Microwave properties of high-purity tetrathiofulvalene-tetracyanoquinodimethan (TTF-TCNQ)

Abstract

An experimental study of the microwave properties of pure crystals of the organic salt tetrathiofulvalene-tetracyanoquinodimethan (TTF-TCNQ) is presented. Included are electron spin resonance and a complete study of the dielectric function, ${\ensuremath{\epsilon}}_{1}\ensuremath{-}i{\ensuremath{\epsilon}}_{2}$, from room temperature to 4.2 K as measured along the principal conducting $b$ axis and the transverse $a$ axis using the highest-purity material synthesized thus far. The spin-resonance line is asymmetric characteristic of a metal with skin depth less than the sample thickness when the rf magnetic field is perpendicular to the $b$ axis, and symmetric (Lorentzian) when the rf magnetic field is parallel to the $b$ axis. These results are understood in terms of the Dyson-Bloembergen theory of resonance line shape as applied to the pseudo-one-dimensional metal. The dielectric measurements ($E\ensuremath{\parallel}b$) are consistent with the skin-depth limiting behavior, and in the purest samples show negligible loss over a relatively wide temperature range ($50<T<100$ K) indicative of very high conductivity. Minimal attempts to reduce sample purity (no gradient sublimation and subsequent reaction in air) led to serious degradation of sample quality as evidenced by the appearance of significant loss throughout the entire metallic regime. The low-temperature dielectric constants appropriate to the $b$ axis (${\ensuremath{\epsilon}}_{1}^{b}$) and $a$ axis (${\ensuremath{\epsilon}}_{1}^{a}$) are found to be ${\ensuremath{\epsilon}}_{1}^{b}=(3.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{3}$ and ${\ensuremath{\epsilon}}_{1}^{a}=6\ifmmode\pm\else\textpm\fi{}2$. The value for ${\ensuremath{\epsilon}}_{1}^{b}$ is consistent with the known plasma frequency and energy gap. Studies of the temperature dependence of ${\ensuremath{\epsilon}}_{1}^{b}$ are used to obtain direct information on the energy gap in the low-temperature phase. The transport properties associated with the transverse ($a$ axis) direction indicate an electrical anisotropy of ${10}^{3}$ at room temperature increasing to about ${10}^{4}$ near 60 K, thus confirming the picture of TTF-TCNQ as a highly anisotropic pseudo-one-dimensional metal. The $a$-axis transport is shown to be diffusive and severely limited as a result of the weak interchain coupling and the phonon thermal disorder. Evidence of excess low-temperature microwave conductivity in the purest samples at low temperatures is presented and discussed in terms of current models of the ground state.