Y. Matsuda, Department of Physics, Kyoto University, Kyoto, Japan

Mercredi 10 Décembre 2014, 10h30
Amphi Boreau, Esc C, 2ème etage

Quantum criticality, BCS-BEC crossover and nematicity in iron-based high-Tc superconductors.

Y. Matsuda
Department of Physics, Kyoto University, Kyoto, Japan

Whether a quantum critical point (QCP) lies beneath the superconducting dome has been a long-standing issue that remains unresolved in many classes of unconventional superconductors, notably cuprates, heavy fermion compounds and most recently iron-pnictides. Recent experiments in BaFe2(As1−xPx)2 have provided the first clear and unambiguous evidence of a second order quantum phase transition lying beneath the superconducting dome [1][2]. Based on these results, a new phase diagram of iron-pnictide is discussed [3].
We also show that in iron-charcogenide FeSe the Fermi energy EF is extremely small and comparable to the superconducting gap energy Δ ; the ratio Δ/ EF 1. High-field thermal conductivity measurements in a clean crystal reveal a new superconducting phase when the Zeeman energy becomes comparable to EF and Δ. Such a superconducting state with three comparable characteristic energy scales has never been realized in any previously studied superconductors. FeSe may pave the way for future study of the highly spin-polarized degenerate Fermi gas, in the crossover regime between weakly coupled Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Einstein-condensation (BEC) limit [4].

[1] T. Shibauchi, A. Carrington, and Y. Matsuda, Annu. Rev. Condens. Matter Phys. 5, 113 (2014).
[2] K. Hashimoto et al. Science 336, 1554 (2012)
[3] S. Kasahara et al. Nature, 486, 382 (2012).
[4] Proc. Natl. Acad. Sci. USA doi:10.1073/pnas.1413477111 (2014).

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