Temporally ordered collective creep and dynamic transition in the charge-density-wave conductor NbSe3

We have observed an unusual form of creep at low temperatures in the charge -density-wave (CDW) conductor NbSe3. This creep develops when CDW motion be comes limited by thermally activated phase advance past individual impuriti es, demonstrating the importance of local pinning and related short-length- scale dynamics. Unlike in vortex lattices, elastic collective dynamics on l onger length scales results in temporally ordered motion and a finite thres hold field. A first-order dynamic phase transition from deep to high-veloci ty sliding produces "switching" in the velocity-field characteristic.