  Novel states of matter in quantum magnets like quantum spin liquids attract
considerable interest recently. Despite the existence of a plenty of candidate
materials, there is no confirmed quantum spin liquid, largely due to the lack
of proper experimental probes. For instance, spectrosocopy experiments like
neutron scattering receive contributions from disorder-induced local modes,
while thermal transport experiments receive contributions from phonons. Here we
propose a thermo-optic experiment which directly probes the mobile magnetic
excitations in spatial-inversion symmetric and/or time-reversal symmetric Mott
insulators: the temperature-gradient-induced nonreciprocal directional
dichroism (TNDD) spectroscopy. Unlike traditional probes, TNDD directly detects
mobile magnetic excitations and decouples from phonons and local magnetic
modes.
