Spectroscopic evidence for topological band structure in FeTe$_{0.55}$Se$_{0.45}$

TL;DR

This study provides spectroscopic evidence confirming the topological nature of the band structure in FeTe$_{0.55}$Se$_{0.45}$, demonstrating the presence of Dirac surface states and band inversion, and highlighting the role of correlations.

Contribution

The paper offers direct ARPES evidence of topological band structure in FeTe$_{0.55}$Se$_{0.45}$ and clarifies the band inversion mechanism through comparison with FeSe and a tight-binding model.

Findings

Persistent Dirac surface states independent of $k_z$

Identification of spectral signatures of band inversion

Reconciliation of band structure with a symmetry-preserving model

Abstract

FeTe$_{0.55}$Se$_{0.45}$(FTS) occupies a special spot in modern condensed matter physics at the intersections of electron correlation, topology, and unconventional superconductivity. The bulk electronic structure of FTS is predicted to be topologically nontrivial thanks to the band inversion between the $d_{xz}$ and $p_z$ bands along $\Gamma$-$Z$. However, there remain debates in both the authenticity of the Dirac surface states (DSS) and the experimental deviations of band structure from the theoretical band inversion picture. Here we resolve these debates through a comprehensive ARPES investigation. We first observe a persistent DSS independent of $k_z$. Then, by comparing FTS with FeSe which has no band inversion along $\Gamma$-$Z$, we identify the spectral weight fingerprint of both the presence of the $p_z$ band and the inversion between the $d_{xz}$ and $p_z$ bands. Furthermore, we propose a reconciling band structure under the framework of a tight-binding model preserving crystal symmetry. Our results highlight the significant influence of correlation on modifying the band structure and make a strong case for the existence of topological band structure in this unconventional superconductor.