ABSENCE OF FRACTIONAL GIANT SHAPIRO STEPS IN DIAGONAL JOSEPHSON-JUNCTION ARRAYS

When rf and dc bias currents are injected diagonally into an N x M Josephson-junction array and a perpendicular magnetic field is applied, fractional giant Shapiro steps are unexpectedly absent, although giant Shapiro steps are still observed. This absence is in direct contrast to observations in the usual case of in-line current injection and to theoretical predictions. Numerical simulations confirm these experimental results. A theoretical argument is presented to explain why diagonal Josephson-junction arrays cannot support fractional giant Shapiro steps.