Neutron Scattering from the Flux Line Lattice in the Cubic (K, Ba)BiO₃ Superconductor

Small Angle Neutron Scattering has been used to observe diffraction from the flux line lattice in the fully isotropic (K, Ba)BiO₃ high T_c superconductor. We show that the flux lines are ordered into a hexagonal lattice at low field (H < H_e ∼ 0.6T ≪ H_g(0) where H_g is the second order vortex glass transition field). The diffracted intensity continuously drops to zero as the field is increased towards H_e. Those observations are in good agreement with recent theoritical works which suggested that the formation of dislocations becomes favorable above He leading to a glassy structure. The field H_e lies close to the onset of the second peak in magnetization measurements indicating that this peak is related to the change in the vortex structure.