F. Marsiglio and J.E. Hirsch, Physica C 159, 157 (1989).
Within the conventional electron-phonon mechanism of superconductivity, normal-insulator-superconductor (N-I-S) tunneling characteristics are expected to be symmetric with respect to the sign of the bias voltage. A recently proposed mechanism of superconductivity based on pairing of hole carriers predicts an asymmetry of universal sign: the tunneling current should be larger for a negatively biased sample. We suggest a search for this asymmetry in conventional superconductors, as well as in 'hole' and 'electron' oxide superconductors: its systematic observation would provide direct evidence in favor of the hole pairing mechanism of superconductivity.