Brillouin scattering study of incommensurate potassium selenate (acoustic anomalies and the Landau free energy).
Item
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Title
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Brillouin scattering study of incommensurate potassium selenate (acoustic anomalies and the Landau free energy).
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Identifier
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AAI9119650
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identifier
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9119650
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Creator
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Li, Gen.
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Contributor
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Adviser: Herman Z. Cummins
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Date
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1991
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Language
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English
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Publisher
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City University of New York.
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Subject
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Physics, Condensed Matter | Physics, Acoustics | Physics, Optics
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Abstract
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Brillouin scattering studies of {dollar}\rm K\sb2SeO\sb4{dollar} in the vicinity of the incommensurate and commensurate phase transitions were performed and analyzed in the following aspects: (1) The longitudinal acoustic mode propagating along the c* axis was investigated both in 90{dollar}\sp\circ{dollar} and 180{dollar}\sp\circ{dollar} scattering geometries. A theoretical derivation of the complex elastic constant {dollar}{lcub}\tilde C{rcub}\sb{lcub}33{rcub}(\omega){dollar} was carried out in the framework of Landau theory including both bilinear (Landau-Khalatnikov) coupling to the amplitude mode, and fluctuation contributions from the {dollar}\Sigma\sb2{dollar} soft mode above {dollar}\rm T\sb i{dollar} and from pairs of amplitude modes and phase modes below {dollar}\rm T\sb i{dollar}. The contribution of phase modes was considered for the first time in this analysis. Comparison between theory and the Brillouin data led to an excellent fit using free energy parameters close to values deduced from previous static and dynamic experimental results. This analysis also indicated that the phason gap {dollar}\Omega\sb\phi{dollar}(0) is at least 100 GHz. Analysis beyond the mean-field approximation was also discussed. (2) Transverse acoustic modes propagating and polarized the crystal axes were investigated in 90{dollar}\sp\circ{dollar} scattering geometries. The C{dollar}\sb{lcub}44{rcub}{dollar} anomalies above and below {dollar}\rm T\sb i{dollar} were tentatively explained by bilinear coupling with the lowest {dollar}\rm B\sb{lcub}3g{rcub}{dollar} optical mode. Raman scattering studies of this {dollar}\rm B\sb{lcub}3g{rcub}{dollar} mode indicated a variation of frequency with temperature which is similar to the variation of C{dollar}\sb{lcub}44{rcub}{dollar}. The C{dollar}\sb{lcub}55{rcub}{dollar} anomaly near {dollar}\rm T\sb c{dollar} was explained by coupling to the phase mode with wavevector of {dollar}\rm K\sb i{dollar} = a*{dollar}\delta{dollar}(T). The C{dollar}\sb{lcub}66{rcub}{dollar} anomaly near {dollar}\rm T\sb i{dollar} was also analyzed with Landau theory. Theoretically predicted transverse acoustic asymmetries due to the coupling with phase modes were also explored.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
