42 lines
2.5 KiB
XML
42 lines
2.5 KiB
XML
== Phonons in Perfect 4H-SiC
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总共有 21 个声子模式。我们把它分成 18 个极性很小的和 3 个极性很强的两类。
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对应于不同拉曼入射配置（正入射、掠入射、边入射）的声子位于倒空间中的不同位置。
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它们都位于 Gamma 点附近，与 Gamma 点的距离由入射光波长决定，而与 Gamma 点的相对位置则由入射方向决定。
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对于每一个 Gamma 附近的点，都有 21 个声子模式（简并模式按简并数计）。
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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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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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// This classification make sense.
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This classification is based on the fact that
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the four Si atoms in the primitive cell of 4H-SiC carry similar positive Born effective charges (BECs),
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and the four C atoms carry similar negative BECs (see @table-bec).
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In the 18 negligible-polar phonons,
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the vibrations of two Si atoms are approximately opposite to those of the other two Si atoms,
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and the same holds for the C atoms,
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leading to cancellations of macroscopic polarity.
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In contrast, in the three strong-polar phonons,
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all Si atoms vibrate in the same direction, and all the C atoms vibrate in the opposite direction,
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resulting in a strong dipole moment.
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#include "table-bec.typ"
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#include "non-polar/default.typ"
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#include "polar/default.typ"
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