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@@ -269,12 +269,10 @@ The method only takes the vibration directions of each atom in each phonon mode,
and the result was summarized in @table-predmode.
In the Raman tensors in @table-predmode,
$a_i$ corresponding to the change of polarizability caused by movement of the Si atoms in A and C layers,
$epsilon_i$ and $eta_i$ corresponding to the difference between the A/C layers and B layers,
and $eta_i$ corresponding the difference between the Si and C atoms.
The absolute values of $a_i$ is expected to be much larger than that of $epsilon_i$, $eta_i$ and $zeta_i$,
$epsilon_i$, $eta_i$ and $eta_i$ corresponding to the difference between different bilayers and different atoms.
Due to the similarity of environment in different bilayers and around different atoms,
the absolute values of $epsilon_i$, $eta_i$ and $zeta_i$ are expected to be much smaller than that of $a_i$,
thus the Raman tensors containing $a_i$ are expected to be much larger than those not containing $a_i$.
It could be seen that,
our prediction is mostly consistent with the first principle calculation and experiment.
// Raman Tensor for A1: line1 xx/yy; line2 zz
// Raman Tensor for E1: x-dirc xz or y-dirc yx
@@ -324,27 +322,29 @@ It could be seen that,
placement: none,
)<table-predmode>]
as well as the frequencies
The Raman tensors and frequencies of the negligible-polar phonons were calculated using first-principles methods,
and the results are compared with experiment and theory (@table-nopol).
Calculated frequencies of these phonons are consistent with the experimental results
with a low-estimated error of about 2% to 5%, which might be due to the PBE functional used in the calculation (cite).
The Raman tensors of these phonons are also consistent with the experimental and theoretical results,
where E#sub[2] mode experimentally at 776 is the most intense phonon mode,
followed by four modes with lesser intensities
(E#sub[2] modes at 195.5 and 203.3, E#sub[1] mode at 269.7, A#sub[1] mode at 609.5).
The Raman scatter of the E#sub[1] mode calculately at 746.91 and E#sub[2] mode calculately at 756.25
are much weaker than the E#sub[2] mode calculated at 756.25 but located near it, according to our calculation,
thus it could not be distinguished from E#sub[2] mode calculated at 756.25,
which explains why they are not observed in experiments.
Moveever, the A#sub[1] mode calculated at 812.87
have a very weak Raman intensity in the basal plane (xx and yy, only 0.01)
but an observable intensity in the zz configuration (1.78).
Thus, this mode could not be observed in most Raman experiments (cite),
but could be observable when incident light propagate not along the z-direction (our experiment),
or the incident light wavelength is near the resonance condition (cite).
,
except for the E#sub[2] mode experimentally at 200 cm#super[-1], which we expected to be at higher frequencies.
// 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。
// 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。
// 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。
// 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。
The Raman tensors of these Raman-active phonons were calculated using first-principles methods,
and the results are summarized and compared with experimental results in @table-nopol.
Two Raman-active modes are not observed in our experiments,
including the E#sub[1] mode at 746.91 cm#super[-1] and the E#sub[2] mode at 764.33 cm#super[-1],
due to their relatively low Raman intensities, broad FWHM values, and their proximity to stronger modes.
The A#sub[1] phonon at 812.87 cm#super[-1] is Raman-active
in both in-plane (xx and xy) and out-of-plane (zz) polarization configurations,
but it is only visible when both the incident and scattered light propagate along the z-direction (zz),
as its Raman intensity in basal plane is too week to be distinguished from the noise.
We also calculated the linewidths of these phonons at 300 K and compared them with experimental results,
as summarized in the @table-nopol.
The atomic vibration amplitudes are listed separately in the Appendix.
Besides, there are other peeks in the experiment.
The peek at 796 and 980 are caused by strong-polar phonons which will be discussed later.
Besides, there are small peeks at xxx,
which could not be explained in perfect 4H-SiC and will be discussed in the next section.
// TODO: 将一部分 phonons 改为 phonon modes
// 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。