Charge-density-wave transport in quasi-one-dimensional conductors. II. ac-dc interference phenomena

We present a detailed experimental study of ac-dc interference phenomena in charge-density-wave (CDW) transport. Harmonic and subharmonic ‘‘steps’’ of constant CDW current are observed when ac and dc voltages are applied together, and the widths of these steps exhibit Bessel-like oscillations as a function of both ac amplitude and frequency. We now observe similar oscillations in the dc threshold field, and thereby complete the analogy between these phenomena and similar effects in Josephson junctions. By minimizing crystal defects and contact effects, we have obtained complete harmonic and subharmonic phase locking in very long ${\mathrm{NbSe}}_3$ crystals under a broad range of conditions, including cases where the total time-dependent voltage remains above the dc threshold $V_T$. We analyze our experimental results with a single-coordinate model used previously to account for subharmonic steps. It is based on the tunneling theory and assumes CDW motion in a highly nonsinusoidal pinning potential. This model provides an excellent and semiquantitative account of all our results; it also provides a detailed interpretation of interference features observed in the ac conductance, electric-field-induced anomalies observed in the CDW elastic response, and the inductive loops observed in phase-space plots of the CDW’s response to ac fields. The quality of the agreement provides strong evidence both for an effective pinning potential of the proposed form and for a single-degree-of-freedom description of CDW transport.