Vortex Viscosity in Superconductors with Short Coherence Length

In superconductors where the coherence length is comparable to the Fermi wavelength, electronic levels within a vortex core are quantized and separated by energies of the order of the superconducting gap. These compounds can easily be in the "ultraclean limit," where the conventional Bardeen-Stephen theory cannot be applied. In addition, a significant range of temperatures exists where transitions between vortex states are suppressed. This effect modifies the energy dissipation due to vortex motion, at sufficiently low temperatures. The minimum energy splitting defines a vortex velocity, below which the vortex viscosity is strongly reduced. In this regime, the $I-V$ characteristics are non-Ohmic.