We have in detail characterized the anisotropic charge response of the dimer Mott insulator κ-(BEDT-TTF)2\-Cu2(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 ρ(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.
@article{arxiv.1407.6252,
title = {Anisotropic charge dynamics in the quantum spin-liquid candidate $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$},
author = {M. Pinteric and M. Culo and O. Milat and M. Basletic and B. Korin-Hamzic and E. Tafra and A. Hamzic and T. Ivek and T. Peterseim and K. Miyagawa and K. Kanoda and J. A. Schlueter and M. Dressel and S. Tomic},
journal= {arXiv preprint arXiv:1407.6252},
year = {2015}
}