An ultrasonic study of hydroxyl ion in single crystals of rubidium chloride
Ho, Wai Kwong Bernard
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https://hdl.handle.net/2142/25379
Description
Title
An ultrasonic study of hydroxyl ion in single crystals of rubidium chloride
Author(s)
Ho, Wai Kwong Bernard
Issue Date
1983
Doctoral Committee Chair(s)
Granato, A.V.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
ultrasound
hydroxyl ion
rubidium chloride
single crystals
Language
en
Abstract
Ultrasonic measurements o~ absorption and velocity changes of sound waves on single crystals of RbCl containing OH ions were made as a function of polarization, frequency, concentration and temperature from 0.6 K to 24 K. A rise in absorption of the low frequency (10 to 70 MHz) Cmode sound waves has been observed at low temperll
atures. A similar anomalous absorption has been observed in other defect systems, but they were treated as a relaxation effect. The anomalous absorption is identified for the first time to be a resonance effect. A six-level model is presented for the evaluation, as a function of stress, of the energy levels and matrix elements. It is found that low frequency resonance absorption occurs in RbCl:OHbecause there are arbitrarily close levels in the six-level system that are coupled by acoustic phonons. To simplify the discussion, a two level system (TLS) is used to obtain results for the ultrasonic absorption and velocity changes due to resonance and relaxation interactions of sound waves with the defect. By means of a TLS, the physics of relaxation and para-effects on the one hand, resonance and dia-effects of defects on the other hand, is clarified. Relaxation or para-effects are the restoration to thermal equilibrium by thermal fluctuation after the system has been disturbed. Resonance or diaeffects are the direct response of the system to external disturbances without regard to thermal equilibrium. A detailed comparison of
measurements with the predictions for a TLS model shows that the relaxation effects measured are as expected, with a relaxation rate changing from a power law to a linear temperature dependence below 3 K. The magnitude of the low temperature rate, however, is about 17 times larger than that obtained from electro-optical measurements for this system. The resonance effects for the modulus change are also as expected, but the absorption has a frequency dependence which lies between that expected for an isolated defect and a glassy state. It is believed that the discrepancies result from the failure of a TLS model to adequately account for a six-level system. A realistic model needs also to take account of distributions of internal strains, electric fields and tunnelling parameters.
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