Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi$_2$Se$_3$ topological insulator

TL;DR

This study combines ab initio and tight-binding methods to analyze how Mn impurities affect the electronic and magnetic properties of Bi$_2$Se$_3$ topological insulators, revealing the interplay between acceptor states and surface Dirac states.

Contribution

It provides a detailed theoretical investigation of Mn impurity states and their interaction with topological surface states, highlighting the role of electronic correlations and hybridization effects.

Findings

Mn acts as a +2 valence acceptor in Bi$_2$Se$_3$.

The Mn-acceptor state is mainly p-character and localized around Mn and neighboring Se atoms.

The Dirac surface state gap opening depends on hybridization with Mn-acceptor states.

Abstract

We investigate the properties of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi$_2$Se$_3$ topological insulator. Combining ab initio calculations with microscopic tight-binding modeling, we identify the effects of inversion-symmetry and time-reversal-symmetry breaking on the electronic states in the vicinity of the Dirac point. In agreement with experiments, we find evidence that the Mn ion is in the ${+2}$-valence state and introduces an acceptor in the bulk band gap. The Mn-acceptor has predominantly $p$-character, and is localized mainly around the Mn impurity and its nearest-neighbor Se atoms. Its electronic structure and spin-polarization are determined by the hybridization between the Mn $d$-levels and the $p$-levels of surrounding Se atoms, which is strongly affected by electronic correlations at the Mn site. The opening of the gap at the Dirac point depends crucially on the quasi-resonant coupling and the strong real-space overlap between the spin-chiral surface states and the mid-gap spin-polarized Mn-acceptor states.