Field-induced superconducting phase of FeSe in the BCS-BEC cross-over

Shigeru Kasahara, Tatsuya Watashige, Tetsuo Hanaguri, Yuhki Kohsaka, Takuya Yamashita, Yusuke Shimoyama, Yuta Mizukami, Ryota Endo, Hiroaki Ikeda, Kazushi Aoyama, Taichi Terashima, Shinya Uji, Thomas Wolf, Hilbert Von Löhneysen, Takasada Shibauchi, Yuji Matsuda

Research output: Contribution to journalArticlepeer-review

284 Citations (Scopus)


Fermi systems in the cross-over regime between weakly coupled Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Einsteincondensate (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 eF 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.

Original languageEnglish
Pages (from-to)16309-16313
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number46
Publication statusPublished - Nov 18 2014
Externally publishedYes


  • BCS-BEC cross-over
  • Exotic superconducting phase
  • Fermi energy
  • Iron-based superconductors
  • Quasiparticle interference

ASJC Scopus subject areas

  • General


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