CONTRIBUTION OF SURFACE TO CURRENT-CARRYING CAPACITY OF TYPE 2 SUPERCONDUCTORS IN MIXED STATE

Experiments on type II superconductors of low defect density reveal that the current-carrying capacity in the mixed state is profoundly influenced by any plane surface parallel to the magnetic field direction. Similar surface effects are found to be present when the force on the flux lines is due to a temperature gradient rather than to a transport current. A temperature gradient localized near to the surface influences the capacity to support transport current in a manner which varies with the polarity of field and current. It is concluded that critical current is controlled by interactions between the fluxoids and the sample surface, and not by a 'resistanceless surface layer'. Crude calculations based on the interaction model provide qualitative explanations of polarity effects in the critical current and of the presence of a strong minimum in the critical current of one polarity.