Shapiro steps in strongly-interacting Fermi gases.

Driven many-body systems exhibit diverse and complex dynamical behaviors. Here, we report the observation of Shapiro steps in periodically driven Josephson junctions between strongly interacting Fermi superfluids of ultracold atoms. The height and the width of the observed quantized plateaus in the current-potential characteristics mirror the external drive frequency and the junction nonlinear response. Direct measurements of the current-phase relationship showcase how Shapiro steps arise from the synchronization between the relative phase of the two reservoirs and the external drive. Such a mechanism is further supported by the detection of periodic phase-slippage processes, in the form of vortex-antivortex pairs. Our results are corroborated by a circuital model and numerical simulations. Our work may open prospects for studying emergent nonequilibrium dynamics in quantum many-body systems under external drives. Editor's summary: In the Josephson effect, a current can flow through an insulating barrier between two superconductors without dissipation. For a high enough current, dissipation does occur, and if microwave radiation is also applied, a series of plateaus—the so-called Shapiro steps—appear in the current-voltage characteristic. Instead of using superconductors, Del Pace et al. and Bernhart et al. observed Shapiro steps in an atomic Josephson junction. The researchers moved a barrier through an ultracold gas at a speed that had a periodically modulated component to emulate microwave radiation. The height of the resulting Shapiro steps in the chemical potential difference was proportional to the frequency of the modulation. —Jelena Stajic [ABSTRACT FROM AUTHOR]