PHONON DENSITY OF STATES AND OXYGEN-ISOTOPE EFFECT IN BA1-XKXBIO3

The phonon densities of states (DOS) of insulating BaBiO3 and superconducting Ba0.6K0.4BiO3 and the variation of the phonon spectrum of the superconducting compound upon oxygen-isotope (O-16, O-18) substitution are determined by inelastic neutron scattering (INS) and molecular-dynamics (MD) simulations. The MD simulations are carried out with an effective interaction potential which includes steric effects, Coulomb interactions, and the charge-dipole interactions due to the electronic polarizability of O2-. The MD results are in good agreement with the INS experiments and electron-tunneling measurements. Partial DOS of Ba, K, Bi, and O in BaBiO3 and Ba0.6K0.4BiO3 are also determined from MD simulations. In the superconducting material, the phonon spectrum softens and is comprised of broad bands around 15, 30, and 60 meV. The partial DOS reveal that phonons above 20 meV are due to oxygen vibrations, whereas phonons below 20 meV are due to Ba, K, and Bi. The reference oxygen-isotope-effect exponent, alpha(Or) = - partial derivative 1n<omega>/partial derivative 1nM(o), of Ba0.6K0.4BiO3 is determined to be alpha(Or) = 0.42 +/- 0.05 from the mass (M(o)) variation of the first moment of the phonon DOS, O-16<omega> and O-18<omega>. This value is close to the oxygen-isotope-effect exponent, alpha(o), determined from the variation of T(c) (0.41 +/- 0.03 by Hinks et al. and 0.35 +/- 0.05 by Kondoh et al.), indicating that Ba0.6K0.4BiO3 is a weak- to moderate-coupling BCS-like superconductor and that the high T(c) (approximately 30 K) results from large electron-phonon matrix elements involving high-energy oxygen-related phonons.