Fractional Shapiro steps in ladder Josephson arrays.

摘要: We study the response of Josephson ladder arrays (1\ifmmode\times\else\texttimes\fi{}N plaquettes) to a combined ac and dc current. For N\ensuremath{\ge}1, are predicted exhibit Shapiro steps at voltages 〈V〉=nN\ensuremath{\Elzxh}\ensuremath{\omega}/(2e) for n=1/2,1,3/2, . ., over broad range fields f=\ensuremath{\Phi}/${\mathrm{\ensuremath{\Phi}}}_{0}$ (\ensuremath{\Phi}=flux per plaquette, ${\mathrm{\ensuremath{\Phi}}}_{0}$=hc/2e). The calculated half-steps persist even in single-plaquette ``arrays'' all f\ensuremath{\ne}0, and, when critical currents parallel applied current sufficiently unequal, f=0. Measurements step widths \ensuremath{\Delta}${\mathit{I}}_{1/2}$(f) \ensuremath{\Delta}${\mathit{I}}_{1}$(f) 1\ifmmode\times\else\texttimes\fi{}600 Nb-Au-Nb junctions good agreement with calculations.