Flux expulsion and penetration in superconductingYBa2Cu3O7−δ

A large difference in the rate of flux flow in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ superconductors is observed between flux expulsion (field cooling then removing the field) and flux penetration (zero-field cooling then switching on a field). The mean activation energy and the width of its distribution for flux expulsion are 14 and 28 meV and for flux penetration are 34 and 67 meV. The smaller activation barrier for flux expulsion relative to flux penetration can be explained quantitatively in terms of flux pinning at the surface image potential and is also consistent with a model of randomly coupled superconducting grains.