By using a recently derived upper bound on the allowed equilibrium current in a ring, it is proved that the magnitude of the group velocity of a Bloch electron in a one-dimensional periodic potential is always less than or equal to the group velocity of the same Bloch state in an empty lattice. Our inequality also implies that each energy band in a one-dimensional crystal always lies below the corresponding free-electron band, when the minima of those bands are aligned.

• Received 24 August 1994

DOI:https://doi.org/10.1103/PhysRevB.51.2616