== Phonons in Perfect 4H-SiC

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。

// There are 21 phonons in total.
// We classified them into two categories: 18 negligible-polar phonons and 3 strong-polar phonons.
The phonons involved in Raman scattering are located in reciprocal space around the #sym.Gamma point,
  and the exact positions are determined by the wavevectors of the incident and scattered light.
At each such position, there are 21 phonon modes (degenerate modes are counted as their multiplicity).
We classify these 21 phonons into two categories based on their polarities.
The 18 of 21 phonons are classified into negligible-polar phonons (i.e., phonons with zero or very weak polarity),
    for which the effect of polarity can be ignored in the Raman scattering process;
  and the other three phonons are strong-polar phonons,
    where the polarity gives rise to observable effects in the Raman spectra.

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。

// This classification make sense.
This classification is based on the fact that
  the four Si atoms in the primitive cell of 4H-SiC carry similar positive Born effective charges (BECs),
  and the four C atoms carry similar negative BECs (see @table-bec).
In the 18 negligible-polar phonons,
  the vibrations of two Si atoms are approximately opposite to those of the other two Si atoms,
  and the same holds for the C atoms,
  leading to cancellations of macroscopic polarity.
In contrast, in the three strong-polar phonons,
  all Si atoms vibrate in the same direction, and all the C atoms vibrate in the opposite direction,
  resulting in a strong dipole moment.

#include "table-bec.typ"

#include "non-polar/default.typ"
#include "polar/default.typ"
