Inclined-field structure, morphology, and pinning of the vortex lattice in microtwinned YBa₂Cu₃O₇

A detailed small-angle neutron scattering study of the vortex lattice in a single crystal of YBa₂Cu₃O₇ was made for a field of 0.5 tesla inclined at angles between 0 and 80 degrees to the crystalline c axis. The vortex lattice is triangular for all angles, and for angles less than or equal to 70 degrees its orientation adjusts itself to maximize the pinning energy to densely and highly regularly spaced twin planes. These observations have important implications for the microscopic flux-pinning mechanism, and hence for the critical current achievable in YBa₂Cu₃O₇. For large angles (about 80 degrees) the vortex lattice consists of independent chains in the orientation predicted by anisotropic London theory.