Signatures of quantum dipole liquid in an organic Mott insulator κ-(BEDT-TTF)Hg(SCN)₂Br

Mott insulators are commonly pictured with electrons localized on lattice sites. Their low-energy physics involves spins only. Recent theoretical work suggests that in molecular systems a new on-site charge degree of freedom can emerge. On a frustrated lattice with charge-spin coupling it would result in a new quantum spin liquid state. We experimentally demonstrate [1] a presence of this fluctuating charge degree of freedom in a molecule-based Mott insulator κ-(BEDT-TTF)₂Hg(SCN)₂Br. When electrons localize on a triangular lattice of molecular dimers of this compound at temperatures below 100 K, they form electric dipoles which do not order at low temperatures and fluctuate, resulting in a so-called quantum dipole liquid state. A frequency of dipole fluctuations of 40 cm^-1 is detected experimentally in our Raman spectroscopy experiments through an observation of a related collective mode. We show that this spectroscopic response of a quantum dipole liquid is qualitatively different from a response of molecular Mott insulators with no on-site charge degree of freedom. The Raman spectra of the latter show two-magnon excitations at frequencies below 500 cm^-1 expected for a S=1/2 antiferromagnet on a triangular lattice with J≈250 K. Our results can be a key to understanding of organic triangular lattice spin liquid candidates.

References:
1. N. Hassan, S. Cunningham, M. Mourigal, E. I. Zhilyaeva, S. A. Torunova, R. N. Lyubovskaya, J. A. Schlueter, N. Drichko. Science, 360, 6393 (2018)