Direct observation of the vortex structure in high-T_c superconductors in tilted magnetic fields. II. Vortex pinning by twins in YBaCuO single crystals

The high resolution Bitter technique was used for observation of the vortex structure in twinned YBaCuO single crystal in the magnetic field tilted with respect both to the c-axis and to the twin planes. The dominating effect in vortex distribution was strong pinning by twin planes which was seen as an alignment of vortices along twins with approximately two times density higher density than in the bulk. This strong pinning remained unchange for a surprisingly broad interval of tilt angles 0<25°. For larger tilt angles, pinning did not disappear abrupity but the signs of a decrease of the effective pinning potential were observed-the difference of vortex density between twin boundaries and the bulk was gradually decreasing and finally disappeared at a critical angle as large as 42°. The observed behaviour is discussed in terms of the formation of zig-zag vortices partly pinned at twin planes.