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The superlattice has an electrically polar structure because mirror symmetry along the stacking direction is broken.

Figure 1 | Breaking nuclear mirror symmetry. a, In a pair of mirror nuclei, the number of protons in one nucleus equals the number of neutrons in the other, and vice versa.

Chirality means handedness: like left and right hands, objects of opposite chirality have mirror symmetry, but cannot be superimposed on each other.

There are more sophisticated examples, such as mirror symmetry.

Just two of the 17 wallpaper groups had the spirals he was looking for—those with mirror symmetry, so that they head off in opposite directions wherever they meet.