Self-enhanced flux penetration into small angle grain boundaries in YBCO thinfilms

Abstract

Grain boundaries in high-temperature superconductors are well known to strongly decreasethe critical current. However, most information about their superconducting properties isbased on transport measurements which always give non-local, averaged data over thewhole sample size.In this paper, we report on the spatially resolved magneto-optical measurementsof the magnetic flux and current density distribution in bicrystallineY Ba2Cu3O7−x thin films with small angle grain boundaries. The current distributions are obtained by amodel-independent inversion of Biot–Savart’s law.In contrast to Bean-like models of the current pattern at the grain boundary,we find an inhomogeneous current density across the grain boundary, which isstrongly influenced by the magnetic stray fields of the currents in the grains. Theself-field effects are also responsible for a self-enhanced flux penetration wherethe penetration depth is significantly increased compared to a Bean-like model.