Linear response of the Chern insulator MnBi$_2$Te$_4$: A Wannier function approach

TL;DR

This paper investigates the optical response of MnBi$_2$Te$_4$ thin films using Wannier functions, revealing insights into their topological properties and clarifying discrepancies in their Chern number classifications.

Contribution

The study combines density functional theory with Wannier functions to accurately compute the topology and optical properties of MnBi$_2$Te$_4$ films, addressing previous conflicting reports.

Findings

Eleven septuple layer films share the same Chern number as five-layer films.

Discrepancies with previous studies on Chern number are discussed and analyzed.

Spin-orbit coupling-driven band inversions are identified as topological indicators.

Abstract

Recent work demonstrated that in the long wavelength limit the linear response of a Chern insulator to finite-frequency electric fields is the sum of two terms: A general frequency-dependent Kubo contribution that is present irrespective of band topology, and a topological Hall term that vanishes for topologically trivial insulators. Motivated by recent experiments and theoretical predictions, we use these expressions to calculate the optical conductivity and susceptibility of intrinsically magnetic MnBi$_2$Te$_4$ thin films with one, four, five, and eleven septuple layers by combining density functional theory with "single-shot" Wannier functions. To characterize the underlying topology of these systems, we compute the two-dimensional Chern number of these films using recently derived global expressions formulated in terms of Bloch energies and velocity matrix elements; the use of these expressions allows us to circumvent numerical issues at band crossings. Films with eleven septuple layers are of particular interest. We find that they have the same Chern number as five septuple layer films, in contrast to the reported "higher Chern-number phase" of these systems in other studies; we discuss a few possible reasons for the discrepancy. We also identify spin-orbit coupling-driven band inversions as a possible indicator of these topological phases.