Flux-Line Lattice Structures in Untwinned YBa2Cu3O
  A small angle neutron scattering study of the flux-line lattice in a large single crystal of untwinned YBa2Cu3O 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 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 re-orientation of the lattice is observed around 3T. 
S. T. Johnson, E. M. Forgan, S. H. Lloyd, C. M. Aegerter, S. L. Lee,   R. Cubitt, P. G. Kealey, C. Ager, S. Tajima, A. Rykov...