Low-Temperature Charge Transport in Tetrathiafulvalene Tetracyanoquinodimethane (TTF-TCNQ) near Commensurability

At low pressure and at a commensurability of 3 (at ∼19 kbar) the low-temperature thermopower of TTF-TCNQ is large, negative, and varying $\sim \frac{1}{T}$. At 17 kbar it is smaller, positive, and independent of temperature. At 22 kbar it is similar to that at 17 kbar but of opposite sign. For $P\lesssim 19$ kbar pulling of the distortion wave vector from $2k_{\mathrm{F}}$ to a commensurate value ($\frac{b^{*}}{3}$) leaves the TCNQ valence band underfilled and TTF valence band overfilled. For $P\gtrsim 19$ kbar the reverse holds. We suggest that the excess carriers are charged solitons or polarons.