Observation of a novel vortex structure driven by magnetic interactions near a sawtooth twin boundary in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have used magnetic decoration to study the pattern of vortices formed near an isolated sawtooth twin boundary in single-crystal ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. When the pitch of the sawtooth is comparable to both the intervortex spacing and the penetration depth, we find an unusual vortex structure which has a reduced symmetry relative to the sawtooth itself. This vortex pattern arises from the magnetic interaction between the vortices and the twin boundary which occurs for an asymmetric twin. This is the first observation of a vortex structure caused by the magnetic interaction with a crystalline defect.