Observation of zero-crossing steps in surface junctions of Bi2Sr2CaCu2O8+δ single crystals

We have studied the ac Josephson effect using an intrinsic Josephson junction fabricated on the surface of Bi2Sr2CaCu2O8+delta single crystals. Irradiated with external microwave of f = 76 and 94 GHz, the I-V characteristics show clear constant-voltage steps crossing the zero-current line. These steps satisfy the ac Josephson relation and the step heights show the Bessel-function behavior with increasing microwave power, corresponding to step numbers up to n = 4. The intrinsic surface junction with a very low critical current density meets the criterion for observing zero-crossing steps.