Microwave Conductivity Measurements of Anisotropic Conductors.
The properties of anisotropic conductors are probed with the microwave perturbation technique. Presented are measurements of Hollandite, Mn doped Hollandite, tantalum trisulphide, tantalum triselenide, niobium trisulphide, zirconium and hafnium pentatelluride, and vanadium pentoxide doped with sodium. The perturbation equations are derived and extended into the Rayleigh limit where the product of the sample diameter and wavevector is small. A criterion for the validity is developed, and regions of accuracy are determined for the application of the microwave perturbation technique. The 1-d superionic conductor Hollandite displayed a low temperature (LT) activation energy of .036 eV, a room temperature (RT) conductivity of .8/(ohm-cm), a RT dielectric constant of 200, a RT conductivity anisotropy ratio of 360, a Mn hyperfine constant of -3.9 G/100 K, and a LT ESR linewidth of 45 G which exhibits the onset of motional narrowing at 345 (+OR-) 20 K. The LT activation energy is explained by a model of correlated ionic motion, and the increased conductivity and slope above room temperature is due to the cross over from the correlated ionic motion model to the model of transport in a disordered system. The ESR linewidth narrowing temperature is explained in terms of a 1-d model; the effects of re-encounters are shown to be more important in 1-d than in 3-d. Niobium trisulphide exhibits a smaller dielectric constant than tantalum trisulphide, and 1-d fluctuations near the transition temperature, 155 K. Tantalum trisulphide exhibits a dielectric constant of 1000, and 1-d fluctuations near the transition temperature, 215 K. Tantalum triselenide exhibits no transition or frequency dependent conductivity above 5 K. The binary pentatellurides exhibit a frequency dependent conductivity anomaly; the ternary pentatellurides, a reduced anomaly. These field and frequency dependent properties are compared to other known CDW materials. The pentatelluride measurements are consistant with the appearance of a CDW state, while the trichalcogenide measurements (in addition to other published evidence) are clearly indicative of a CDW state.
- Pub Date:
- Physics: Condensed Matter