Two-phonon bound states in imperfect crystals |
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Authors: | S N Behera Sk Samsur |
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Institution: | (1) Institute of Physics, A/105 Saheed Nagar, 751 007 Bhubaneswar, India |
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Abstract: | The question of the occurrence of two-phonon bound states in imperfect crystals is investigated. It is shown that the anharmonicity
mediated two-phonon bound state which is present in perfect crystals gets modified due to the presence of impurities. Moreover,
the possibility of the occurrence of a purely impurity mediated two-phonon bound state is demonstrated. The bound state frequencies
are calculated using the simple Einstein oscillator model for the host phonons. The two-phonon density of states for the imperfect
crystal thus obtained has peaks at the combination and difference frequencies of two host phonons besides the peaks at the
bound state frequencies. For a perfect crystal the theory predicts a single peak at the two-phonon bound state frequency in
conformity with experimental observations and other theoretical calculations. Experimental data on the two-phonon infrared
absorption and Raman scattering from mixed crystals of GA1−c
Al
c
P and Ge1−c
Si
c
are analysed to provide evidence in support of impurity-mediated two-phonon bound states. The relevance of the zero frequency
(difference spectrum) peak to the central peak observed in structural phase transitions, is conjectured;
This work is a part of the thesis to be submitted by one of the authors (SS) in partial fulfilment of the degree of Doctor
of Philosophy to Utkal University, Bhubaneswar, India. |
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Keywords: | Two-phonon bound states imperfect crystals anharmonic crystals phonon-phonon interactions combination modes |
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