Charge-density waves in tetrathiafulvalene-tetracyanoquinodimethane

The observed structural phase transitions in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) are analyzed using a Landau-Ginzburg-type free energy. It is shown that because of a coupling between the components $q_a$ and $q_c$ of the wave vector associated with the charge-density wave, the latter is expected to be nonzero for $38<T<\mathrm{}47\mathrm{}$ °K, contrary to previous expectations.