Interaction of vortices with oriented twin boundaries in single-crystal YBa2Cu3O7-delta

We have studied the response of the vortex lattice (VL) in a YBa2Cu3O7-delt a sample with oriented twin boundaries (TB's) and in an untwinned sample. I n the twinned sample the density of twin boundaries is estimated from decor ation experiments. The TB's affect the liquid-to-solid transition in the vo rtex structure, which is of first order in the twin free crystal and to a B ose glass in the twinned sample. We measure the ac susceptibility, and find that the response strongly depends on the orientation of the probing ac fi eld with respect to the TB's. The ac field produces a tilting stress on the vortices. With the applied de magnetic field H-dc parallel to the c axis, the response of the VL is weaker when the tilt is parallel to the TB's than when it is perpendicular or at 45 degrees to the TB's. The results are exp lained by the fact that the VL is locked to the twin boundaries for small d eviations of H-dc from the c axis and is partially pinned to them for large r angles. By measuring over the whole angular range we estimate the angles over which each regime is realized for different temperatures. The results of the detwinned sample are used for comparison.