Classical versus quantum models of charge-density-wave depinning in quasi-one-dimensional metals

A critique is given of classical models of depinning and transport by sliding charge-density waves (CDW) in quasi-one-dimensional metals. A considerable body of evidence supports a theory based on quantum tunneling over macroscopic distances, particularly on ${\mathrm{NbSe}}_3$ and ${\mathrm{TaS}}_3$. No sound prediction of the classical approach is consistent with these data. All evidence indicates that it is necessary to treat CDW metals as macroscopic quantum systems with quantum tunneling as an essential feature. Classical concepts are useful for memory, hysteresis, and low-frequency response in the pinned state below threshold. They can only be used for phenomena on length scales larger than the Lee-Rice domain length in the sliding regime above threshold.