Mott insulators are commonly pictured with electrons localized on lattice sites, with their low-energy degrees of freedom involving spins only. Here, we observe emergent charge degrees of freedom in a molecule-based Mott insulator κ-(BEDT-TTF)Hg(SCN)Br, resulting in a quantum dipole liquid state. Electrons localized on molecular dimer lattice sites form electric dipoles that do not order at low temperatures and fluctuate with frequency detected experimentally in our Raman spectroscopy experiments. The heat capacity and Raman scattering response are consistent with a scenario in which the composite spin and electric dipole degrees of freedom remain fluctuating down to the lowest measured temperatures.

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Strong electronic interactions can drive a system into a state with a symmetry breaking. Lattice frustration or competing interactions tend to prevent a symmetry breaking, leading to quantum disordered phases. In spin systems frustration can produce a spin liquid state. Frustration of a charge degree of freedom also can result in various exotic states, however, experimental data on these effects is scarce. In this work we demonstrate how a charge ordered ferroelectric looses the order on cooling to low temperatures using an example of a Mott insulator on a weakly anisotropic triangular lattice κ-(BEDT-TTF) 2 Hg(SCN) 2 Cl. Typically, a low temperature ordered state is a ground state of a system, and the demonstrated re-entrant behavior is unique. Raman scattering spectroscopy finds that this material enters an insulating ferroelectric "dipole solid" state at T = 30 K, but below T = 15 K the order melts, while preserving the insulating energy gap. The resulting phase diagram is relevant to other quantum paraelectric materials.Frustration of a charge degree of freedom can result in charge glass [1,2] or a quantum paraelectric state, where electric dipoles fluctuate down to the lowest temperatures [3][4][5]. Such quantum dipole liquid was observed experimentally in a band insulator on a triangular lattice [4] and in a Mott insulator [6]. In a band insulator fluctuations of polarization are predicted to lead to a multiferroic effect [7]. In a Mott insulator charge-spin coupling is predicted to result in a spin liquid state [5,8]. An experimental realization of a system where electrical dipoles form on lattice sites of a Mott insulator at this point is limited to molecular-based systems [3,5,6]. However, exotic multiferroicity [7,9] which can result from an interplay of a quantum paraelectric and a spin liquid states is of interest to a broad community working on materials with strong electron-electron interactions. Also, notable is an analogy of a charge degree of freedom on the orbital of molecular dimer (BEDT-TTF) 2 to the orbital degree of freedom and orbital liquid in atomic crystals, and as a way to produce novel spin liquid states [8].Organic Mott insulators where electronic ferroelectricity and quantum dipole liquid are observed are layered charge-transfer crystals based on BEDT-TTF [10] molecule. Layers responsible for the interesting physical properties of these materials are formed by dimers (BEDT-TTF) +1 2 . They alternate with layers which serve as charge reservoirs and define the exact structural parameters of the BEDT-TTF layers. A compound discussed in this work κ-(BEDT-TTF) 2 Hg(SCN) 2 Cl (k-Hg-Cl) shows a structure where (BEDT-TTF) +1 2 sites form a slightly anisotropic triangular lattice within the layer. In this compound electronic ferroelectricity is observed in a charge ordered state below 30 K [11,12]. In this work we experimentally detect a gradual melting of this charge order as the material is cooled down below 15 K.Typically, if a system undergoes a phase transition into a broken symmetry sta...