F. Ham- merath, D. Paar, A. Alfonsov, V. Kataev, J. Werner, G. Behr, TT 40.4 Thu 14:45 HSZ 03 and B. Buchner¨ — Leibniz-Institute for Solid State and Materials Thermodynamic study of the Co-doped Ba-122 iron pnic- Research, IFW Dresden, 01171 Dresden, Germany tide — • ´ ´ 1, 1, 1, Frederic Hardy Christoph Meingast Thomas Wolf We present a detailed study of the intrinsic electronic phase diagram 1, 1, 1, 1, Rolf Heid Peter Adelmann Peter Schweiss Doris Ernst of the oxypnictide superconductors in the normal state based on the and Hilbert v. Lohneysen¨ 1,2 — 1Forschungszentrum Karl- analysis of the electrical resistivity ρ of both LaO1−xFxFeAs and Thursday SmO1−xFxFeAs for a wide range of doping. Our data give clear-cut ported. Within the resonance regime of the photon energy profile an evidence for unusual normal state properties in these new materials. intensity enhancement centered at E = 3.6 eV is observed which can be As a function of doping ρ of LaO1−xFxFeAs shows a clear transition identified as the many body satellite. The energy position of the satel- from pseudogap to Fermi liquid-like behavior, mimicking the phase lite matches the expectations based on previously extracted parameters diagram of the cuprates. Moreover, our data reveal a correlation be- from Fe L x-ray absorption spectroscopy and Fe 2p x-ray photoemis- tween the strength of the pseudogap signatures and the stability of the sion spectroscopy. The results show, that the Hubbard repulsion U, superconducting phase. The pseudogap signatures, which are clearly although smaller than the bandwidth, preserves a clear physical mean- connected with the structural and magnetic transitions of the parent ing giving rise to local electron phenomena in an otherwise itinerant material, become stronger in SmO1−xFxFeAs where superconductiv- environment. ity is enhanced and vanish when superconductivity is reduced in the doping region with Fermi liquid-like behavior [1]. We further present TT 40.11 Thu 16:45 HSZ 03 evidence for the connection between the pseudogap signatures in elec- Momentum dependence of the superconducting gap in 1 1,2 trical transport and the slowing-down of spin fluctuation. Ba1−xKxFe2As2 — •D. V. Evtushinsky , D. S. Inosov , V. [1] C. Hess et al., Preprint at http://arxiv.org/abs/0811.1601 B.
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