Abstract: 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 show how to use Raman spectroscopy as a tool to distinguish Mott insulators with an on-site charge degree of freedom from simple dimer Mott insulators. Raman spectra of the latter show two-magnon excitations expected for a S=1/2 antiferromagnet on a triangular lattice [1]. We demonstrate a presence of the charge degree of freedom in molecule-based Mott insulators k-(BEDT-TTF)2Hg(SCN)2X (X=Br,Cl). In k-(BEDT-TTF)2Hg(SCN)2Br, when electrons localize on a triangular lattice of molecular dimers 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 1.3 THz is detected experimentally in our Raman spectroscopy experiments through an observation of a related collective mode [2]. In k-(BEDT-TTF)2Hg(SCN)2Cl, these electric dipoles order below 30 K, however we demonstrate that at lower temperatures this charge order is melting, and the material shows paramagnetic behaviour suggestive of a spin liquid.

References

[1] N. Hassan rt al. Crystals (2018)

[2] N. Hassan et al. Science (2018)