In quantum Hall effect experiments, there is a critical measuring current, beyond which dissipation becomes apparent. Using samples with a narrow constriction in the middle, we have observed critical current densities which are much higher than previously reported values.

Large dc supercurrents at nonzero voltages are observed in dc current-biased small Josephson tunnel junctions of the type PbIn- (PbIn)xOy-Pb with critical current densities j0 ranging from 2×102 to 2×105 A/cm2. The dependence of the hysteresis on j0 and the form of the subharmonic energy gap structure are compared with the theoretical results of McDonald et al.

By observing constant-voltage steps from Josephson junctions at voltages as high as 17 mV we deduce that junctions can generate harmonics up to frequencies as high as 8200 GHz. In consonance with this, submillimeter wave laser detection, harmonic generation, and mixing are demonstrated. These results suggest a model for the upper frequency limit of the Josephson effect.

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