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 YBa₂Cu₃O_7-x crystals by varying the angle θ between the field and the twin planes. Two critical angles θL and θK are found: For θ<θL, vortices lock in to the twin planes; for θL<θ<θK, a kinked vortex structure occurs; for θ>θK, hysteresis loops reproduce the magnetic response of the untwinned regions where pinning is produced by random point defects. The angles θK and θL depend only on the disorder in the untwinned regions and scale with [ΔM(θK)]^-0.5, where ΔM(θK) is the hysteresis width for θ>θK.