Frequency Dependent Transport in Charge Density Wave Systems.

This dissertation is an investigation of the electrodynamic response of the charge density waves (CDW) found in several inorganic compounds. A CDW is a collective state formed when the lattice in a quasi-one-dimensional metal spontaneously distorts below a transition temperature. In the distorted state a gap occurs in the electron spectrum and the material is a semiconductor. An electric field will drive a collective motion of the CDW but interactions with impurities lead to pinning and a response only occurs at finite fields or finite frequencies. In these materials the response occurs at microwave and millimeter-wave frequencies. Two experimental techniques are used in this study. In the first technique we place a sample in a resonant cavity and calculate the complex conductivity from the change in resonant frequency and quality factor. We developed the second technique for this study. We use a millimeter -wave bridge to measure the complex impedance of a section of waveguide containing the sample, and then calculate the complex conductivity from the impedance. These measurements were performed on four materials NbSe_3, Orthorhombic TaS _3, (TaSe_4) _2I, and K_{0.3} MoO_3 and in each case we identified all the fundamental parameters of the response. One parameter, the effective mass, agrees with theory. A doping study of TaS_3 showed that the linewidth was independent of impurity concentration (i.e. intrinsic), while the pinning frequency was found to increase sharply with impurity concentration in agreement with the concept of impurity pinning. At high temperatures the resonance contains a shoulder on the low frequency edge that is understood to arise from quasi-particle backflow within the Macmillan -Littlewood theory. At low temperature the resonance is also not harmonic oscillator and evolves to a gap-like appearance. The linewidth also has a temperature and impurity independent component. These features imply that the spectrum may be that of a band above a bandgap, and require a fundamentally quantum mechanical theory, such as that proposed by Bardeen.