Anisotropic charge dynamics in the quantum spin-liquid candidate $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$

Abstract

We have in detail characterized the anisotropic charge response of the dimer Mott insulator $\kappa$-(BEDT-TTF)${}_2$\-Cu${}_2$(CN)${}_3$ by dc conductivity, Hall effect and dielectric spectroscopy. At room temperature the Hall coefficient is positive and close to the value expected from stoichiometry; the temperature behavior follows the dc resistivity $\rho (T)$. Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also the relaxation levels off. The observed behavior bears features of relaxor-like ferroelectricity. Because heterogeneities impede its long-range development, only a weak tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.

arXiv comment14 pages, 13 figures

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• From charge- and spin-ordering to superconductivity in the organic charge-transfer solids
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• Anions effects on the electronic structure and electrodynamic properties of the Mott insulator $\kappa$-(BEDT-TTF)${}_2$Ag${}_2$(CN)${}_3$
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• Ingredients for Generalized Models of $\kappa$-Phase Organic Charge-Transfer Salts: A Review
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• Importance of van der Waals interactions and cation-anion coupling in an organic quantum spin liquid
• Resonant inelastic x-ray scattering probes the electron-phonon coupling in the spin-liquid kappa-(BEDT-TTF)2Cu2(CN)3
• Transition of a prestine Mott insulator to a correlated Fermi liquid: Pressure-dependent optical investigations of a quantum spin liquid
• Novel Dipole-Lattice coupling in the Quantum-Spin-Liquid Material $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$
• Charge glass in an extended dimer Hubbard model
• Charge-Transfer Complex $\kappa$-(BEST)${}_2$Cu${}_2$(CN)${}_3$ Analogous to Organic Spin Liquid Candidate
• Emergence of a Fluctuating Ground State in Y-kapellasite under Pressure