Do surface currents in superconductors only arise due to an externally applied magnetic field?

Question

On the topic of surface currents or (screening currents) which are responsible for non decaying eddy currents that expel an externally applied magnetic field, does this phenomenon only occur when applying an external field? In other words if I have a superconducting rod with current running through it and I do NOT apply an external field, therefore I only have the current induced field, would the current density remain uniform? Or does the current become concentrated near the surface which causes the induced field also get expelled? There isn’t much clarity on whether this effect occurs on induced fields or strictly on externally applied fields

Answer 1

After extensive research it seems that it depends on the geometry of the conductor, as well as the magnitude of the current. For a cylindrical conductor the current is uniformly distributed by symmetry, however in rectangular conductors there is a critical current value above which the current distribution is uniform, and below the critical current the current does in fact cancel itself out in the center and flow mostly on the surface within a few London penetration depths. This means the meissner effect (expulsion of magnetic fields does not only apply for externally applied fields, but also for self field currents (no external field) for which the current is below the critical current. So strangely enough, a superconductor can eject it’s own internal self induced magnetic field just as it expels the externally applied field which is famously seen in magnetic levitation of superconductors.