The exciton state in an insulator is defined as an excited state of the fully interacting electronic system. Its motion is governed by the homogeneous Bethe-Salpeter equation. From this we derive the effective-mass approximation in the limit of small binding energy, small total momentum, and large spatial extent of the exciton. The corrections to the effective-mass approximation are briefly indicated.

• Received 22 November 1965

DOI:https://doi.org/10.1103/PhysRev.144.708