Poincaré sections of charge-density-wave dynamics: Mode locking

This Letter reports the first measurements of Poincaré sections of charge-density-wave (CDW) dynamics. High-quality crystals of ${\mathrm{NbSe}}_3$ were excited by combined dc and ac currents. Mode-locked and -unlocked behavior are identified by Poincaré sections consisting of points and closed curves, respectively. We analyze our results in terms of a dynamical phase transition. We argue that multiply degenerate attractors exist in subharmonic mode-locked phases. Dynamics of such phases are dominated by domain walls separating sections of the CDW with different attractors.