Experimental and theoretical studies are made of the effect of a microwave on dc current-voltage characteristics of long Josephson junctions. In the case of zero external dc current, dc voltages were observed when the microwave was applied to the junctions. By comparing the observed magnetic-field and microwave-power dependences with theoretical results, it is concluded that these dc voltages resulted from vortex motion which was induced by the applied microwave. When the junctions were in a flux-flow state caused by the external current, a good agreement between theory and experiment concerning the microwave-power and magnetic-field dependences of Shapiro steps demonstrates that an almost ideal traveling-wave type coupling between the applied microwave and a moving vortex array took place in the long Josephson junctions.
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