Scaling near mode locking in a charge density wave conductor

A striking evolution of the I-V curves is observed in a single sample (i.e., for a given realization of quenched disorder) of the charge density wave conductor ${\mathrm{NbSe}}_3$ in the presence of an external rf field of varying amplitude. This yields qualitatively different ‘‘depinning transitions’’ from the pinned and the mode-locked states. In some regimes the transition is apparently continuous and shows scaling behavior in agreement with some recent theoretical results. In other regimes the transition turns first order at both harmonic and subharmonic steps, but with a marked asymmetry.