Studies of Surface Transport Currents in Type-II Superconductors; a Surface-Flux-Pinning Model

The critical surface transport current of type-II films is measured as a function of magnetic field ($H_{c1\mathrm{}}<H<\mathrm{}{H}_{c3\mathrm{}}$), of the angle that the magnetic field makes with the surfaces, of surface condition, and of film thickness. The results show that the critical surface current (1) is much smaller than that predicted by the Abrikosov-Park model, (2) does not vary systematically with film thickness as predicted by certain recent theories, (3) increases as the surface is roughened, (4) decreases sharply as the perpendicular component of the applied magnetic field is increased, and (5) increases sharply as the applied magnetic field is lowered through $H_{c2\mathrm{}}$. These results are interpreted as evidence for surface flux pinning, i.e. of a surface-critical-state model, rather than as evidence for any of the published theoretical models. In our model, quantized flux threads or spots intercepting the surface of the sample are pinned at surface pinning sites. When a transport current is applied, a Lorentz force is exerted on these surface flux threads or spots. At a transport current level below the intrinsic theoretical limit, the Lorentz force exceeds the pinning force; flux moves across the surface, a steady voltage is detected, and a critical surface current is thereby defined.