Vortex Pinning by Competing Disorder: Bose-Glass to Vortex-Glass Crossover

Magnetization measurements have been used to study the Bose-glass to vortex-glass crossover in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ crystals by varying the angle $\theta$ between the field and the twin planes. Two critical angles ${\theta }_L$ and ${\theta }_K$ are found: For $\theta <{\theta }_L$, vortices lock in to the twin planes; for ${\theta }_L<\theta <{\theta }_K$, a kinked vortex structure occurs; for $\theta >{\theta }_K$, hysteresis loops reproduce the magnetic response of the untwinned regions where pinning is produced by random point defects. The angles ${\theta }_K$ and ${\theta }_L$ depend only on the disorder in the untwinned regions and scale with $\left[\Delta M\right({\theta }_K){]}^{-0.5\mathrm{}}$, where $\Delta M\left({\theta }_K\right)$ is the hysteresis width for $\theta >{\theta }_K$.