Band structure reconstruction across nematic order in high quality FeSe single crystal as revealed by optical spectroscopy study

TL;DR

This study uses optical spectroscopy to reveal how the electronic band structure of high-quality FeSe single crystals changes across the nematic transition at 90 K, showing gap formation, band splitting, and spectral weight transfer.

Contribution

It provides detailed optical evidence of band structure reconstruction and correlation effects in FeSe across the nematic transition, complementing ARPES findings.

Findings

Observation of a gap-like feature at 400 cm$^{-1}$ below $T_s$

Reduction in plasma frequency indicating lower carrier density

Spectral weight transfer suggesting strong electronic correlations

Abstract

We perform an in-plane optical spectroscopy measurement on high quality FeSe single crystals grown by a vapor transport technique. Below the structural transition at $T_{\rm s}\sim$90 K, the reflectivity spectrum clearly shows a gradual suppression around 400 cm$^{-1}$ and the conductivity spectrum shows a peak at higher frequency. The energy scale of this gap-like feature is comparable to the width of the band splitting observed by ARPES. The low-frequency conductivity consists of two Drude components and the overall plasma frequency is smaller than that of the FeAs based compounds, suggesting a lower carrier density or stronger correlation effect. The plasma frequency becomes even smaller below $T_{\rm s}$ which agrees with the very small Fermi energy estimated by other experiments. Similar to iron pnictides, a clear temperature-induced spectral weight transfer is observed for FeSe, being indicative of strong correlation effect.