The Landau theory of phase transitions in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane)

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Abstract
The observed low-temperature charge-density-wave (CDW) transitions in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) are analyzed in the framework of the Landau theory of second-order phase transitions. A careful treatment of the symmetry requirements is given. It is found that the (commensurate) transition at 54 K involves CDW distortions on both TCNQ and TTF chains. The possibilities for the subsequent (noncommensurate) transition near 49 K are analyzed. A continuous transition involving a phase modulation of the first transition may happen for a range of values of the Landau parameters. The next (commensurate) transition, at 38 K is discontinuous. The formulation requires a thorough group-theoretical analysis: We discuss the validity of the usual practice of using only a simple basis set for an irreducible representation associated with a transition. A discussion of the absence or presence of commensurability energies of the various transitions is also given.