Metal-insulator transition in the dimerized organic conductor $\kappa -\left(\text{BEDT-TTF}{)}_2\mathrm{Hg}\right(\mathrm{SCN}{)}_2\mathrm{Br}$

The organic charge-transfer salt $\kappa -\left(\text{BEDT-TTF}{)}_2\mathrm{Hg}\right(\mathrm{SCN}{)}_2\mathrm{Br}$ is a quasi-two-dimensional metal with a half-filled conduction band at ambient conditions. When cooled below $T=80\mathrm{K}$, it undergoes a pronounced transition to an insulating phase where the resistivity increases many orders of magnitude. In order to elucidate the nature of this metal-insulator transition, we have performed comprehensive transport, dielectric and optical investigations. The findings are compared with other dimerized $\kappa -\left(\text{BEDT-TTF}\right)$ salts, in particular the Cl analog, where a charge-order transition takes place at $T_{\mathrm{CO}}=30\mathrm{K}$.

Figure 1

Temperature dependence of the normalized dc resistivity of $\kappa$-(BEDT-TTF)${}_2\mathrm{Hg}$(SCN)${}_2\mathrm{Br}$ measured within the highly conducting $\left(bc\right)$ plane. By cooling below $T_{\mathrm{MI}}=80\mathrm{K}$ a metal to insulator transition occurs. For comparison, the normalized dc resistivity of sibling compound $\kappa$-(BEDT-TTF)${}_2\mathrm{Hg}$(SCN)${}_2\mathrm{Cl}$ is plotted as dashed line; due to charge ordering the system becomes insulating below $T_{\mathrm{CO}}=30\mathrm{K}$.

Figure 2

(a) Sketch of the bis-(ethylenedithio)tetrathiafulvalene molecule, called BEDT-TTF. (b) The view on the BEDT-TTF layers along the out-of-plane $a$ direction illustrates the dimer structure (dimers are denoted by shaded circles). (c) The molecular layers along the conducting $bc$ plane are separated by the Hg(${\mathrm{SCN})}_2{\mathrm{Br}}^{-}$ anions. Note that within a dimer the two molecules are displaced along the molecular axis, leading to a reduced transfer integral $t_d$.

Figure 3

(a) Electrical resistivity of $\kappa$-(BEDT-TTF)${}_2\mathrm{Hg}$(SCN)${}_2\mathrm{Br}$ as a function of inverse temperature measured within the $\left(bc\right)$ plane and perpendicular to the plane, i.e., $E\parallel a$. (b) The phase transition around $T_{\mathrm{MI}}\approx 80\mathrm{K}$ is identified as the maximum in the derivative.

Figure 4

(a) Dielectric strength $\mathrm{\Delta }\varepsilon$, (b) distribution width of relaxation times $1-\alpha$, and (c) mean relaxation time ${\tau }_0$ of $\kappa$-(BEDT-TTF)${}_2\mathrm{Hg}$(SCN)${}_2\mathrm{Br}$ as a function of inverse temperature measured perpendicular to the $\left(bc\right)$ plane. (Inset) Real and imaginary parts of the measured dielectric response (points) and fit (lines, see text) at a select temperature.

Figure 5

Real part of the dielectric constant $\varepsilon \left(T\right)$ of $\kappa$-(BEDT-TTF)${}_2\mathrm{Hg}$(SCN)${}_2\mathrm{Br}$ determined (left panel) within the $\left(bc\right)$ plane and (right panel) for $E\parallel a$ (perpendicular to the plane) at various frequencies from 1 kHz up to 1 MHz as indicated. Note the logarithmic scale.

Figure 6

Temperature dependence of the absorption feature related to the molecular vibration ${\nu }_{27}\left({\mathrm{b}}_{1u}\right)$, which is seen as a local probe of the charge per BEDT-TTF molecule. In the upper panel, a clear splitting is observed in $\kappa$-(BEDT-TTF)${}_2\mathrm{Hg}$(SCN)${}_2\mathrm{Cl}$ as the temperature is reduced below the charge order transition $T_{\mathrm{CO}}=34\mathrm{K}$. The inset displays the peak frequency as a function of temperature. The lower panel shows the ${\nu }_{27}$ vibrational mode of $\kappa$-(BEDT-TTF)${}_2\mathrm{Hg}$(SCN)${}_2\mathrm{Br}$ for different temperature in a waterfall plot, i.e., the curves are shifted with respect to each other. Except of a gradual development of a side peak, no splitting due to charge order is found.

Figure 7

Optical reflectivity of $\kappa$-(BEDT-TTF)${}_2\mathrm{Hg}$(SCN)${}_2\mathrm{Br}$ measured at different temperatures as indicated (a) for light polarized along the $c$ direction and (c) for $E\parallel b$. [(b) and (d)] The corresponding optical conductivity as obtained by Kramers-Kronig analysis for the two different orientations. The insets show the low-frequency conductivity ${\sigma }_1\left(\omega \right)$ on an enlarged scale.