Nature of phonons and isotope effect in Ba1-xKxBiO3

The phonon densities-of-states of orthorhombic insulating BaBiO3 and cubic superconducting Ba0.6K0.4BiO3 are calculated using the molecular dynamics (MD) method. The results are compared with the recent inelastic neutron scattering and electron-tunneling experiments. The exponent of the oxygen-isotope effect is estimated from the first moment of the phonon density-of-states (the weak coupling limit) and from the solution to Eliashberg gap equation, and it agrees with experiments. Inelastic neutron scattering, tunneling, and isotope-effect experiments together with the MD calculations suggest that this material is a normal BCS superconductor with strong coupling of the carriers to high-energy oxygen phonons. (author). 14 refs, 8 figs, 3 tabs