ASYMMETRIC CURRENT-VOLTAGE CHARACTERISTICS IN TYPE-II SUPERCONDUCTORS

We report asymmetric current-voltage characteristics in superconductin g films in a parallel magnetic field. The critical current, I(c), is f ound to differ by as much as 30% for negative and positive currents, a nd beyond I(c), the magnitude of the voltage is also different for pos itive and negative currents, \V(+I)\not-equal\V(-I)\. The critical cur rent difference \I(c+)\-\I(c-)\ has complicated behavior, changing sig n as temperature and magnetic field are varied. We propose a model bas ed on the Bean-Livingston surface barrier and inhomogeneous bulk pinni ng that accounts for all observed behavior.