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 β′-EtMe3\-Sb\-[Pd(dmit)2]2, we discover enhanced conductivity below 175cm−1, comparable to the energy of the magnetic coupling J≈250 K. For ω→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.
@article{arxiv.1803.01553,
title = {Low-Energy Excitations in Quantum Spin-Liquids Identified by Optical Spectroscopy},
author = {A. Pustogow and Y. Saito and E. Zhukova and B. Gorshunov and R. Kato and T. -H. Lee and S. Fratini and V. Dobrosavljević and M. Dressel},
journal= {arXiv preprint arXiv:1803.01553},
year = {2018}
}