Evidence for a quantum dipole liquid state in an organic quasi–two-dimensional material

Abstract

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.

Raman Scattering as a Probe of the Magnetic State of BEDT-TTF Based Mott Insulators

“…The reason for this difference is an "active" intra-dimer degree of freedom. Indeed, in contrast to the compounds discussed in the previous section, κ-(BEDT-TTF) 2 Hg(SCN) 2 Br shows an evidence of fluctuating intra-dimer charge detected by vibrational spectroscopy [17]. Respectively, a different collective excitation with a maximum at around 40 cm −1 is found in the spectra of this compound.…”

“…In this paper, we would like to give an overview of our results on the Raman spectra of magnetic excitations which we observed in a number of κ-phase organic conductors. They demonstrate a striking difference between materials which can be described by a model of a Mott insulator on a frustrated square or triangular lattices with S = 1/2 per lattice site, and a more complicated situation, when the intra-dimer charge degree of freedom becomes active, resulting in a quantum dipole liquid [17]. In Section 4 we will compare our results to the existing literature data on other organic spin liquid candidates.…”

“…The number of spin liquid candidates [5][6][7][8][9][10][11][12] with S = 1/2 on a triangular lattice found among these materials is larger than among inorganic compounds. Moreover, intriguing ferroelectric properties of spin liquid candidates [13][14][15][16], and recently a quantum dipole liquid state [17] were observed for the κ-phase BEDT-TTF-based compounds, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. These results suggest an importance of a charge degree of freedom in the Mott insulator state.…”

“…With high values of electronic correlations U/t, where U is on-cite repulsion and t is transfer integral, these compounds are Mott insulators, and their properties are described by the Hubbard model on 2D square or triangular lattice at half-filling [4,22]. The latter model ignores a possibility of an active intra-dimer degree of freedom, which recently has been found important in some cases [17,18,20]. In a limiting case, the presence of the intra-dimer charge degree of freedom breaks the half-filled model, and a system has to be regarded as 1/4-filled [19].…”

Strongly Correlated Aromatic Molecular Conductor

“…Thus, we further study the evolution of the distribution of charge on the lattice of these systems around the MIT. The charge distribution is probed by measuring the frequency of the central CC molecular bond vibration, which changes by about 50 cm −1 after cooling form 300 to 120 K (Figure c) . This is a result of a lengthening of the central CC bond of the molecule, in which the charge occupies the highest occupied molecular orbital as shown in the theoretical calculation (Figure S6, Supporting Information).…”