AC Quantum Interference Effects in Nanopatterned Nb Microstrips

The mixed-state dc voltage response of nanopatterned Nb microstrips is investigated by combined microwave (0.5 MHz-1 GHz) and dc electrical resistance measurements. The nanopatterns are arrays of symmetric grooves on the film surface fabricated by focused ion beam milling. They provide a pinning potential of the washboard type for the vortex motion. When subject to a microwave stimulus, the dc current-dc voltage curves of the microstrips exhibit Shapiro steps. The steps occur at voltages V=n V (0)=n N f I broken vertical bar (0), where n is an integer, N is the number of vortex rows between the voltage leads, f is the microwave frequency, and I broken vertical bar (0) is the magnetic flux quantum. These steps arise as an interference effect when one or a multiple of the hopping period of the coherently moving Abrikosov vortices coincides with the period of the ac drive. When tuning the field value away from the matching field, the steps disappear due to lacking coherence in the motion of the Abrikosov vortex lattice. Dependencies of the dc critical (depinning) current on the microwave power and frequency are reported.