ac-induced dc conductivity in the charge-density-wave state in NbSe3

We report the observation of dc conductivity induced by ac excitation of large amplitude in the charge-density-wave state in Nb${\mathrm{Se}}_3$. We find a crossover from strong ac-dc coupling at low frequencies to weak coupling at high frequencies. These observations are in clear contrast with carrier excitations across a (small) gap, and we account for our findings by arguments based on depinning of charge-density waves.