NONLOCAL EFFECTS AND VORTEX LATTICE TRANSITIONS IN YNI2B2C

High resolution, neutron small angle scattering studies have been perf ormed to investigate the low field (B < 250 mT) region of the phase di agram for the vortex lattice in the superconducting state of YNi2B2C. The data present clear evidence for a vortex lattice reorientation tra nsition from a state with the diagonal of the rhombic unit cell along a [110] direction to a [100] direction. Above this transition the latt ice distorts under the influence of the applied field until the apex a ngle becomes constant at some higher field. For B parallel to c a squa re lattice configuration is formed. These experiments confirm qualitat ively many of the predictions of a general model based on nonlocal cor rections to the London model as applied to this and similar materials.