Small angle neutron scattering and magnetization measurements in the cubic(K, Ba)BiO3 superconductor

We report on both small angle neutron scattering and magnetization measurem ents in the cubic (K,Ba)BiO3 superconductor. We show that the flux lines ar e ordered into a hexagonal lattice at low field (B < B-e similar to 0.6 T) and that the diffracted intensity continuously drops to zero as the magneti c field is increased towards B-e. This intensity fall is attributed to a st atic Debye-Waller factor associated with disorder induced longitudinal fluc tuations of the vortex lines. We show that B-e lies close to the onset of t he anomalous "peak effect" in magnetization measurements, suggesting that t his peak is related to the change in the structure of the vortex solid. [S0 031-9007(99)09328-4].