Flux lattice behavior in high-T_c materials studied by neutron depolarization

The depolarization of a neutron beam passing through a sample of the high-Tc superconductor YBa2Cu3O 7-δ has been measured as a function of temperature and applied field. The difference in behavior between field-cooled and zero-field-cooled states, the observation of hysteresis correlated with Hc1, and the disappearance of the effect near 55 K (below Tc) suggest an explanation in terms of vortex line lattice formation with possible connection to recently proposed flux line entanglement and melting.