Electronic transport inCs2(TCNQ)3

Abstract

Transport measurements are reported for ${\mathrm{Cs}}_2$ ${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_3$ single crystals, conductivity from 100 to 500 K and thermopower from 200 to 400 K, both for the preferentially conducting $\overrightarrow{\mathrm{b}}$ axis and for directions in the (010) plane. Results for $T>\mathrm{}300\mathrm{}$ K indicate intrinsic behavior, with ${\epsilon }_i=(0.715-5\mathrm{}\times {10}^{-4\mathrm{}}T)$ eV, and with ${\mu }_n=1.9\mathrm{}{\mu }_p=0.65\mathrm{}$ ${\mathrm{cm}}^2$/V sec for $\overrightarrow{\mathrm{b}}$-axis conduction. At high temperatures, the thermopower is positive for direction $\overrightarrow{\mathrm{a}}$, but negative for directions $\overrightarrow{\mathrm{b}}$ and ${\overrightarrow{\mathrm{c}}}^{*}$. Below room temperature, the thermopower becomes large and positive for all directions ($\Theta >+1000\mathrm{}$ μV/K for $T<\mathrm{}250\mathrm{}$ K), with significant intersample differences. The conductivity is similarly sample dependent below 300 K, with a slight reduction of activation energy. An analysis of $\overrightarrow{\mathrm{b}}$-axis conduction and thermopower shows that this can be accounted for with a narrow-band model (using Fermi-Dirac one-electron statistics and one-dimensional density of states factors) with low-temperature behavior controlled by ∼${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ of deep-level acceptors (${\epsilon }_a=0.303\mathrm{}$ eV), and partial compensation that varies from sample to sample. Transport in directions normal to [010] is expected to have temperature dependences reflective of the statistics of the $\overrightarrow{\mathrm{b}}$ axis, even though band motion is not valid in these directions.