Persistent swinging of electrons between atomic sites in crystals
This crystal lattice consists of a large number of unit cells with an identical atomic arrangement. In the elementary adiabatic picture, the motions of electrons in the crystal follow the motion of the atomic nuclei instantly, i.e., atomic nuclei and electrons are moving as a single entity. While this physical picture is valid for the inner, so-called core electrons of an atom it fails for the valence electrons,…