Observation of a Quantum Dipole Liquid State in an Organic Quasi-Two-Dimensional Material

We demonstrated experimentally using inelastic (Raman) scattering technique that new charge degrees of freedom emerge in a molecule-based Mott insulator $\kappa$-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Br resulting in quantum dipole liquid state. Electric dipoles localized on molecular dimer lattice sites do not order at low temperatures and fluctuate with frequency of about 40 $cm^{-1}$. Such fluctuations were detected by analyzing the line shape of charge-sensitive $\nu_2$ vibration of BEDT-TTF molecule, and by a direct observation of a collective mode related to dipole fluctuations at 40 $cm^{-1}$. The shape of $\nu_2$ vibrational band was modeled using the two-site jump model to calculate the fluctuation frequency. The low energy collective mode associated with those fluctuations shows temperature dependence that is in agreement with vibrational band behavior. The heat capacity shows a low-temperature linear term $\gamma\approx$ 13 mJ K$^{-2}$mole$^{-1}$. Heat capacity and Raman scattering response support a scenario where the composite spin and electric dipole degrees of freedom remain fluctuating down to the lowest temperatures. Such Fluctuations of electric dipoles coupled to S=1/2 spins on a triangular lattice of dimers have been suggested as a mechanism for spin-liquid behaviour.