Mechanisms for Sub-Gap Optical Conductivity in Herbertsmithite

TL;DR

This paper explores the mechanisms behind sub-gap optical conductivity in Herbertsmithite, a Kagome lattice spin-liquid candidate, focusing on emergent fermionic spinons, magneto-elastic effects, and spin-orbit coupling.

Contribution

It introduces new mechanisms for optical absorption in Herbertsmithite, including magneto-elastic effects and spin-orbit coupling, and provides a microscopic basis for existing theories in the Dirac spin-liquid scenario.

Findings

Identified magneto-elastic effects as a new absorption mechanism.

Linked microscopic origins to slave-particle theories in Dirac spin-liquid.

Analyzed sub-gap conductivity in two spin-liquid scenarios.

Abstract

Recent terahertz conductivity measurements observed low-power-law frequency dependence of optical conduction within the Mott gap of the Kagome lattice spin-liquid candidate Herbertsmithite. We investigate mechanisms for this observed sub-gap conductivity for two possible scenarios in which the ground-state is described by: 1) a U(1) Dirac spin-liquid with emergent fermionic spinons or 2) a nearly critical Z2 spin-liquid in the vicinity of a continuous quantum phase transition to magnetic order. We identify new mechanisms for optical-absorption via magneto-elastic effects and spin- orbit coupling. In addition, for the Dirac spin-liquid scenario, we establish an explicit microscopic origin for previously proposed absorption mechanisms based on slave-particle effective field theory descriptions.