Higher order Shapiro steps in ac-driven Josephson junctions
  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 $\pi$-periodic effective potential for the phase, manifests itself clearly in the parameter regions where the usual Shapiro steps are expected to vanish. 
Kim O. Rasmussen, Vadim Zharnitsky, Igor Mitkov, and Niels   Gr{\o}nbech-Jensen