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

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 ss'-EtMe3Sb[Pd(dmit)(2)](2), we discover enhanced conductivity below 175 cm(-1), comparable to the energy of the magnetic coupling J approximate to 250 K. For omega -> 0, these low-frequency excitations vanish faster than the charge-carrier response subject to Mott-Hubbard correlations, resulting in a dome-shaped band peaked at 100 cm(-1). Possible relations to spinons, magnons, and disorder are discussed.