DIELECTRIC THEORY OF PLASMA OSCILLATIONS AND ITS EXTENSION TO THE EXCITON PROBLEM

A new form of the microscopic expression for the dielectric constant is derived and used to obtain the dispersion relation for plasma modes. It is found that the usual dispersion relation for plasma modes derived using the random phase approximation contains higher-order correlation effects than is usually believed. The dielectric approach to the plasma problem is extended to the exciton problem by introducing a nonlocal charge density instead of the local charge density appearing in the case of the plasma modes. The same equation determining the energy of the exciton states as derived in a previous paper is obtained.