Ab initio investigation of impurity-induced in-gap states in Bi$_2$Te$_3$ and Bi$_2$Se$_3$

TL;DR

This study combines first-principles calculations and an Anderson impurity model to show that impurity-induced in-gap states in Bi$_2$Te$_3$ and Bi$_2$Se$_3$ originate from hybridization with bulk bands, challenging previous assumptions.

Contribution

It reveals that in-gap states from transition metal impurities in topological insulators come from bulk band hybridization, not surface states, providing new insights into magnetic doping effects.

Findings

In-gap states originate from bulk band hybridization.

Impurity states are not primarily from topological surface states.

Contradicts previous models attributing in-gap states to surface state hybridization.

Abstract

We investigate in-gap states emerging when a single $3d$ transition metal impurity is embedded in topological insulators (Bi$_2$Te$_3$ and Bi$_2$Se$_3$). We use a combined approach relying on first-principles calculations and an Anderson impurity model. By computing the local density of states of Cr, Mn, Fe and Co embedded not only in surfaces of Bi$_2$Te$_3$ and of Bi$_2$Se$_3$ but also in their bulk phases, we demonstrate that in-gap states originate from the hybridization of the electronic states of the impurity with bulk bands and not with the topological surface states as it is usually assumed. This finding is analyzed using a simplified Anderson impurity model. These observations are in contradiction with the prevailing models used to investigate the magnetic doping of topological insulators [R. R. Biswas et al. PRB 81, 233405 (2010), A. M. Black-Schafer et al. PRB 91, 201411 (2015)], which attribute the origin of the in-gap states to the hybridization with the topological surface states.