Massive Dirac surface states in topological insulator/magnetic insulator heterostructures

TL;DR

This study investigates the effects of magnetic insulator heterostructures on topological insulator surface states, revealing a significant energy gap induced by magnetic exchange coupling, with potential implications for topological quantum phenomena.

Contribution

The paper identifies MnSe as an optimal magnetic insulator for topological insulator heterostructures and demonstrates through first-principles calculations that it induces a sizable gap in surface states.

Findings

MnSe is an effective magnetic insulator for TIs.

Exchange coupling induces a 54 meV gap in surface states.

Surface state gap depends on Mn ion distance.

Abstract

Topological insulators are new states of matter with a bulk gap and robust gapless surface states protected by time-reversal symmetry. When time-reversal symmetry is broken, the surface states are gapped, which induces a topological response of the system to electromagnetic field--the topological magnetoelectric effect. In this paper we study the behavior of topological surface states in heterostructures formed by a topological insulator and a magnetic insulator. Several magnetic insulators with compatible magnetic structure and relatively good lattice matching with topological insulators ${\rm Bi_2Se_3}, {\rm Bi_2Se_3}, {\rm Sb_2Te_3}$ are identified, and the best candidate material is found to be MnSe, an anti-ferromagnetic insulator. We perform first-principles calculation in ${\rm Bi_2Se_3/MnSe}$ superlattices and obtain the surface state bandstructure. The magnetic exchange coupling with MnSe induces a gap of $\sim$54 meV at the surface states. In addition we tune the distance between Mn ions and TI surface to study the distance dependence of the exchange coupling.