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

TL;DR

This paper reports the discovery of a field-induced superconducting phase in FeSe, where the Fermi energy, superconducting gap, and Zeeman energy are comparable, providing new insights into the BCS-BEC crossover regime.

Contribution

It demonstrates that FeSe enters a novel regime with small Fermi energy and reveals a new superconducting phase induced by magnetic fields in the BCS-BEC crossover.

Findings

FeSe has an extremely small Fermi energy with a large gap ratio.

A distinct phase line appears below the upper critical field where Zeeman energy matches Fermi energy and gap.

The observed phase offers insights into spin-polarized Fermi liquids in the BCS-BEC crossover.

Abstract

Fermi systems in the crossover 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 crossover has been of considerable interest both in the fields of condensed matter and ultracold atoms. One of the most challenging issue 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 $\varepsilon_{\rm F}$, superconducting gap $\Delta$ and Zeeman energy, become comparable. Through the superfluid response, transport, thermoelectric response, and spectroscopic-imaging scanning tunneling microscopy, we demonstrate that $\varepsilon_{\rm F}$ of FeSe is extremely small, with the ratio $\Delta/\varepsilon_{\rm F}\sim1 (\sim0.3)$ 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 $\varepsilon_{\rm F}$ and $\Delta$. 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 crossover regime.