Motion of Magnetic Flux Quanta in Type-II Superconductors – Some Open Questions

Abstract The motion of the magnetic flux quanta in the mixed state of a type-II superconductor, due to the Lorentz force of an electric transport current, represents the key mechanism for generating resistive losses. Whereas the most common result is Joule heating of the superconductor, also purely electronic non-equilibrium effects can play a dominant role in the case of effective cooling of the sample. The latter situation can be realized by means of very thin superconducting films (having a large surface/ volume ratio). In this paper we discuss experiments with Nd2−xCexCuOy (NCCO) films performed some time ago, yielding evidence for electronic non-equilibrium effects due to the energy dependence of the quasi-particle density of states in the mixed state of the films. The films were imbedded within super-fluid helium for cooling. The recent advances in the fabrication of epitaxial ultra-thin superconducting films promise to contribute to further clarification of these electronic non-equilibrium effects.