Flux-line lattice structures in untwinned YBa2Cu3O7-d

A small angle neutron scattering study of the flux-line lattice in a large untwinned single crystal of YBa2Cu3O7-delta is presented. In fields parallel to the c axis, diffraction spots are observed corresponding to four orientations of a hexagonal lattice, distorted by the a-b anisotropy, A value for the anisotropy, the penetration depth ratio, of lambda(a)/lambda(b) = 1.18(2) was obtained. The high quality of the data is such that second-order diffraction is observed. indicating a well ordered FLL. With the field at 33 degrees to c, a field dependent reorientation of the lattice is observed around 3 T. [S0031-9007(99)08812-2].