Observation of a Quantum Dipole Liquid State in Organic Mott Insulators

Mott insulators are commonly pictured with electrons localized on lattice sites, with their low-energy degrees of freedom involving spins only. Molecular Mott insulators allow to achieve electric dipoles on molecular lattice sites, resulting in the coupling of charge and magnetic degrees of freedom. We observe emergent charge degrees of freedom in a molecule-based triangular lattice Mott insulator κ-(BEDT-TTF)₂Hg(SCN)₂Br, resulting in a quantum dipole liquid state and exotic magnetic properties. Electrons localized on molecular (BEDT-TTF)₂ lattice sites form electric dipoles that do not order at low temperatures and fluctuate with frequency detected experimentally in our Raman spectroscopy experiments. These fluctuations prevent an ordering of spins at low temperatures, resulting in dynamic ferromagnetic-like response. A small modification of the crystal structure achieved in κ-(BEDT-TTF)₂Hg(SCN)₂Cl leads to an appearance of an ordered ferroelectric state in this material below 30 K, and a spin singlet formation at 25 K. We discuss a possible phase diagram to describe these states and suggest experimental control parameters allowing to tune the ground states of these systems.