Unusual band evolution and persistence of topological surface states in high-T_C magnetic topological insulator

TL;DR

This study investigates the electronic structure and topological surface states in high-Curie-temperature ferromagnetic topological insulators, revealing persistent Dirac states and unusual band evolution with doping, advancing understanding of ferromagnetism in TIs.

Contribution

It provides new insights into the band evolution and persistence of topological surface states in high-T_C magnetic TIs through ARPES measurements, highlighting the interplay between doping, band topology, and ferromagnetism.

Findings

Dirac cone persists up to high T_C despite doping

Unusual band filling and evolution with Cr doping

Magnetic gap opens below T_C, indicating strong coupling

Abstract

Understanding the mechanism of ferromagnetism in ferromagnetic topological insulators (TIs) is a key to realize exotic time-reversal-symmetry-broken quantum phases. However, electronic states relevant to the ferromagnetism are highly controversial. Here we report angle-resolved photoemission spectroscopy on (CrxSb1-x)2Te3 thin films, high-Curie-temperature (T_C) ferromagnetic TIs, spanning the non-doped (T_C=0 K) to highly-doped (T_C=192 K) region. We found that, upon Cr doping to Sb2Te3, the bulk valence-band valley exhibits filling-in behavior while retaining band inversion, leading to the formation of a nearly-flat band in high-T_C regime and evolution from a six-petal flower to a Star-of-David Fermi surface. Despite the weakening of spin-orbit coupling with Cr doping, the Dirac-cone state persists up to the highest-T_C sample, and shows a clear magnetic-gap opening below TC accompanied with an unexpected band shift, signifying its strong coupling with spontaneous ferromagnetism. The present result lays the foundation for understanding the interplay between band topology and ferromagnetism in TIs.