Infrared markers of topological phase transitions in quantum spin Hall insulators

TL;DR

Infrared optical response can effectively distinguish topological from trivial phases in quantum spin Hall insulators, with observable spectral discontinuities linked to changes in Born effective charges, as demonstrated in germanene and jacutingaite.

Contribution

This study introduces infrared spectroscopy as a novel method to identify topological phase transitions in 2D QSH insulators, supported by first principles calculations and low-energy modeling.

Findings

Infrared spectrum shows discontinuities across topological transitions.

Born effective charges exhibit large discretized jumps at the transition.

Dynamical effects influence phonon resonance profiles in different materials.

Abstract

Using first principles techniques, we show that infrared optical response can be used to discriminate between the topological and the trivial phases of two-dimensional quantum spin Hall insulators (QSHI). We showcase germanene and jacutingaite, of recent experimental realization, as prototypical systems where the infrared spectrum is discontinuous across the transition, due to sudden and large discretized jumps of the value of Born effective charges (up to 2). For these materials, the topological transition can be induced via the application of an external electrostatic potential in the field-effect setup. Our results are rationalized in the framework of a low-energy Kane-Mele model and are robust with respect to dynamical effects which come into play when the energy gap of the material is of the same order of the infrared active phonon frequency. In the small gap QSHI germanene, due to dynamical effects, the in-plane phonon resonance in the optical conductivity shows a Fano profile with remarkable differences in the intensity and the shape between the two phases. Instead, the large gap QSHI jacutingaite presents several IR-active phonon modes whose spectral intensities drastically change between the two phases.