Low-Energy Excitations in Quantum Spin-Liquids Identified by Optical Spectroscopy

TL;DR

This study uses optical spectroscopy to identify low-energy excitations in organic quantum spin liquids, revealing in-gap states linked to the quantum spin liquid phase and providing insights into their magnetic and electronic properties.

Contribution

The paper reports the first observation of distinct in-gap excitations in organic spin liquids via optical spectroscopy, connecting these features to the quantum spin liquid state.

Findings

Enhanced low-frequency conductivity below 175 cm^{-1} in the spin liquid.

In-gap excitations vanish faster than charge response at low temperatures.

Dome-shaped spectral feature peaked at 100 cm^{-1}.

Abstract

The electrodynamic response of organic spin liquids with highly-frustrated triangular lattices has been measured in a wide energy range. While the overall optical spectra of these Mott insulators are governed by transitions between the Hubbard bands, distinct in-gap excitations can be identified at low temperatures and frequencies which we attribute to the quantum spin liquid state. For the strongly correlated $\beta^{\prime}$-EtMe$_3$\-Sb\-[Pd(dmit)$_2$]$_2$, we discover enhanced conductivity below $175~{\rm cm}^{-1}$, comparable to the energy of the magnetic coupling $J\approx 250$ K. For $\omega\rightarrow 0$ these low-frequency excitations vanish faster than the charge-carrier response subject to Mott-Hubbard correlations, resulting in a dome-shape band peaked at 100~\cm. Possible relations to spinons, magnons and disorder are discussed.