2014/11/05 by S. Kasahara, T. Watashige, Tatsuya Watashige +25 · 384 citations
Materials Science · Physics and Astronomy · #Condensed matter physics #Field (mathematics) #Iron-based superconductors research #Phase (matter) #Physics #Physics of Superconductivity and Magnetism #Quantum mechanics #Rare-earth and actinide compounds #Superconductivity #cond-mat.quant-gas #cond-mat.str-el #cond-mat.supr-con
paper · pdf · doi:10.1073/pnas.1413477111
published in Proceedings of the National Academy of Sciences 111(46), 16309-16313 (National Academy of Sciences) · Accepted for publication in PNAS
arxiv created 2014/11/05 · openalex publication_date 2014/11/06 · arxiv updated 2014/11/13 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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.