Symmetry of modulated phases in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ): Four- and five-dimensional superspace groups

The three modulated ordered phases in tetrathiafulvalene-tetracyanoquinodimethane are described in terms of four- and five-dimensional superspace groups. The 47-K transition is driven by a soft excitation polarized in the fourth "phase" direction. Extinction rules are derived, and the absence of certain $4k_F$ spots observed by Kagoshima et al. are explained. Additional experiments are suggested to completely determine the space groups.