Magnetodielectric effects and spin-charge coupling in the spin-liquid candidateκ-(BEDT-TTF)2Cu2(CN)3

Abstract

Microwave measurements of the in-plane dielectric function of the spin-liquid candidate $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(CN)${}_{3}$ revealed anomalies below 300 K that indicate that charge and spin degrees of freedom are correlated down to 1.8 K. If the first anomaly around 100 K can be explained partly by a Debye relaxation model, it signals also the approach of an inhomogeneous high-temperature quantum critical phase (QC${}_{H}$) extending down to 6 K, where a second anomaly is observed at the crossover to the intermediate quantum critical phase (QC${}_{M}$) within which a third anomaly is detected near 3--4 K. The low-temperature anomalies are not only dependent on microwave frequency and power, but they are also strongly modified in a highly anisotropic way by a magnetic field. These dielectric results confirm that a scenario of coupled spin and charge degrees of freedom is indeed valid in this material at low temperatures, as suggested by several theoretical approaches.