Measurements have been made of the X-ray scatter from an MgO single crystal at low temperatures irradiated by an infra-red laser beam of wavelength 10.6μm. The purpose of these measurements was to obtain information about the anharmonic coupling between the lattice vibrations. The modes investigated were the transverse acoustic phonons in the [100] direction, which it has been suggested have anomalously long lifetimes at very low temperatures. The above method was chosen after considering a variety of possible excitation methods including electron-phonon interactions via the piezoelectric effect and the deformation potential. These other processes had to be rejected because they either excited phonons which were unsuitable for X-ray measurement or required pulsed experiments. The dispersion curves for MgO, obtained from a deformable shell model for the lattice were analysed to provide information as to the most likely positions in reciprocal space for phonon population enhancement to occur. The decay routes for the primary phonons excited directly by the infra-red radiation were predicted from the dispersion curves. A major part of the work involved the design and construction of the helium cryostat on which the sample crystal was mounted and kept at as low a temperature as possible. The standard X-ray detection technique of a scintillating crystal-photomultiplier combination was employed and the infra-red beam was produced by a CO2 gas laser with an output power of up to lOW. Because of the nature of the measurements, the results obtained for the change in X-ray scatter by the MgO crystal when the infra-red beam was shone on the crystal required analysis by statistical methods in order to provide the maximum information. It was possible to set an upper limit to the effects found of a few percent, and to compare the orders of magnitude of the coupling for two different processes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:592112 |
| Date | January 1971 |
| Creators | Barron, Hugh Wilson Taylor |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=227101 |
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