K Transition in Ttf — Tcnq Viewed

The d.c conductivity, static dielectric constant and thermopower of TTF—TCNQ have been calculated for temperatures below 54K within a self-consistent effective medium approximation. TTF—TCNQ is assumed to consist of small regions which are semiconducting and others which are highly conducting. The conducting regions have a large negative dielectric constant, a large d.c. conductivity and a negative thermopower; the semiconducting portions have a large positive dielectric constant, an activated d.c. conductivity and a large positive thermopower. The volume fraction of conductor increases from zero at T = 0 to unity at 54K. The model is the first to reproduce the observed metal—insulator transition (e.g. the d.c. conductivity anomaly) near 38 K. The dielectric constant is calculated to rise initially with temperature and then become negative as the temperature increases above 38 K. The thermopower changes sign below 54K and increases very rapidly at 38 K.