On Energy Bands of Crystals with Lowered Point Symmetry

The special case of those subgroups of space groups that have the same translational symmetry but lowered point symmetry is discussed from the crystallographic, group‐theoretical and physical points of view. Consequences are studied concerning the energy band structure (degeneracies, zero slopes of E(k), dipole selection rules, etc.). It is shown preliminarily that the theory of space subgroups [3] is important here and that the case of lowered point symmetry is especially simple and physically interesting.