Excited State Effects in the Dielectric Function of a 2D Boson System

Abstract

We present calculations for the dielectric function and collective modes of a two-dimensional interacting gas of bosons (excitons) in the xy plane which can be polarized by the application of an electric field in the z direction. Previous work has explored the response of the density fluctuations while excitons lie in their intrinsic ground state. We extend the formalism here by taking into account the internal degrees of freedom of the excitons, and their possible interplay with the density-fluctuation modes of the system. In the limit of low temperature, we find collective normal modes related to the various degrees of freedom. Our calculations of the corresponding intrinsic oscillator strength show that optical-like modes associated to the excited states become increasingly damped when temperature is raised, as the occupation factors change the overall response.