Phonons, Oxygen Isotope Effect and Superconductivity in Ba 1-x K x BiO 3
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PHONONS, OXYGEN ISOTOPE EFFECT AND SUPERCONDUCTIVITY IN Baj.xKxBiO 3 W. Jin°+, C.K. Loongs, D.G. Hinks+, P. Vashishta+, R.K. Kalia+, M. H. Degani+, D.L. Price+, J.D. Jorgensen+, and B. Dabrowski* + * Science and Technology Center for Superconductivity + Materials Science Division # Intense Pulsed Neutron Source Argonne National Laboratory, Argonne, IL,60439 ABSTRACT The phonon densities-of-states (DOS) of superconducting Bal-xKxBi 1 60 3 and Bai.xKxBi 18 0 3 (x=0.4) are determined using molecular dynamics (MD) simulations and inelastic neutron scattering measurements. The reference isotope-effect exponent is obtained from the mass variation of the first frequency moment of the phonon DOS. The energy gap, oxygen isotope-effect exponent and electron tunneling characteristics are calculated within the framework of Eliashberg theory of electron-phonon coupling. INTRODUCTION Since the discovery 1 ,2 of superconductivity in Bal.xKxBiO3, experimental evidence has been accumulating that this compound is simpler than the high-Tc cupric oxides. Showing none of the planar structures observed in other high-Tc compounds, the superconducting phase of Baj~xKxBiO 3 (0.37 < x < 0.5) in fact displays a cubic perovskite structure. 3 As the K concentration is reduced, superconductivity disappears when the structure changes from cubic to orthorhombic. Furthermore, BaBiO 3 is nonmagnetic 4 ,5 while the other high-Tc related materials, in the parent nonsuperconducting phases, display antiferromagnetism. 6 Hall effect measurements in BajixKxBiO 3 indicate that the carriers are electrons 7 whereas in the cuprates, with the exception of RE2.xCexCuO 4 (RE=Nd, Pr), the carriers are holes. 8 The importance of electron-phonon coupling 9 in Bal-xKxBiO 3 was revealed by measurements of the oxygen isotope effect from the superconducting transition temperature Tc, Tc - Mo -"o, where Mo is the mass of the oxygen isotope and oto the corresponding exponent. The exponent aoo was found to be 0.41 + 0.05 by Hinks et al. 10 In contrast, the exponent ao is much smaller ( - 0.1) in the high-Tc cuprates. 8 Infrared reflectivity measurements 11 of the superconducting gap A0 in Ba1-xKxBiO 3 give the dimensionless ratio 2Ao/kBTc = 3.5 ± 0.4, consistent with the BCS Mat. Res. Soc. Symp. Proc. Vol. 209. ©1991 Materials Research Society
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value of 3.52 for weak-coupling superconductors. Electron tunneling measurements on Bal-xKxBiO 3/In junctions 12 and on point contact junctions 13 show features at 35, 42 and 60 meV in the tunneling second derivative, in agreement with the phonon 14 DOS obtained by neutron scattering experiments. PHONON DENSITY-OF-STATES We have performed molecular dynamics simulations with effective interatomic interactions consisting of Coulomb potentials due to charge-transfer effects, steric repulsion, and a charge-dipole interaction due to the large electronic polarizability of oxygen ions. The calculations were done with 540 particles for orthorhombic BaBiO 3 and 625 particles for cubic Ba0 .6K0o 4 BiO 3 at the experimental number dens
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