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

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 $175{\mathrm{cm}}^{-1}$, comparable to the energy of the magnetic coupling $J\approx 250$ K. For $\omega \to 0$ 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.