Mechanism for subgap optical conductivity in honeycomb Kitaev materials

TL;DR

This paper develops a theoretical framework to explain subgap optical conductivity in honeycomb Kitaev materials, specifically addressing terahertz absorption in $ ext{RuCl}_3$, by analyzing electric and magnetic dipole transitions.

Contribution

It introduces a second-order polarization operator mechanism and calculates associated dynamical correlation functions using the exact Kitaev model, linking theory to recent experiments.

Findings

Identifies a polarization mechanism for subgap optical absorption.

Calculates electric dipole transition contributions.

Provides theoretical support for terahertz absorption observations.

Abstract

Motivated by recent terahertz absorption measurements in $\alpha$-RuCl$_3$, we develop a theory for the electromagnetic absorption of materials described by the Kitaev model on the honeycomb lattice. We derive a mechanism for the polarization operator at second order in the nearest-neighbor hopping Hamiltonian. Using the exact results of the Kitaev honeycomb model, we then calculate the polarization dynamical correlation function corresponding to electric dipole transitions, in addition to the spin dynamical correlation function corresponding to magnetic dipole transitions.