The Role of Electrostatic Screening in Charge-Density Wave Dynamics

A moving charge-density wave (CDW) is subject to damping forces resulting from interactions of the CDW with its environment. Thus far the damping has been measured two ways, by analyzing the high frequency conductivity at frequencies high enough to see the inertial roll-off due to the CDW mass and by an analysis of the temperature dependence of the dc conductivity. In an apparent paradox, the two methods give contradictory answers. The high-frequency measurements give a temperature independent damping while the dc results indicate that the damping is diverging at low temperatures. The two results can be reconciled by answering the effects of electrostatic screening. At low frequencies, motion of the CDW is accompanied by time-dependent fluctuations and screening currents of normal electrons. This leads to the conclusion that the dominant low-frequency loss mechanism is the resistive dissipation due to the normal electrons. As the temperature is lowered and the number of activated carriers goes to zero, the damping diverges. At high frequencies, there are not sufficient carriers to screen the applied field and all parts of the CDW see the same electric field. Consequently, local distortions of the CDW play less of a role at high frequencies and damping results from other mechanisms, e.g. thermally activated phasons. These high-frequency damping processes are not well understood, but they likely have much weaker temperature dependence than the damping resulting from screening.