67 lines
3.9 KiB
XML
67 lines
3.9 KiB
XML
== Phonons in Perfect 4H-SiC
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不同方向入射光的声子位置不同。
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声子的性质与它们在倒空间中的位置（即波矢）有关，而参与拉曼散射的声子波矢则由激光波长和拉曼入射配置（正入射、掠入射、边入射）决定。
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在我们的实验（绿光）中，参与拉曼散射的声子都位于 Gamma 点附近（距离仅 xxx 埃），而与 Gamma 点的相对方向则根据入射配置而定。
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对于正入射，声子大致位于 Gamma-A 线上；对于边入射，声子大致位于 Gamma-K 线上。对于掠入射，声子不位于任何高对称线上。
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Properties of phonons depend on their positions in reciprocal space (i.e., wavevectors),
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and the wavevectors of phonons participating in a Raman scattering process
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are determined by the Raman experiment configuration.
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In our experiments (back-scattering with 532 nm laser light),
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phonons involved in Raman scattering are all located near the #sym.Gamma point
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(about $5.13 times 10^(-3) angstrom$ from the #sym.Gamma point),
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and their relative directions from the #sym.Gamma point depend on the incidence configurations.
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For normal and edge incidence,
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phonons participating in Raman process are approximately located along the #sym.Gamma–A and #sym.Gamma–K lines,
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respectively, as illustrated in @figure-discont.
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For grazing incidence, phonons are not located on any high-symmetry lines, and not shown in the figure.
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#include "figure-discont.typ"
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每个点都有 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 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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这个分类基于这样的事实：
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在 4H-SiC 的原胞中，四个 Si 原子携带类似的正的 Born 有效电荷（BEC），
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而四个 C 原子携带类似的负的 BEC（见 @table-bec）。
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在 18 个弱极性声子中，
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两个 Si 原子的振动方向大致与另外两个 Si 原子相反，
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C 原子也是如此，
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导致宏观极性的抵消。
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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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