Flux-line lattice structures in untwinned YBa2Cu3O7-delta

A small angle neutron scattering study of the flux-line lattice in a large untwinned single crystal of YBa2Cu3O7-delta is presented. In fields paralle l to the c axis, diffraction spots are observed corresponding to four orien tations of a hexagonal lattice, distorted by the a-b anisotropy, A value fo r the anisotropy, the penetration depth ratio, of lambda(a)/lambda(b) = 1.1 8(2) was obtained. The high quality of the data is such that second-order d iffraction is observed. indicating a well ordered FLL. With the field at 33 degrees to c, a field dependent reorientation of the lattice is observed a round 3 T. [S0031-9007(99)08812-2].