Experiment versus the classical model of deformable charge-density waves: Interference phenomena and mode locking

The differential resistance of ${\mathrm{NbSe}}_3$ measured in the presence of combined ac and dc electric fields exhibits interference features. Our experimental results for the dependence of these features on ac amplitude and frequency are in complete and dramatic disagreement with predictions of the classical model of deformable charge-density waves, and prove that pinning by impurities does not vanish at large fields and frequencies.