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)BiO3high Tcsuperconductor. We show that the flux lines are ordered into a hexagonal lattice at low field (H < He∼ 0.6T ≪ Hg (0) where Hgis the second order vortex glass transition field). The diffracted intensity continuously drops to zero as the field is increased towards He. Those observations are in good agreement with recent theoritical works which suggested that the formation of dislocations becomes favorable above Heleading to a glassy structure. The field Helies close to the onset of the second peak in magnetization measurements indicating that this peak is related to the change in the vortex structure.