Thermal conductivity of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) near the metal-insulator transition

The thermal conductivity of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) has been determined through a simultaneous measurement of the specific heat and thermal diffusivity using an ac method. A steplike anomaly has been observed which coincides in temperature with anomalies in the specific heat and electrical conductivity. Interpreting ths step to be due to the loss of the electronic contribution to the thermal conductivity at the metal-insulator transition, we show that the Lorentz number is very close to the free-electron value for conductivity enhancement factors in the range 10-30. A mean-field calculation of the thermal current due to paraconductive fluctuations demonstrates that this contribution is negligible. The conclusion follows, assuming the free-electron Lorentz number to be appropriate for this material, that all of the observed electrical conductivity is the result of normal heat-carrying electrons.