Low-temperature dielectric response of beta-alumina at 16 GHz
Smith, Douglas Lowell
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https://hdl.handle.net/2142/21452
Description
Title
Low-temperature dielectric response of beta-alumina at 16 GHz
Author(s)
Smith, Douglas Lowell
Issue Date
1989
Doctoral Committee Chair(s)
Stapleton, H.J.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, General
Physics, Condensed Matter
Language
eng
Abstract
Microwave techniques are used to measure the dielectric response of M$\sp+$ beta alumina at 16 GHz for temperatures between 0.3 and 20K. (M$\sp+$ refers to the monovalent cations Na, K, Ag, or Li.) These measurements include cw observations of the absorption and the dispersion, and the recovery of the absorption signal following a high power pulse of microwaves. The cw measurements at 16 GHz are shown to exhibit qualitative features similar to those observed at audio frequencies (and at 10 GHz for Na) in beta alumina, which have been shown to be characteristic of highly disordered systems.
In this temperature range the dielectric response in glasses is typically explained by modeling the structural disorder in terms of a broad energy independent distribution of localized atomic tunneling centers. In the case of beta alumina's unique structure this tunneling occurs within disordered planes separated by crystalline blocks of alumina. The tunneling theory has been used to explain both the frequency and temperature dependence of the observed dielectric response.
We demonstrate that although the theory can be used to fit either the loss or dispersion at 16 GHz, they cannot be fit simultaneously with the same set of parameters. Since the theory predicts a linear and causal response, this indicates that the theoretical relationship between the absorption and the loss (based on the Kramers-Kronig relationships) is inconsistent with the observed results. Our measurements of the pulsed saturation recoveries, which probe the mechanism that mediates the dielectric response and therefore should clarify the problem, are shown to be inconclusive. They are, however, consistent with results obtained at radio frequencies which have been explained in terms of spectral diffusion. Finally, we demonstrate that the extension of the theory to the dielectric data above $\sim$0.5K is not supported by other low temperature properties for which the theory was originally created. This indicates either the theory needs to be modified or additional physical processes need to be included.
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