Nonlinear ac dynamics of pinned charge-density waves: Third-harmonic generation

We report the observation of third-harmonic generation in ac-field-driven charge-density waves pinned by impurities. Over a broad range of fields E0 and frequencies, the third-harmonic response (3) is a universal function of and E0, (3)g(E0) with <1, and consequently (3) diverges as 0. The relation of these findings to calculations based on random-field models will be discussed.