ASYMMETRIC CURRENT-VOLTAGE CHARACTERISTICS IN TYPE-II SUPERCONDUCTORS

We report asymmetric current-voltage characteristics in superconducting films in a parallel magnetic field. The critical current, I(c), is found to differ by as much as 30% for negative and positive currents, and beyond I(c), the magnitude of the voltage is also different for positive and negative currents, \V(+I)\not-equal\V(-I)\. The critical current difference \I(c+)\-\I(c-)\ has complicated behavior, changing sign as temperature and magnetic field are varied. We propose a model based on the Bean-Livingston surface barrier and inhomogeneous bulk pinning that accounts for all observed behavior.