We have performed Bitter decoration and ac susceptibility measurements in single-crystal ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ with oriented twin boundaries. The twin boundaries (TB’s) pin vortices over approximately 65% of the sample. The pinned areas are unevenly distributed and some relatively large TB-free regions are present. The Bitter decorations were performed in a 52 Oe dc magnetic field 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 performed in the same sample. Additional dynamical information was obtained from ac susceptibility measurements. Results show that for small tilts the vortices remain locked to the c-axis direction, for angles greater than 12° they form a staircase pattern, and in this case pinning by the twin boundaries remains effective up to $75°.$ We observe vortex chains in twin-free zones of the sample for tilted fields at $65°$ and $40°.$ Due to our particular experimental arrangement, in the twinned regions the interplay of the potentials giving rise to the chains and the pinning potential produces a structure with a disordered square symmetry. The data allow us to estimate the dominant energy of the vortex system for some inclinations of the applied magnetic field.

• Received 25 March 1999

DOI:https://doi.org/10.1103/PhysRevB.62.3534