Twin-boundary effects on flux entry and lower critical fields in single-crystal YBa2Cu3O7−δ

Abstract

We report measurements of the field of first magnetic flux entry and the derived values of the lower critical field ${\mathit{H}}_{\mathit{c}1}$ for different orientations of the applied field with respect to the twin boundaries. For a field orientation perpendicular to the twin boundaries, increased values of ${\mathit{H}}_{\mathit{c}1}$ are found. The results are interpreted in terms of an increased vortex creation energy when the vortex intersects a twin boundary. In an untwinned crystal, ${\mathit{H}}_{\mathit{c}1}$ saturates at low temperatures in agreement with measurements of penetration depths from other experiments. We find no significant a-b anisotropy in ${\mathit{H}}_{\mathit{c}1}$ in the untwinned crystal.