Field-induced superconducting phase of FeSe in the BCS-BEC cross-over.
Fermi systems in the cross-over regime between weakly coupled Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Einstein-condensate (BEC) limits are among the most fascinating objects to study the behavior of an assembly of strongly interacting particles. The physics of this cross-over has been of considerable interest both in the fields of condensed matter and ultracold atoms. One of the most challenging issues in this regime is the effect of large spin imbalance on a Fermi system under magnetic fields. Although several exotic physical properties have been predicted theoretically, the experimental realization of such an unusual superconducting state has not been achieved so far. Here we show that pure single crystals of superconducting FeSe offer the possibility to enter the previously unexplored realm where the three energies, Fermi energy εF, superconducting gap Δ, and Zeeman energy, become comparable. Through the superfluid response, transport, thermoelectric response, and spectroscopic-imaging scanning tunneling microscopy, we demonstrate that εF of FeSe is extremely small, with the ratio Δ/εF ~ 1(~0.3) in the electron (hole) band. Moreover, thermal-conductivity measurements give evidence of a distinct phase line below the upper critical field, where the Zeeman energy becomes comparable to εF and Δ. The observation of this field-induced phase provides insights into previously poorly understood aspects of the highly spin-polarized Fermi liquid in the BCS-BEC cross-over regime.
Department of Physics, Kyoto University, Kyoto 606-8502, Japan;
Shigeru
S
Kasahara
Department of Physics, Kyoto University, Kyoto 606-8502, Japan; RIKEN Center for Emergent Matter Science, Wako, Saitama 351-0198, Japan;
Tatsuya
T
Watashige
RIKEN Center for Emergent Matter Science, Wako, Saitama 351-0198, Japan;
Tetsuo
T
Hanaguri
RIKEN Center for Emergent Matter Science, Wako, Saitama 351-0198, Japan;
Yuhki
Y
Kohsaka
Department of Physics, Kyoto University, Kyoto 606-8502, Japan;
Takuya
T
Yamashita
Department of Physics, Kyoto University, Kyoto 606-8502, Japan;
Yusuke
Y
Shimoyama
Department of Physics, Kyoto University, Kyoto 606-8502, Japan; Department of Advanced Materials Science, University of Tokyo, Chiba 277-8561, Japan;
Yuta
Y
Mizukami
Department of Physics, Kyoto University, Kyoto 606-8502, Japan;
Ryota
R
Endo
Department of Physics, Kyoto University, Kyoto 606-8502, Japan;
Hiroaki
H
Ikeda
Department of Physics, Kyoto University, Kyoto 606-8502, Japan; The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8501, Japan;
Kazushi
K
Aoyama
National Institute for Materials Science, Tsukuba, Ibaraki, 305-0003 Japan; and.
Taichi
T
Terashima
National Institute for Materials Science, Tsukuba, Ibaraki, 305-0003 Japan; and.
Shinya
S
Uji
Institute of Solid State Physics, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany.
Thomas
T
Wolf
Institute of Solid State Physics, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany.
Hilbert
H
von Löhneysen
Department of Physics, Kyoto University, Kyoto 606-8502, Japan; Department of Advanced Materials Science, University of Tokyo, Chiba 277-8561, Japan;
Takasada
T
Shibauchi
Department of Physics, Kyoto University, Kyoto 606-8502, Japan; matsuda@scphys.kyoto-u.ac.jp.
Yuji
Y
Matsuda
BCS-BEC cross-over; Fermi energy; exotic superconducting phase; iron-based superconductors; quasiparticle interference