Engineering quantum anomalous Hall phases with orbital and spin degrees of freedom

TL;DR

This paper explores how intrinsic spin-orbit coupling and orbital degrees of freedom in buckled honeycomb lattices can induce quantum anomalous Hall phases, with detailed analysis on Bi bilayers and potential tuning via alloying and doping.

Contribution

It demonstrates the emergence of multiple QAH phases driven by orbital and spin interactions, and proposes methods to tune topological properties in Bi bilayers.

Findings

Multiple QAH phases identified in Bi bilayers.

Coexistence of quantized charge and spin Hall conductivities.

Tuning of topological phases via alloying and doping methods.

Abstract

Combining tight-binding models and first principles calculations, we investigate the quantum anomalous Hall (QAH) effect induced by intrinsic spin-orbit coupling (SOC) in buckled honeycomb lattice with sp orbitals in an external exchange field. Detailed analysis reveals that nontrivial topological properties can arise utilizing not only spin but also orbital degrees of freedom in the strong SOC limit, when the bands acquire non-zero Chern numbers upon undergoing the so-called orbital purification. As a prototype of a buckled honeycomb lattice with strong SOC we choose the Bi(111) bilayer, analyzing its topological properties in detail. In particular, we show the emergence of several QAH phases upon spin exchange of the Chern numbers as a function of SOC strength and magnitude of the exchange field. Interestingly, we observe that in one of such phases, namely, in the quantum spin Chern insulator phase, the quantized charge and spin Hall conductivities co-exist. We consider the possibility of tuning the SOC strength in Bi bilayer via alloying with isoelectronic Sb, and speculate that exotic properties could be expected in such an alloyed system owing to the competition of the topological properties of its constituents. Finally, we demonstrate that 3d dopants can be used to induce a sizeable exchange field in Bi(111) bilayer, resulting in non-trivial Chern insulator properties.