Frequency-dependent dielectric constant and conductivity of tetrathiafulvalene tetracyanoquinodimethane and the phonon-drag theory

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Abstract
The frequency-dependent conductivity and dielectric constant of tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ) were obtained from the ir reflectivity by Jacobsen and recently by Tanner et al. The data cannot be fitted by a single-particle theory. The phonon-drag theory is applied to calculate these constants, with the use of parameters derived from the dc electrical conductivity following the "libron" theory. This theory accounts in a natural way for the large negative dielectric constant observed in the far ir. In order to obtain agreement with the ir data and microwave measurements of the dielectric constant by the Tanner group, the "broken-strand" model must be used, in which chains near the surface are assumed to be damaged. This model is extended with the use of an effective-medium theory. When the phonon-drag theory and the broken-strand model are used, the conductivity and dielectric constant are calculated as a function of frequency and temperature, and excellent agreement with experiment is obtained.