We consider the formation of exciton bound states between electrons and holes in a 2D layer. The medium modifications of two-particle properties are obtained from a systematic Green function approach within a cluster mean field approximation. Contributions from correlations (excitons) in the medium are given. The importance of Pauli blocking on the shift of the ionization energy of excitons in comparison to screening is shown.
Effective wave equation.Electrons and holes in a two-dimensional quantum well structure interact via the Coulomb potential V eh (q) = −e 2 /(2Ω 0 r 0 q) and can form bound states, the excitons. A rigorous approach to this electron-hole-exciton system embedded into a surrounding medium in equilibrium (density n, temperature T ) can be given by the method of thermodynamic Green functions. We will follow the approach according to the monography [1], see also [2,3], where the Bethe-Salpeter equation for the propagation of a two-particle cluster was investigated. The following effective wave equation (in-medium Schrödinger equation) for the electron-hole system has been derived yielding the eigenstates Ψ αP (1, 2) and the corresponding energy eigenvalues E αP :The single-particle states are given by the quantum numbers {1} = { p 1 , σ 1 , c 1 } describing linear momentum (two dimensional), spin and species (e, h), respectively. The bound states (excitons) are characterized by the total linear momentum P and internal quantum number α. The influence of the surrounding matter on the two-particle complex is described by the plasma HamiltonianThere are two important many-particle effects. Firstly, the interaction between the free particles as well as the bound states is considered in Born approximation, taking into account the full antisymmetrization of all fermionic states, also denoted as cluster-mean field approximation (mf). Secondly, the Coulomb interaction is dynamically screened by free as well as bound states (scr) as shown in [4]. The cluster-mean field terms (the Hartree terms vanish because of charge neutrality) are given as [5]V cc (12, 1 2 )Ψ α P (12)Ψ * α P (1 2 ) g eh (E α P ), V cc (13, 1 3 )Ψ * α P (23 )Ψ α P (2 3) g cc (E α P ) ,