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我们将这 21 个声子模式分为两类:18 个极性可以忽略的声子模式(即极性为零或非常弱的声子),在拉曼散射过程中它们的电极性造成的效应可以忽略;
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和 3 个强极性声子模式,在拉曼光谱中电极性效应是可观测的。
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Phonons involved in different Raman incidence configurations (normal, grazing, and edge incidence)
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are located at different positions in reciprocal space,
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all of which are near the #sym.Gamma point,
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with the distance from the #sym.Gamma point determined by the wavelength of the incident light,
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and the relative position to the #sym.Gamma point determined by the direction of incidence.
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For each such position around the #sym.Gamma point,
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there are 21 phonon modes (degenerate modes are counted according to their multiplicity).
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We classify these 21 phonon modes into two categories:
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18 negligible-polar phonons (i.e., phonons with zero or very weak electrical polarity),
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for which the effect of polarity can be ignored in the Raman scattering process;
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and the other three strong-polar phonons,
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where the polarity gives rise to observable effects in the Raman spectra.
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Phonons involved in Raman scattering under different incidence configurations (normal, grazing, and edge incidence)
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are located at distinct positions in reciprocal space,
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all close to the #sym.Gamma point.
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The distance from the #sym.Gamma point is determined by the wavelength of the incident light,
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and the direction from the #sym.Gamma point is determined by the incidence configurations.
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At each such position near the #sym.Gamma point,
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there exist 21 phonon modes (with degeneracies counted accordingly).
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These modes can be categorized into two groups:
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18 negligible-polar phonons (i.e., zero or very weak electrical polarity),
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whose electrical polarity can be disregarded in the Raman scattering process;
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and three strong-polar phonons,
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whose electrical polarity leads to observable effects in the Raman spectra.
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这个分类是有意义的。从微观上讲,它也和原子的振动方向有关。
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