== Definition of the axes

坐标轴的定义。

坐标轴在我们的研究中需要被明确定义，因为 4H-SiC 的各向异性，以及在不同文献中坐标轴的不同定义。
4H-SiC 晶体中存在镜面（mirror plane）和滑移面（glide plane），如图所示。
4H-SiC 的原子结构沿镜面反演后不变，但沿滑移面反演后还需要沿 z 轴平移半个晶格常数才能保持不变。
我们定义 x 轴平行于滑移面，y 轴与 x 轴垂直并平行于一个镜面。
同时，我们定义 a 轴与 x 轴平行，b 轴与 a 轴夹角为 120 度。
在此定义下，晶圆的切边沿 x 方向（即 [$11 overline(2) 0$] 方向），
  台阶流外延过程中台阶台阶边沿平行于 y 方向（即 [$1 overline(1) 00$] 方向），生长方向为 x 方向（即 [$11 overline(2) 0$]）。
此定义与 material projects 数据库以及大部分文献中的定义一致，而与其它一些文献中的定义不一致。
（最后一行补充参考文献）。

The axes in our study need to be clearly defined due to the anisotropy of 4H-SiC.
In the 4H-SiC crystal, there are three mirror planes and three glide planes, as shown in @figure-axis.
The atomic structure of 4H-SiC remains unchanged under reflection across a mirror plane,
  but it requires a translation of $1/2c$ to remain unchanged under reflection across a glide plane.
We define the x-axis parallel to the glide plane,
  and the y-axis perpendicular to the x-axis and parallel to a mirror plane.
Concurrently, we define the a-axis parallel to the x-axis,
  and the b-axis at an angle of 120#sym.degree to the a-axis.
Under this definition, the wafer edge is along the x-axis (i.e. [$11 overline(2) 0$]),
  the step edge during step-flow epitaxy is parallel to the y-axis (i.e. [$1 overline(1) 00$]),
  and the growth direction is along the x-axis (i.e. [$11 overline(2) 0$]).
This definition is consistent with most literature and the Materials Project database,
  while it differs from some other literature.

#include "fig-axis.typ"

依据此定义，K在Gamma点的x方向，M在Gamma点的y方向。（应该也需要画个图）