Real-space BCS-BEC crossover in FeSe monolayer

TL;DR

This study provides direct real-space spectroscopic evidence of the BCS-BEC crossover in a FeSe monolayer, revealing how band structure shifts induce the crossover and demonstrating the phenomenon at the atomic scale.

Contribution

First direct real-space spectroscopic observation of BCS-BEC crossover in a 2D material, supported by theoretical modeling and Zeeman splitting analysis.

Findings

Spectroscopic evidence of BCS-BEC crossover in FeSe monolayer

Crossover driven by band structure shift relative to Fermi level

Zeeman splitting consistent with condensate characteristics

Abstract

The quantum many body states in the BCS-BEC crossover regime are of long-lasting interest. Here we report direct spectroscopic evidence of BCS-BEC crossover in real-space in a FeSe monolayer thin film by using spatially resolved scanning tunneling spectra. The crossover is driven by the shift of band structure relative to the Fermi level. The theoretical calculation based on a two-band model qualitatively reproduces the measured spectra in the whole crossover range. In addition, the Zeeman splitting of the quasi-particle states is found to be consistent with the characteristics of a condensate. Our work paves the way to study the exotic states of BCS-BEC crossover in a two-dimensional crystalline material at the atomic scale.