Anomalous dielectric response in the dimer Mott insulator $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2{\text{Cu}}_2{\left(\text{CN}\right)}_3$

We have measured and analyzed the dielectric constant of the dimer Mott insulator $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2{\text{Cu}}_2{\left(\text{CN}\right)}_3$, which is known as a playground for a spin-liquid state. Most unexpectedly, this particular organic salt has nontrivial charge degrees of freedom, being characterized by a relaxor-like dielectric relaxation below around 60 K. This is ascribed to the charge disproportionation within the dimer due to the intersite Coulomb repulsion. A possible microscopic model is suggested and discussed.

Figure 1

(Color online) (a) Schematic of the BEDT-TTF layer in $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2{\text{Cu}}_2{\left(\text{CN}\right)}_3$. The dotted ellipsoids represent dimerized molecules. (b) A triangular lattice of dimers, where closed circles represent identified with the dimerized molecules. (c) A triangular lattice of magnetic dipoles (spins). (d) A triangular lattice of electric dipoles, where open and closed circles represent positive and negative point charges, respectively. This is identical to Fig. 5f.

Figure 2

(Color online) (a) The dielectric constant of a single crystal of $\kappa \text{−}{\left(\text{BEDT-TTF}\right)}_2{\text{Cu}}_2{\left(\text{CN}\right)}_3$ along the $a$ axis (cross-plane) direction at various frequencies as a function of temperature. The randomly oriented electric dipoles appear below 60 K. The dotted curve indicates the peak temperature $T_{\text{max}}$. (b) The ac electrical conductivity of the same crystal.

Figure 3

(Color online) The measurement frequency $f$ plotted as a function of $1/\left(T_{\text{max}}-T_c\right)$. $T_{\text{max}}$ is the peak temperature at which the dielectric constant goes through a broad maximum, and $T_c$ is assumed to be 6 K. The solid line corresponds to the fitting curve, where $f_0$ and $E_g/k_B$ are evaluated to be $2.5\times {10}^8\text{Hz}$ and 250 K, respectively.

Figure 4

(Color online) (a) The inverse dielectric constant plotted as a function of temperature. The temperature-independent part (the dielectric constant at 1.2 K) has been subtracted. The dotted line indicates that the dielectric constant obeys the Curie-Weiss law with a Curie temperature $T_c=6\text{K}$. (b) The low-temperature part of the dielectric constant. A frequency-independent cusp is observed near $T_c$. The inset shows the electric displacement $D$ plotted as a function of external electric field $E$.

Figure 5

(Color online) (a) Schematic of a quantum electric dipole. [(b)–(e)] Schematics of two neighboring quantum electric dipoles. The arrows represent the electric dipoles. (b) and (c) represent the cases when two electrons are come close to one another, whereas (d) and (e) represent the cases when the two electrons are far apart. [(f) and (g)] Possible short-range domains of the electric dipoles fluctuating collectively.