Anomalous thermopower in the organic compound $\theta \text{−}{\left(\text{BEDT-TTF}\right)}_2\text{Rb}M{\left(\text{SCN}\right)}_4$ $\left(M=\text{Zn},\text{Co}\right)$

We have investigated the thermopower in the organic compound $\theta \text{−}{\left(\text{BEDT-TTF}\right)}_2\text{Rb}M{\left(\text{SCN}\right)}_4$ $\left(M=\text{Zn},\text{Co}\right)$ in both its normal and rapidly cooled states. The anomalous temperature dependence of the thermopower is found to extend into the rapidly cooled state and reveals that the charge carriers exhibit both localized and itinerant behaviors. We propose a simple model that captures this feature on the basis of an extended Hubbard model.

Figure 1

(Color online) Arrangement of the ET molecules along the $b$ axis. The dotted rectangle represents one unit cell of the I222 crystal structure (Ref. 9).

Figure 2

(Color online) (Top) Temperature dependence of the $c$ axis resistivity, $\rho$ of $\theta \text{-RbZn}$ and $\theta \text{-RbCo}$. When slowly cooled, both organic compounds go through a first-order phase transition at $T_c\approx 195\text{K}$, where $\rho$ increases abruptly. (Bottom) The temperature dependence of the $c$ axis thermopower $S_b$ of $\theta \text{-RbZn}$ and $\theta \text{-RbCo}$. The values of $\rho$ and $S_b$ below 195 K are related to the quenched high-temperature state of $\theta \text{-RbZn}$. The sample was measured between 130 and 50 K in both cooling and warming stage. The quenching temperature is from 210 to 130 K where the metastable state is considered stable during several hours (see Fig. 3). Dotted line is the linear fit of the thermopower data. The green (solid) and magenta (dash-dotted) lines are theoretical fittings (see text). (Bottom inset) Doping dependence of Eq. (5).

Figure 3

(Color online) Time dependence of the $c$ axis resistivity (top) and $S_b$ (bottom) of $\theta \text{-RbZn}$ at 145 K during the spinodal decomposition from the $q_1^{\prime }$ state to the $q_2$ state. The $60\mu \text{V}/\text{K}$ dashed red line is the value of the Heikes formula for localized charges carriers with full spin degeneracy.

Figure 4

(Color online) Example of a filling pattern of NB charges. The number represents the order we randomly put the NB charges (full green sites) starting from the top-left corner and going down each vertical line from left to right. The striped blue sites are forbidden sites for NB charges due to nearest-neighbor Coulomb repulsion and the filling pattern.