Melting of charge order in the low-temperature state of an electronic ferroelectric-like system

Abstract

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...

Molecular quantum materials: electronic phases and charge dynamics in two-dimensional organic solids

“…The problem might be due to melting of charge order at low temperatures. Results from a recent Raman scattering study suggest that the charge-ordered state formed at T CO = 30 K is not the ground state; rather it persists only down to T = 15 K and gradually melts below [287]. In Figure 41 the temperature evolution of the Raman spectra is plotted in the frequency range of the charge-sensitive ν 2 (a g ) stretching vibration that involves the central C=C bond of the BEDT-TTF molecules.…”

“…In the metallic state at T = 45 K BEDT-TTF 0.5+ band is observed, while in the charged ordered state at 20 K two bands BEDT-TTF 0.4+ and BEDT-TTF 0.6+ are clearly resolved. On further cooling at T = 10 K and 4 K signatures of charge order melting are detected: the two bands BEDT-TTF 0.4+ and BEDT-TTF 0.6+ broaden and a BEDT-TTF 0.5+ band emerges (after [287]).…”

Absence of Superconductivity in the Hubbard Dimer Model for κ-(BEDT-TTF)2X

“…What is perhaps very relevant in this context is that transition from CO to SC is far more common in CT solids, including in the β, β , β and θ-families [14]. Even among the κ-phase materials, CO has been detected in X = Hg(SCN) 2 Cl [47][48][49] while in the related material X = Hg(SCN) 2 Br there occurs a dipolar liquid phase [50] the formation of which is related to the mechanism of CO formation [51,52]. While these latter materials are also strongly correlated, CO requires an effective 1 4 -filled (or 3 4 -filled) description of CT solids, within which intra-dimer charge dimer degrees of freedom are explicitly taken into account [14,[53][54][55][56].…”

Absence of superconductivity in the Hubbard dimer model for kappa-(BEDT-TTF)_2X

“…What is perhaps very relevant in this context that transition from CO to SC is far more common in CT solids, including in the β, β , β and θ-families [14]. Even among the κ-phase materials, CO has been detected in X = Hg(SCN) 2 Cl [45][46][47] while in the related material X = Hg(SCN) 2 Br there occurs a dipolar liquid phase [48] the formation of which is related to the mechanism of CO formation [49,50]. While these latter materials are also strongly correlated, CO requires an an effective 1 4 -filled (or 3 4 -filled description of CT solids, within which intra-dimer charge dimer degrees of freedom are explicitly taken into account [14,[51][52][53][54].…”