Small Angle Neutron Scattering and Magnetization Measurements in the Cubic (K,Ba)BiO₃ Superconductor

We report on both small angle neutron scattering and magnetization measurements in the cubic (K,Ba)BiO₃ superconductor. We show that the flux lines are ordered into a hexagonal lattice at low field ( B<B_e∼0.6 T) and that the diffracted intensity continuously drops to zero as the magnetic field is increased towards B_e. This intensity fall is attributed to a static Debye-Waller factor associated with disorder induced longitudinal fluctuations of the vortex lines. We show that B_e lies close to the onset of the anomalous “peak effect” in magnetization measurements, suggesting that this peak is related to the change in the structure of the vortex solid.