Neutron measurements of the vortex lattice in YBa₂Cu ₃O₇ (invited)

Neutron diffraction has been used to measure the vortex lattice scattering for YBa₂Cu₃O₇. A square pattern is found when the field is applied along the c axis, while a triangular pattern is found when the field is applied well away from the c axis. High-resolution measurements for the square pattern show that the vortex lattice has long-range orientational order but only short-range positional order. The temperature dependence of the penetration depth is not that expected for a superconductor with a conventional s-wave BCS-type gap. Preliminary measurements for temperatures near the irreversibility line are consistent with the occurrence of melting of the vortex lattice or glass phase.