Large Gap Quantum Anomalous Hall Effect in a Type-I Heterostructure Between a Magnetically Doped Topological Insulator and Antiferromagnetic Insulator

Abstract

Heterostructures between topological insulators (TI) and magnetic insulators represent a pathway to realize the quantum anomalous Hall effect (QAHE). But major challenges remain, typical heterostructures have weak magnetic exchange interactions across the interface, resulting in small inverted band gaps. Magnetic doping can further increase the band gap due to the additional Zeeman effect, but gaps remain small at low concentrations of magnetic dopants, with higher concentrations resulting in shifting of the Fermi-level, that can render the heterostructure metallic. In addition, chemical bonding across interfaces can lead to imperfect interfaces that might induce metallicity or further lower the gap size. Using density functional theory based systematic screening and investigation of thermodynamic, magnetic and topological properties of heterostructures, we demonstrate that forming a type-I heterostructure between a wide gap antiferromagnetic insulator Cr2O3 and a TI-film, such as Sb2Te3, can lead to pinning of the Fermi-level at the center of the gap, even when magnetically doped. Cr-doping in the heterostructure increases the gap to ∼ 64.5 meV, with a large Zeeman energy from the interfacial Cr dopants, thus overcoming potential metallicity due to band bending effects. By fitting the band-structure around the Fermi-level to a 4-band k.p model Hamiltonian, we show that Cr doped Sb2Te3/Cr2O3 is a Chern insulator with a Chern number C = -1. Transport calculations further show chiral edge-modes localized at the top/bottom of the TI-film to be the dominant current carriers in the material. Our predictions of a large interfacial magnetism due to Cr-dopants, that coupled antiferromagnetically to the AFM substrate is confirmed by our polarised neutron reflectometry measurements on MBE grown Cr doped Sb2Te3/Cr2O3 heterostructures, and is consistent with a positive exchange bias measured in such systems recently. Consequently, Cr doped Sb2Te3/Cr2O3 heterostructure represents a promising platform for the development of functional topological magnetic devices, with high tunability.