We demonstrate that the well known phase-locking mechanism leading to Shapiro steps in ac-driven Josephson junctions is always accompanied by a higher order phase-locking mechanism similar to that of the parametrically driven pendulum. This effect, resulting in a π-periodic effective potential for the phase, manifests itself clearly in the parameter regions where the usual Shapiro steps are expected to vanish.
Cite
@article{arxiv.patt-sol/9809003,
title = {Higher order Shapiro steps in ac-driven Josephson junctions},
author = {Kim O. Rasmussen and Vadim Zharnitsky and Igor Mitkov and Niels Grønbech-Jensen},
journal= {arXiv preprint arXiv:patt-sol/9809003},
year = {2009}
}