Special phonons and superconductivity in the hexagonal tungsten bronzes

Neutron scattering measurements on single crystals of hexagonal tungsten bronzes M/sub 0.33/WO/sub 3/ reveal low-frequency, relatively dispersionless phonon branches which can be associated with the vibrations of the metal atoms M in the large open channels they occupy in the crystal structure. An analysis based on Eliashberg theory shows that the strong dependence of the superconducting transition temperature on the species M arises from these special phonons.