Step-by-step advancement of the charge density wave in the rf-synchronized modes and oscillations of the width of Shapiro steps with respect to the rf power applied

The sliding of the room-temperature charge density wave (CDW) in the monoclinic phase of $\mathrm{Nb}{\mathrm{S}}₃$ under rf power is studied. The threshold field, ${E}_{\mathrm{t}}$, and Shapiro steps' width, \ensuremath{\delta}E, show aperiodic Bessel-type oscillations as a function of rf voltage. Here we demonstrate experimentally that, if presented as a function of CDW path length in each half period of the rf voltage, ${E}_{\mathrm{t}}$ and \ensuremath{\delta}E show periodic oscillations, the period being equal to the CDW wavelength. The result is found to be universal for different compounds and gives clear understanding of the synchronization effects in terms of forced oscillations of a particle in a periodic potential.