Charge-Density Transport in Quasi - One-Dimensional Conductors.

A detailed experimental study of the remarkable collective-transport properties of charge-density-wave (CDW) conductors is presented. Measurements of current oscillations observed in response to applied dc voltages and of mode-locking phenomena observed in response to combined ac and dc voltages indicate a nearly complete analogy with similar effects observed in Josephson junctions. Even though these effects result from CDW interaction with randomly -distributed impurities, they are extraordinarily clean and beautiful: current-oscillation spectra with 23 harmonics and spectral widths of one part in 30,000 and literally hundreds of subharmonic mode-locked features in the dc I-V characteristic are observed. However, crystal defects other than impurities can significantly degrade the measured properties, and play a determining role in the generation of broad-bandwidth noise. A single-coordinate model which assumes overdamped CDW motion in a nonsinusoidal periodic impurity potential provides a good qualitative and semi-quantitative account of current oscillation and ac-dc interference phenomena, and also allows a transparent physical interpretation. Calculations based on the classical deformable-medium model reproduce some qualitative features of experiment, but some experiments are in complete and dramatic disagreement with these calculations. The quantum tunneling model provides a detailed and quantitative account of a wide variety of experiments. Predicted forms for the dc and ac conductances are in excellent agreement with measurements over two orders of magnitude in field and frequency and over a broad temperature range. Values of microscopic parameters deduced from these measurements using tunneling model expressions are consistent with values deduced from independent measurements. Most importantly, a number of experiments clearly establish that the magnitude of the periodic impurity potential is independent of field and frequency. This finding appears to be in fundamental disagreement with the classical deformable-medium model, but confirms the basic assumption of the tunneling model: that CDW interaction with impurities cannot be treated perturbatively, but must be included in zero order.