Electrical conductivity of tetrathiofulvalinium tetracyanoquinodimethan (TTF) (TCNQ)

Abstract

The temperature dependence of the dc electrical conductivity of tetrathiofulvalinium tetracyanoquinodimethan (TTF)(TCNQ) measured along the crystallographic $a$, $b$, and $c^{*}$ axes and the corresponding anisotropies $\frac{{\sigma }_{\parallel }^b}{{\sigma }_{\perp }^a}$ and $\frac{{\sigma }_{\parallel }^b}{{\sigma }_{\perp }^{c^{*}}}$ are reported. Standard four-probe measurements are supplemented by applying the technique of Montgomery to the question of inhomogeneous currents in this anisotropic conductor. We have observed that the intrinsic anisotropy contains two maxima which provide a direct internal method for unambiguously testing the validity of four-probe measurements of the conductivity along the principal conducting $b$ axis. The transverse transport properties are diffusive and yield an electrical anisotropy greater than 500 at room temperature which increases to greater than ${10}^4$ near 58 K. The extreme sensitivity of this one-dimensional metal to crystalline imperfections is experimentally demonstrated through temperature cycling studies. The dc results confirm (TTF)(TCNQ) is a one-dimensional metal exhibiting a strongly temperature-dependent $b$-axis conductivity that reaches maximum values exceeding ${10}^5$ ${\left(\mathrm{\Omega }\mathrm{c}\mathrm{m}\right)}^{-}$. These data are taken as confirming evidence that above 58 K the metallic state exhibits strong electron correlations associated with a many-body collective state in which the conductivity can greatly exceed the limitations of single-particle scattering.