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@@ -125,7 +125,7 @@ Phonons at the #sym.Gamma point were used
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This approximation is widely adopted and justified by the fact that,
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although the phonons participating in Raman processes are not these strictly located at the #sym.Gamma point,
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their dispersion near the #sym.Gamma point is continuous with vanishing derivatives,
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and their wavevector is very small (about 0.01 nm#sub([-1]) in this paper,
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and their wavevector is very small (about 0.01 nm#super[-1] in this paper,
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which corresponds to only 1% of the smallest reciprocal lattice vector of 4H-SiC),
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as shown by the orange dotted line in @phonon.
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Therefore, negligible-polar phonons involved in Raman processes
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@@ -138,12 +138,25 @@ There are 21 distinct phonons at the #sym.Gamma point in 4H-SiC.
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Among them, 18 phonons are classified as negligible-polar phonons.
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This classification is based on the fact that
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the four Si atoms in the primitive cell carry similar Born effective charges (BEC),
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as do the four C atoms. // TODO: refer
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as do the four C atoms, as shown in @bec.
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In these 18 modes, the vibrations of two Si atoms are approximately opposite to those of the other two Si atoms,
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and similarly for the C atoms,
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leading to cancellations of macroscopic polarity.
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These phonon modes correspond to twelve irreducible representations of the $\mathrm{C_{6v}}$ point group: $\mathrm{3A_1 + 4B_1 + 3E_1 + 4E_2}$.
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#figure(
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table(columns: 4, align: center + horizon,
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table.cell(colspan: 2)[], table.cell(colspan: 2)[*BEC*],
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table.cell(colspan: 2)[], [x / y direction], [z direction],
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table.cell(rowspan: 2)[Si atom], [A/C layer], [2.667], [2.626],
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[B layer], [2.674], [2.903],
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table.cell(rowspan: 2)[C atom], [A/C layer], [-2.693], [-2.730],
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[B layer], [-2.648], [-2.800],
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),
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caption: [Born effective charges of Si and C atoms in A/B/C/B layers of 4H-SiC.]
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)<bec>
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// These phonon modes correspond to twelve irreducible representations of the $\mathrm{C_{6v}}$ point group: $\mathrm{3A_1 + 4B_1 + 3E_1 + 4E_2}$.
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18 of them are classified as negligible-polar phonons,
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