ABSTRACT
Two limits. If the nuclei are positioned on the sites of some regular cubic or hexagonal crystal lattice, the electrons no longer move in a spherical electric field of a single nucleus which would give rise to the atomic bound state orbits, rather the electrons experience a periodic electric potential due to the nuclei on the lattice. You may imagine some set of energy wells with a characteristic depth -V0 separated by a distance a. To get an idea of what may happen in this situation we can approach it from two sides as I indicated in Figure III.3.1.
