29 lines
1.6 KiB
XML
29 lines
1.6 KiB
XML
(There are 21 phonons in total.
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We classified them into two categories: 18 negligible-polar phonons and 3 strong-polar phonons.)
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// 拉曼活性的声子模式对应于 Gamma 点附近的声子模式。
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// 根据这些声子模式的极性，我们将这些声子分成两类。
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The phonons involved in Raman scattering are located in reciprocal space around the #sym.Gamma point,
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at the exact positions are determined by the wavevectors of the incident and scattered light.
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At each such position, there are 21 phonon modes (degenerate modes are counted as their multiplicity).
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We classify these 21 phonons into two categories based on their polarities.
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The 18 of 21 phonons are classified into negligible-polar phonons (i.e., phonons with zero or very weak 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 phonons are strong-polar phonons,
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where the polarity gives rise to observable effects in the Raman spectra.
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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 "bec.typ"
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