Small Angle Neutron Scattering and Magnetization Measurements in the Cubic $(\mathrm{K},\mathrm{Ba}){\mathrm{BiO}}_3$ Superconductor

We report on both small angle neutron scattering and magnetization measurements in the cubic $(\mathrm{K},\mathrm{Ba}){\mathrm{BiO}}_3$ superconductor. We show that the flux lines are ordered into a hexagonal lattice at low field ( $B<\mathrm{}{B}_e\sim 0.6\mathrm{T}$) 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.