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

TL;DR

This study investigates the anisotropic charge dynamics in the quantum spin-liquid candidate $ppa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, revealing relaxor-like ferroelectric behavior and complex temperature-dependent conductivity and dielectric properties.

Contribution

It provides a detailed characterization of the anisotropic charge response, highlighting the interplay between dielectric relaxation, conductivity, and potential ferroelectricity in this material.

Findings

In-plane dc conductivity follows variable-range hopping; out-of-plane does not.

Detected broad in-plane dielectric relaxation below 60 K, with a pronounced anomaly at 17 K.

Dielectric behavior suggests relaxor-like ferroelectricity with heterogeneity-induced short-range order.

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.