Switching and charge-density-wave transport inNbSe3. I. dc characteristics

The phenomenon of switching is studied in the lower charge-density-wave (CDW) state of ${\mathrm{NbSe}}_3$ by measurements of dc conductivity. Results show that switching is distinguished from nonswitching CDW transport by discontinuities in CDW current, large polarization before the onset of CDW motion, and an unusual temperature dependence of the electric fields necessary to initiate CDW motion. Furthermore, switching behavior is found to originate within the crystalline bulk of ${\mathrm{NbSe}}_3$. Switching is not caused by surface defects or external electrical contacts, but is instead produced by strong, nonuniform pinning of a CDW. Standard models of CDW transport cannot explain the experimental results. In particular such models neglect fluctuations of the CDW amplitude, but the observation of current discontinuities indicates that amplitude fluctuations occur within switching crystals. We suggest that amplitude fluctuations, caused by order-parameter oscillations at phase-slip centers, produce a breakdown in pinning that leads to switching behavior. We discuss the possible origin of phase-slip centers in ${\mathrm{NbSe}}_3$, and define two length scales that, in conjunction with the Fukuyama-Lee-Rice phase coherence length, set a criterion for the occurrence of switching.