Metallic state and charge-order metal-insulator transition in the quasi-two-dimensional conductor $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl

We present a study of optical and dc properties of a highly frustrated organic conductor $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl in the 300–10 K temperature range. At temperatures above 30 K, the material shows properties of a half-filled metal with strong electron-electron correlations. At 30 K, the compound undergoes a metal-insulator transition which we identify as a charge-ordering transition. We find that properties of $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl are well explained by a model of a paired electron crystal.

Figure 1

Crystal structure of $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl at 100 K. Unit cell borders are marked with a dashed line. (a) Packing diagram illustrating the layered structure along the $a$ axis; (b) Cation BEDT-TTF conducting layer. Dimers [BEDT-TT]${}_2^{+}$ are marked by circles. Red lines indicate intermolecular $S...S$ interactions less than 3.6 Å. (c) Polymeric anion layer Hg(SCN)${}_2$Cl.

Figure 2

Reflectivity (a), (c) and conductivity (b), (d) spectra of the conducting plane of $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl ($E\parallel c$ left panel, $E\parallel b$ right panel) at temperatures between 300 and 20 K. The inset in (a) shows dc resistivity of the $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl measured in the conducting plane. The insets in lower panels (b) and (d) show the low-frequency conductivity spectra of the metallic state, where the Drude peak is observed at temperatures between 150 and 40 K.

Figure 3

Comparison of the room temperature conductivity spectra of $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl, $E\parallel b$ (red line) and $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br${}_{0.9}$Cl${}_{0.1}$, $E\parallel a$ (black line). The comparison demonstrates that the spectra of the studied compounds are typical for the $\kappa$ phase.

Figure 4

Temperature dependence of the Drude (a) scattering rate $\Gamma$ and (b) plasma frequency ${\Omega }_p$ determined from a Drude-Lorentz fit of the reflectivity and conductivity spectra of $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl. A clear distinction between a high-temperature regime above 150 K, with a weak overdumped Drude response and a Drude-type coherent carriers response below 150 K is observed. Panel (c) shows a resistivity $\rho /{\rho }_0\left(T^2\right)$ dependence. The region between 50 and 170 K follows the $T$${}^2$ behavior, typical for the Fermi liquid.

Figure 5

The figure presents (a) optical conductivity spectra of $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl in $E\parallel b$ polarization at temperatures between 300 and 50 K in the low-frequency region; (b) frequency-dependent scattering rate ${\Gamma }_1\left(\omega \right)$ at temperatures between 100 and 50 K; and (c) frequency-dependent effective mass of charge carriers $m$${}^{*}$/$m$${}_b$.

Figure 6

Conductivity spectra of $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl above (50 K) and below (20 K) the metal-insulator transition for $E\parallel b$ (upper panel) and $E\parallel c$ (lower panel). The insets show the corresponding reflectivity spectra.

Figure 7

Temperature dependence of the conductivity spectra of $\kappa$-(BEDT-TTF)${}_2$Hg(SCN)${}_2$Cl perpendicular to the conducting plane in the region of $B$${}_{1u}$(${\nu }_{27}$) mode. There is a single mode above 30 K which splits into two components below this temperature. The temperature behavior of the frequency of this vibration is shown in the inset.

Figure 8

Possible charge-order patterns shown. (a) Shows anisotropy in agreement with the anisotropy of the conductivity spectra (P2/c symmetry of the unit cell); (b) shows anisotropy reversed to that of the conductivity spectra (P2${}_1$/c symmetry of the unit cell); (c), (d) show a doubling of the unit cell and anisotropy lower than observed in the conductivity spectra.