Thermoelectric transport properties of the quasi-one-dimensional dimer-Mott insulator ${\beta }^{\prime }{\text{-(BEDT-TTF)}}_2{\mathrm{ICl}}_2$

Low-dimensional materials, in which the electronic and transport properties are drastically modified in comparison to those of three-dimensional bulk materials, yield a key class of thermoelectric materials with high conversion efficiency. Among such materials, the organic compounds may serve peculiar properties owing to their unique molecular-based low-dimensional structures with highly anisotropic molecular orbitals. Here we present the thermoelectric transport properties of the quasi-one-dimensional dimer-Mott insulator ${\beta }^{\prime }$-(BEDT-${\mathrm{TTF})}_2{\mathrm{ICl}}_2$, where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. We find that the thermopower exhibits typical activation-type temperature variation expected for insulators but its absolute value is anomalously large compared to the expected value from the activation-type temperature dependence of the electrical resistivity. Successively, the Jonker-plot analysis, in which the thermopower is usually scaled by the logarithm of the resistivity, shows an unusual relation among such transport quantities. We discuss a role of the low dimensionality for the enhanced thermopower along with recent observations of such a large thermopower in several low-dimensional materials.

Figure 1

(a) The crystal structure of ${\beta }^{\prime }$-(${\mathrm{ET})}_2{\mathrm{ICl}}_2$ drawn by VESTA [41]. Hydrogen atoms are not drawn for clarity. (b) The highest occupied molecular orbital of the dimerized ET molecules in ${\beta }^{\prime }$-(${\mathrm{ET})}_2{\mathrm{ICl}}_2$. (c) ET molecule arrangement viewed along the long axis of ET molecules. The dotted ovals represent dimerized ET molecules. (d) Photograph of the ${\beta }^{\prime }$-(${\mathrm{ET})}_2{\mathrm{ICl}}_2$ single crystal for the transport measurement.

Figure 2

(a) Temperature dependence of the resistivity of ${\beta }^{\prime }$-(${\mathrm{ET})}_2{\mathrm{ICl}}_2$ single crystals. The inset shows the temperature dependence of the resistivity normalized by the value at $T=300\mathrm{K}$. (b) Arrhenius plot for the resistivity. (c), (d) 1D and 2D VRH plots for the resistivity.

Figure 3

(a) Temperature dependence of the thermopower. (b) Thermopower as a function of the inverse temperature. (c) Comparison of the activation energies, ${\mathrm{\Delta }}_S/2$ and ${\mathrm{\Delta }}_{\rho }/2$.

Figure 4

The Jonker plot for the measured samples of ${\beta }^{\prime }$-(${\mathrm{ET})}_2{\mathrm{ICl}}_2$. The thermopower is plotted as a function of $\sigma T^3$ to adapt the temperature-dependent mobility.

Figure 5

The Jonker plot for various low-dimensional materials. In ${\beta }^{\prime }$-(${\mathrm{ET})}_2{\mathrm{ICl}}_2$, the thermopower is shown as a function of the normalized $\sigma T^3$. For ${\beta }^{\prime }$-(${\mathrm{ET})}_2{\mathrm{AuCl}}_2$ [24, 25], $\alpha$-(${\mathrm{ET})}_2{\mathrm{I}}_3$ [26], and ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ [56], the horizontal axis represents the normalized conductivity $\sigma /{\sigma }_0$. For the cuprate superconductors [57], ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ [58], and ZnO [59], the horizontal axis represents the normalized carrier concentration $n/n_0$. The normalization factor is the horizontal intersect of the extrapolated slope line for each material.