STRUCTURE AND SUPERCONDUCTIVITY IN ALKALI TUNGSTEN BRONZES

The anomalous concentration dependence of the superconducting transition temperature of alkali tungsten bronzes MxWO3 is shown to be consistent with electron pairing induced by exchange of acoustic plasmons. Structural changes in these metals yield significant band-structure splitting which is shown to be essential to the formation of well-defined acoustic-plasmon branches in the dielectric function. Our results explain why cubic HxMO3 is not superconducting even for large metallic densities x→1, and furthermore, demonstrate that the hexagonal phase is favorable to superconducting pairing in agreement with experiments on RbxWO3 and CsxWO3. Possible neutron and Brillouin scattering experiments are proposed to directly measure the acoustic-plasmon dispersion and damping. © 1979 The American Physical Society.