Subharmonic Shapiro Steps and Devil's-Staircase Behavior in Driven Charge-Density-Wave Systems

Subharmonic steps in the dc I-V characteristics are reported in the model compound NbSe₃, when subjected to combined dc and ac drives. The steps occur at currents which correspond to frequencies ωᵢₙₜ=(p/q)ωₑₓₜ, with p and q integers. These steps cannot be explained in terms of the familiar classical equation of motion in a single degree of freedom, because the inertial term is too small. The observed behavior is compared with the devil's staircase, and a fractal dimension of 0.91 ±{} 0.03 is extracted.