Mott insulators are commonly pictured with electrons completely localized on lattice sites. Their low-energy physics involves spins only. In addition, it was shown theoretically that new charge degrees of freedom can emerge in molecule-based Mott insulators as electrons occupy extended molecular orbitals, and can result in a quantum dipole liquid state.
We probe few BEDT-TTF-based 2D Mott insulators using Raman scattering technique. We identify magnetic excitation in an antiferromagnetic compound κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl and a spin liquid candidate κ-(BEDT-TTF)₂Cu₂(CN)₃. We show that their spectrum of excitations is very different from that of a new triangular lattice Mott insulator κ-(BEDT-TTF)₂Hg(SCN)₂Br. Our data demonstrate an existence of quantum dipole liquid in this compound. Here, when in the Mott insulating state electrons localize on dimer (BEDT-TTF)₂ lattice sites, they form electric dipoles which do not order at low temperatures. We experimentally detect charge fluctuation with frequency of about 50 cm-¹ using Raman spectra of vibrations of BEDT-TTF molecule. In addition, in the low frequency Raman scattering response we detect a collective mode at about 50-¹ cm due to respective dipole fluctuations. Heat capacity of　κ-(BEDT-TTF)₂Hg(SCN)₂Br demonstrates a linear term at low temperatures, supporting a scenario where the composite spin and electric dipole degrees of freedom remain fluctuating down to the lowest temperatures.