ENHANCED AC JOSEPHSON EFFECT

The amplitude of the current steps induced by a rf field in a Josephson junction is strictly related to the stability of the phase-locked state. We show how the ac Josephson effect can be enhanced by means of a proper drive: when two phased ac fields of angular frequencies ω and 2ω are added together, the rf-induced steps have maximum normalized amplitudes larger than the Bessel functions as predicted for a sinusoidal drive. Furthermore, in the limiting case of a driving signal that consists of an impulse train, the stability of the phase-locked state can reach its maximum value, i.e., rf-induced steps as large as the critical current can be obtained. Experimental evidence of the enhanced ac Josephson effect is reported for a highly hysteretic Nb/Al-Al2O3/Nb Josephson tunnel junction. The enhanced ac Josephson effect may open new ways for the understanding of the nonlinear dynamics of Josephson junctions and for the design of more efficient Josephson voltage standard devices.