Probes on the Lower Charge Density Wave State of Niobium Triselenide: Magnetic Field, Temperature Gradient, and Pulsed Electric Field Effects

An investigation of the lower-temperature charge -density-wave (CDW) state of NbSe_3 was performed using three different techniques. An investigation of the anomalous magnetoresistance seen in NbSe_3 was performed. Specifically, measurements were made to detect possible magnetic-field -induced conversion between normal and CDW carriers. To this end, the frequency-dependent conductivity was measured as a function of the magnetic field. Also examined was the proportionality constant between the narrow-band-noise (NBN) frequency and the CDW current; this constant is expected to depend on the CDW carrier concentration. In a separate experiment, the resistance increase caused by application of uniaxial stress, which has characteristics similar to the magnetic field effects, was examined in conjunction with a magnetic field for possible interactions between the two phenomena. In crystals with a temperature difference DeltaT across the sample, a detailed investigation was performed on the splitting of a sliding CDW into velocity "sub-domains". Phase slip centers (PSC) are expected to occur between these sub-domains. A theory is developed based on the minimization of CDW phase strain and PSC energies. It predicts scaling behavior between the number of sub-domains and DeltaT and that the dynamic properties of the CDW are sensitive to the relative directions of the heat and electrical currents. Experiments testing this model were performed and the results show very good agreement with the model. The transient voltage response to current pulses was measured. In isothermal samples, the transient response consists of a ringing at the NBN frequency. The ringing amplitude was measured as a function of the initial bias level and shows a sharp peak centered slightly above zero initial bias. In another experiment, the ringing amplitude was measured as a function of zero bias duration. The results indicate that the CDW repins in approximately 80 nanoseconds. The transient response also was measured in samples with a temperature gradient in order to examine the formation of sub-domains and PSC's. Detailed analysis is performed in both the time and frequency domains and shows that sub -domains form extremely fast or already "pre-exist" prior to the application of the pulse.