FLUX-CREEP IN THE PRESENCE OF TWIN BOUNDARIES

The flux-creep rate associated with twin boundary pinning is calculate d in the framework of a Brownian motion model. Systematic elimination of the velocity degrees of freedom exposes the relevant small paramete r and validates the strong-friction limit for a ''typical'' high-tempe rature superconductor. The corresponding expression for the vortex-esc ape probability is obtained in a closed form. For a piecewise linear p inning potential approximation this expression yields an analytic resu lt. The ensuing critical current strongly depends not only on the pinn ing-potential maximum depth, but also on the existence of deep, steep features in its shape. For a ''realistic'' twin boundary pinning poten tial, the calculated vortex-escape probability is compared to the Ande rson-Kim theory. The comparison shows that the Anderson-Kim expression greatly overestimates the vortex-escape probability. Inter-defect dif fusion is found to be particularly important in the ''weak'' current, low-temperature regime.