Neutron diffraction from the flux line lattice near the entanglement transition in (K, Ba)BiO3

Small Angle Neutron Scattering has been used to investigate the structure o f the flux line lattice in the isotropic (K, Ba)BiO3 high T-c compound. The flux lines are well ordered into a hexagonal lattice at low field (H < H-e <similar to> 0.6T much less than H-g(0) where H-g is the second order vort ex glass transition field) but dislocations become energetically favorable above H-e leading to a glassy structure. Moreover, the orientationnal order is partially lost in the vicinity of the entanglement transition but can b e recovered by slightly oscillating the field during the cooling process.