THRESHOLD ANALYSIS FOR THE INVERSE AC JOSEPHSON EFFECT

The inverse ac Josephson effect involves rf-induced (Shapiro) steps th at cross over the zero-current axis; the phenomenon is of interest in voltage standard applications. The standard analysis of the step heigh t in current, which yields the well-known Bessel-function dependence o n an effective ac drive amplitude, is valid only when the drive freque ncy is large compared with the junction plasma frequency or the drive amplitude is large compared with the zero-voltage Josephson current. U sing a first order Krylov-Bogoliubov power-balance approach we derive an expression for the threshold value of the drive amplitude for zero- crossing steps that is not limited to the large frequency or large amp litude region. Comparison with numerical solutions of the RSJ differen tial equation shows excellent agreement for both fundamental and subha rmonic steps. The power-balance value for the threshold converges to t he Bessel-function value in the high-frequency limit.