The intrinsic pinning in twinned and untwinned single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is observed using a novel crossed-magnetic-field technique for obtaining angular resolutions of Δθ<0.005°. The pinning occurs for magnetic-field directions within a critical angle ${\mathrm{\theta }}^{\mathrm{*}}$<1° of the Cu-O planes. The data are discussed in the context of a lock-in transition of vortices in the a-b plane of the crystal.

• Received 29 April 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.390