Observations of the configuration of the high-field vortex lattice in YBa2Cu3O7: Dependence upon temperature and angle of applied field

We have mapped out the high-field phase diagram of the vortex lattice in lightly twinned YBa2Cu3O7 by employing small-angle neutron scattering. This extends previous measurements [S. P. Brown , Phys. Rev. Lett. 92, 067004 (2004)] in which we reported a transition in local flux-line coordination from distorted triangular to square as a function of field applied parallel to the crystal c axis. At high fields, we observe a transition back to triangular coordination with increasing angle between the applied field and c axis or with increasing temperature. At lower fields, our results may be interpreted in terms of a field-induced reduction of the basal-plane a-b anisotropy. Tilting the flux lines away from the c axis reduces the pinning by twin planes, changes the orientation of the vortex lattice nearest-neighbor directions, and the transition between square and hexagonal vortex lattices becomes first order.