Interplay between pinning energy and vortex interaction in YBa2Cu3O7-deltawith oriented twin boundaries in tilted magnetic fields: Bitter decorationand tilt-modulus measurements

We have performed Bitter decoration and ac susceptibility measurements in s ingle-crystal YBa2Cu3O7-delta with oriented twin boundaries. The twin bound aries (TB's) pin vortices over approximately 65% of the sample. The pinned areas are unevenly distributed and some relatively large TB-free regions ar e present. The Bitter decorations were performed in a 52 Oe de magnetic fie ld rotated off the c axis so that the plane defined by the field direction and the c axis is perpendicular to the TB's. Several decorations were perfo rmed in the same sample. Additional dynamical information was obtained from ac susceptibility measurements. Results show that for small tilts the vort ices remain locked to the c-axis direction, for angles greater than 12 degr ees they form a staircase pattern, and in this case pinning by the twin bou ndaries remains effective up to 75 degrees. We observe vortex chains in twi n-free-zones of the sample for tilted fields at 65 degrees and 40 degrees. Due to our particular experimental arrangement, in the twinned regions the interplay of the potentials giving rise to the chains and the pinning poten tial produces a structure with a disordered square symmetry. The data allow us to estimate the dominant energy of the vortex system for some inclinati ons of the applied magnetic field.