Microwave-induced constant voltage steps in surface junctions of Bi2Sr2CaCu2O8+δ single crystals

We have observed the zero-crossing steps in a surface junction of a mesa structure microfabricated on the surface of a Bi2Sr2CaCu2O8+delta single crystal. With the application of microwave of frequencies 76 and 94 GHz, the current-voltage characteristics show clear voltage steps satisfying the ac Josephson relation. Increasing the microwave power, the heights of the steps show the Bessel-function behavior up to step number n=4. We confirm that the intrinsic surface junction meets the criterion for the observation of zero-crossing steps.