Influence of grains on the electromagnetic ac response of superconducting materials

Type II superconducting materials are technologically interesting since they are characterized by the existence of a superconducting phase even after the penetration of the magnetic field inside them. In fact, while type I superconductors are characterized by a net transition from the perfect diamagnetic state (Meissner state) to the normal state, the magnetic field penetrates type II superconductors above a lower magnetic critical field in form of tubes of quantized magnetic flux, which are surrounded by vortices of superconducting currents, placed in a regular hexagonal lattice (mixed state or Abrikosov state). In this mixed state the superconductor can sustain the magnetic field and carry a current larger than in a type I superconductor. The superconducting phase persists below an upper magnetic critical field and a critical current density both depending on the temperature. In particular, the interaction of the superconducting vortices with the shielding currents and the external transport currents can generate dynamical phenomena which produce dissipation, and then increase the temperature and suppress the critical current density of the superconductor. On the other hand, these motion processes are hindered by the presence of defects which act as pinning centers of the vortices inside the lattice making the material able to sustain a current density below the critical value... [edited by Author]