Tetrathiafulvalene Tetracyanoquinodimethane (TTF-TCNQ): A Zero-Bandgap Semiconductor?

We present numerical calculations based on a tight-binding model for the organic conductor tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ). The model consists of anisotropic planar TTF and TCNQ bands coupled together by an interplanar transfer integral, which produces a nearly zero-bandgap semiconductor. The calculations are compared with experiments and it is found unlikely that all of the data can be explained simultaneously by a noninteracting electron band structure. We then discuss how many-body effects may affect the model and lead to a metal-insulator transition as observed.