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 th e flux line lattice in the fully isotropic (K, Ba)BiO3 high T-c superconduc tor. We show that the flux lines are ordered into a hexagonal lattice at lo w field (H < H-e similar to 0.6T << Hg(0) where H-g is the second order vor tex 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 H, leading to a glassy structure. The field H-e lies close to the onset of the second peak in magnetization m easurements indicating that this peak is related to the change in the vorte x structure.