Abstract
We consider a model of TTF-TCNQ (tetrahiafulvalenium-tetracyanoquinodimethanide) and similar charge-transfer linear conductors in which there are two chains. The electrons on the acceptor (TCNQ) chains have a usual metallic band, but the electrons on the donor (TTF) chains have an inverted or "hole" band. The electrons on each chain interact via the forward and backward scattering and processes, respectively. Electrons on different chains interact via the analogous processes and . The model is also applicable to two metallic chains, with the signs of and reversed. Using the techniques of Luther and Emery and of Chui and Lee we show that the problem may be mapped onto a similar problem of three coupled classical two-dimensional Coulomb gases. In addition to gaps in the spin degrees of freedom that were present in the single-chain problem, we find that the antisymmetric form of the resultant charge degrees of freedom may have a gap. We then investigate the predominant two-, four-, six-, and eight-particle response functions and calculate their temperature dependence. In addition to the four divergent two-particle response functions present in the single chain, there is a region for weak interaction strengths in which the interchain Cooper pairing response is the most divergent as . The excitonic insulator response may also be important at high temperatures. For the single chain, either the response or a four-particle response is the most divergent for strong coupling and repulsive , depending upon the sign of . For the two-chain system, the most divergent response as for strongly attractive interactions ( and ) is the response to the formation of excitonic "molecules" or exciton pairs. For strong coupling and repulsive either or octet responses may be observable. We discuss these results in conjunction with our previous renormalization-group treatment, which indicates that for certain values of , , and the line may be crossed. We use these results to interpret the recent x-ray data on TTF-TCNQ showing a crossover from to behavior above the crossover from one-dimensional to three-dimensional behavior.
- Received 13 June 1977
DOI:https://doi.org/10.1103/PhysRevB.20.823
©1979 American Physical Society