Possible origins of resistive tails and critical currents in high-temperature superconductors in a magnetic field

Abstract

The universal lack of a Lorentz-force dependence on dissipation for fields parallel to the ${\mathrm{CuO}}_{2}$ planes of the highly anisotropic high-temperature superconductor ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathit{x}}$ questions whether flux motion is the cause of this dissipation. We report measurements over a wide range of current densities, in the broadened resistive transitions, current-voltage characteristics I(V), and critical current densities ${\mathit{J}}_{\mathit{c}}$. We rule out the suggestion that this effect is caused by vortices in the ${\mathrm{CuO}}_{2}$ planes, due to a small misalignment of fields parallel to these planes: That model requires a significantly larger field component perpendicular to the planes than is reasonable, based on the measured alignment of the samples and crystal axes. Instead, we consider a Josephson-coupling model that is consistent with the broadened resistive transitions and the lack of Lorentz-force dependence. A detailed comparison of the predictions of these models is made: The Josephson-coupling model is consistent with the temperature dependences of the activation energy U and ${\mathit{J}}_{\mathit{c}}$, and is better matched to the weak-field dependence of ${\mathit{J}}_{\mathit{c}}$; while the flux-creep model fits the experimental result for U, but it predicts a much stronger temperature and field dependence of ${\mathit{J}}_{\mathit{c}}$ than is found. Possible origins of Josephson junctions in high-quality films and single crystals are discussed. For the data with the field parallel to the c axis, a conventional flux-flow explanation is also quite reasonable.