Enhancement of the magnetic susceptibility of TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) by Coulomb correlations

In order to gain some insight into the nature of the interactions in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane), we compare the magnitude of the observed magnetic susceptibility χ with the predictions of two simple models. First we consider a tight-binding band of noninteracting electrons and find that the magnitude of the observed high-temperature χ is large compared to that calculated using the expected bandwidth. The same conclusion is also reached by a more general consideration of the magnitude of the product Tχ. It is concluded that at high temperatures χ is enhanced. We then compare the observed χ to calculations using a Hubbard model with on-site Coulomb repulsion energy U. χ is calculated in both the low-U and large-U limits and, in either limit, the large magnitude of χ can be readily accounted for. Using these two results, a relationship between the two parameters U and the bandwidth 4 t is obtained, knowing the magnitude of χ at 300 K. From independent estimates of 4 t, it is concluded that U is comparable or larger than 4 t, at least on one stack. Additional evidence for and against the presence of Coulomb interactions in TTF-TCNQ is summarized. © 1977 The American Physical Society.