Alternating transport-current flow in superconductive films: The role of a geometrical barrier to vortex motion

Abstract

${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films grown on both ${\mathrm{SrTiO}}_{3}$ single-crystal and rolling-assisted biaxially textured (Ni) substrates (RABiTS) carry high critical current densities ${J}_{c}.$ A geometrical barrier to vortex motion raises the apparent ${J}_{c},$ but also increases power loss associated with ac transport current through such tape above that expected from Norris's 1970 theory of hysteretic energy loss. Present theoretical estimates of the geometrical barrier have insufficient magnitude to account for our observations at low ac current levels. Evidence is reported that the ferromagnetic Ni substrate makes no significant contribution to the tape self-field loss. Loss is enhanced, and ${J}_{c}$ modestly reduced by the presence of low-angle grain boundaries. Application of dc magnetic field further lowers ${J}_{c}$ and raises loss in films.