Flux-Line Lattice Structures in UntwinnedYBa2Cu3O7−δ

A small angle neutron scattering study of the flux-line lattice in a large untwinned single crystal of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ 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 ${\lambda }_a/{\lambda }_b=1.18\mathrm{}\left(2\right)$ 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° to $c$, a field dependent reorientation of the lattice is observed around 3 T.