The two-chain problem: A model of TTF-TCNQ

Abstract

The two-chain, inverted band model for TTF-TCNQ is solved by the exact methods developed by Luther and Emery and by Chui and Lee. On each chain, there are intrachain interactions g₁ and g₂, and there are interchain interactions w₁ and w₂. We investigate all of the possible two-particle correlation functions, of which the new divergent ones not present in the single chain problem are of the excitonic insulator type. In addition, we investigate essentially all of the many possible four-particle correlation functions. For the single chain, there are two divergent ones; the 4k_F response found by Emery, and a four-particle condensation of two Cooper pairs. For the two-chain problem, there are many others, the most unusual of which are of the excitonic molecule type. In addition, the 4k_F response is enhanced by the interchain coupling, and is divergent for weak intrachain coupling as well as for strong coupling. Using renormalization group techniques, we find that the essential features of the temperature dependence of the relative size of the 4k_F and 2k_F excitations recently observed can be explained in terms of a crossover from a region in which the 4k_F excitation is dominant to one in which the 2k_F charge density wave response is dominant.