Search

Fermi systems in the cross-over regime between weakly coupled Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Ein-stein-condensate (BEC) limits are among the most fascinating objects to study the behavior of an assembly of strongly interact ing particles. The physics of this cross-over has been of consider able 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 un usual 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 Zee-man energy, become comparable. Through the superfluid response, transport, thermoelectric response, and spectroscopic-imaging scan ning 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. [ABSTRACT FROM AUTHOR]