A microscopic model for the three low-temperature phases of TTF-TCNQ

Abstract

We use a microscopic model Hamiltonian to study the phase transitions of tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ). The model consists of alternating sheets of chains of TTF and TCNQ molecules with the electrons occupying half-filled tight-binding bands and interacting via short-range direct Coulomb and exchange interactions of wave vectorq. Conduction-electron-phonon couplings and phonon vibrations along the chain axis are also included. Using mean-field theory, we determine the order parameters of the system. By solving for these order parameters numerically, we show that the effect of the interchain coupling is to produce three transitions of the charge-density wave (CDW) type and spin-density wave (SDW) type. The temperature interdependence of the components ofq and the order parameters is due to the monoclinic structure of the lattice. In our description, the three transitions are second order; any discontinuous changes inq lead to unphysical discontinuities in the order parameters. The physical basis of the ordering on the chains is discussed.