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== 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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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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@@ -24,6 +42,12 @@ These modes can be categorized into two groups:
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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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@@ -16,8 +16,6 @@ This approximation is widely adopted (cite) and justified by the fact that,
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Therefore, negligible-polar phonons involved in Raman processes
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have nearly indistinguishable properties from those at the #sym.Gamma point.
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#include "figure-discont.typ"
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gamma 点处 18 个声子的表示。它们的拉曼张量的形状可以确定,但大小无法确定。
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// Representation of these 18 phonons, and the shape of their Raman tensors could be determined in advance.)
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