Dr. Taka Shibauchi , Department of Physics, Kyoto University

Jeudi 27 Mai à 16 h 00 Amphi Holweck, Esc. C, 1er étage

Non-fermi liquid transport properties and non-universal gap
structure in iron-pnictide High-Tc superconductors

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-arsenide compounds [1,2] 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 normal-state transport properties [3] as well as bulk measurements of the magnetic penetration depth and heat transport in high-quality single crystals of BaFe2(As1-xPx)2 [4]. In the normal state at the critical doping near the SDW phase boundary, we find the non-Fermi liquid transport properties including T-linear resistivity, strong temperature dependence of Hall coefficient, and violation of Kohlerʻs rule in magneto-resistance. These anomalous properties become less pronounced with x, which points to an important role of
antiferromagnetic fluctuations in the non- Fermi-liquid transport properties in the Fe-pnictides. In the superconducting state, 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 [2] 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 pairingstates, with and without nodes, which have similar Tc.

[1] K. Hashimoto et al., Phys. Rev. Lett. 102, 017002 (2009).
[2] K. Hashimoto et al., Phys. Rev. Lett. 102, 207001 (2009).
[3] S. Kasahara et al., arXiv:0905.4427.
[4] K. Hashimoto et al., arXiv:0907.4399.

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