Measurements of the thermopower of the organic conductor tetrathiofulvalinium tetracyanoquinodimethane have been made in the region 10-300°K. At high temperatures ($T>\mathrm{}140\mathrm{}°$K) the samples are metallic showing increased electron correlations as the temperature is lowered to about 56°K. At this point the thermopower is zero and a discontinuity marks the transition to the insulating state.

• Received 11 June 1973

DOI:https://doi.org/10.1103/PhysRevLett.31.601