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@@ -269,12 +269,10 @@ The method only takes the vibration directions of each atom in each phonon mode,
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and the result was summarized in @table-predmode.
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In the Raman tensors in @table-predmode,
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$a_i$ corresponding to the change of polarizability caused by movement of the Si atoms in A and C layers,
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$epsilon_i$ and $eta_i$ corresponding to the difference between the A/C layers and B layers,
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and $eta_i$ corresponding the difference between the Si and C atoms.
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The absolute values of $a_i$ is expected to be much larger than that of $epsilon_i$, $eta_i$ and $zeta_i$,
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$epsilon_i$, $eta_i$ and $eta_i$ corresponding to the difference between different bilayers and different atoms.
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Due to the similarity of environment in different bilayers and around different atoms,
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the absolute values of $epsilon_i$, $eta_i$ and $zeta_i$ are expected to be much smaller than that of $a_i$,
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thus the Raman tensors containing $a_i$ are expected to be much larger than those not containing $a_i$.
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It could be seen that,
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our prediction is mostly consistent with the first principle calculation and experiment.
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// Raman Tensor for A1: line1 xx/yy; line2 zz
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// Raman Tensor for E1: x-dirc xz or y-dirc yx
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@@ -324,27 +322,29 @@ It could be seen that,
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placement: none,
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)<table-predmode>]
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as well as the frequencies
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The Raman tensors and frequencies of the negligible-polar phonons were calculated using first-principles methods,
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and the results are compared with experiment and theory (@table-nopol).
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Calculated frequencies of these phonons are consistent with the experimental results
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with a low-estimated error of about 2% to 5%, which might be due to the PBE functional used in the calculation (cite).
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The Raman tensors of these phonons are also consistent with the experimental and theoretical results,
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where E#sub[2] mode experimentally at 776 is the most intense phonon mode,
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followed by four modes with lesser intensities
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(E#sub[2] modes at 195.5 and 203.3, E#sub[1] mode at 269.7, A#sub[1] mode at 609.5).
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The Raman scatter of the E#sub[1] mode calculately at 746.91 and E#sub[2] mode calculately at 756.25
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are much weaker than the E#sub[2] mode calculated at 756.25 but located near it, according to our calculation,
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thus it could not be distinguished from E#sub[2] mode calculated at 756.25,
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which explains why they are not observed in experiments.
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Moveever, the A#sub[1] mode calculated at 812.87
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have a very weak Raman intensity in the basal plane (xx and yy, only 0.01)
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but an observable intensity in the zz configuration (1.78).
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Thus, this mode could not be observed in most Raman experiments (cite),
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but could be observable when incident light propagate not along the z-direction (our experiment),
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or the incident light wavelength is near the resonance condition (cite).
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,
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except for the E#sub[2] mode experimentally at 200 cm#super[-1], which we expected to be at higher frequencies.
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// 我们计算了拉曼活性声子的频率及拉曼张量,并与实验对比,如表如图所示。
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// 其中有几个声子的拉曼活性较弱,有几个比较强。强的都可以在实验上看到;但弱的能否看到则取决于它是否恰好位于强模式的附近。
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// 其中,xxx 和xxx 位于强模式的附近,它们在实验上无法看到;xxx 只在 z 方向入射/散射时可以看到;xxx 则在任意方向都能看到。
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// 我们同样计算了这些声子在 300K 下的展宽,并与实验对比,结果如表所示。原子的振幅另外列于附录中。
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The Raman tensors of these Raman-active phonons were calculated using first-principles methods,
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and the results are summarized and compared with experimental results in @table-nopol.
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Two Raman-active modes are not observed in our experiments,
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including the E#sub[1] mode at 746.91 cm#super[-1] and the E#sub[2] mode at 764.33 cm#super[-1],
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due to their relatively low Raman intensities, broad FWHM values, and their proximity to stronger modes.
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The A#sub[1] phonon at 812.87 cm#super[-1] is Raman-active
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in both in-plane (xx and xy) and out-of-plane (zz) polarization configurations,
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but it is only visible when both the incident and scattered light propagate along the z-direction (zz),
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as its Raman intensity in basal plane is too week to be distinguished from the noise.
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We also calculated the linewidths of these phonons at 300 K and compared them with experimental results,
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as summarized in the @table-nopol.
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The atomic vibration amplitudes are listed separately in the Appendix.
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Besides, there are other peeks in the experiment.
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The peek at 796 and 980 are caused by strong-polar phonons which will be discussed later.
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Besides, there are small peeks at xxx,
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which could not be explained in perfect 4H-SiC and will be discussed in the next section.
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// TODO: 将一部分 phonons 改为 phonon modes
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// 在论文中我们这样来称呼:phonon 对应某一个特征向量,而 modes 对应于一个子空间。
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