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Application of strain to a crystal reduces its symmetry. The Bravais lattice basis vectors of a crystal under homogeneous strain are obtained by deforming the vectors of the unstrained crystal
Generally, the strain tensor (3.5) determines only the deformation of the primitive cell as a whole, but the relative displacements of the atoms within the cell differ for the atoms forming the basis. This has no impact on the lattice symmetry but has to be accounted for in band structure calculations. In Section 3.8.2 it is shown how to take this internal displacement into account properly.
Points and lines of the BZ can be classified according to their symmetry. For each lattice there are symmetry operations that leave the lattice and hence the BZ invariant. These operations bring the BZ into coincidence with itself. In such an operation a specific vector is not necessarily projected onto itself or + . For each vector there exists a set of symmetry operations fulfilling
(3.32) |
In Figure 3.5b the symmetry points (filled circles) and symmetry lines (open circles) of the fcc lattice are shown. Strictly, the given points and are not symmetry points according to the definition of (3.33), since they have the same symmetry as the points along the symmetry lines and . Hence, they can be included in these symmetry lines.
Since the center point of the BZ is mapped onto itself at any point operation of the crystal lattice, all symmetry operations of the lattice are included in the point group . Thus, this group determines the shape and volume of the irreducible wedge. The number of symmetry elements of determines the volume of the irreducible wedge as [Nowotny98]
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