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physica status solidi (b)

Excited States and Collective Excitations in a Dense Gas of Excitons

Günter Manzke, Volkhard May, K. Henneberger

Published Oct 1, 1984• 15 citations
Publisher

Abstract

Abstract Following the concept of Hanamura the Bose formalism of Usui is used to investigate the collective properties of a dense gas of excitons. The correlation self‐energy for excitons is calculated with the help of a special concept of pure excitonic screening. The dielectric function and the collective modes are studied in detail. It is shown, that the consideration of excited exciton states leads to a lowering of the 1s exciton level with increasing exciton density. Furthermore the damping of the 1s exciton due to exciton–exciton collisions is investigated.

DOI
10.1002/pssb.2221250230

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Abstract Following the concept of Hanamura the Bose formalism of Usui is used to investigate the collective properties of a dense gas of excitons. The correlation self‐energy for excitons is calculated with the help of a special concept of pure excitonic screening. The dielectric function and the collective modes are studied in detail. It is shown, that the consideration of excited exciton states leads to a lowering of the 1s exciton level with increasing exciton density. Furthermore the damping of the 1s exciton due to exciton–exciton collisions is investigated.
Abstract The spectral properties of a dense exciton gas are discussed using an exciton self‐energy in second collisional Born approximation with respect to exciton‐exciton interaction. The shift of the continuum edge and the ls exciton as well as the increase of the damping of the ls exciton are calculated linearly in the exciton density. The distribution of ls excitons is assumed to be either a macro‐occupation of the k = 0 state or a Boltzmann distribution with effective temperature varying between 10 and 60 K. A weakening of the level shift is obtained with increasing temperature. In the case of macro‐occupation damping is calculated self‐consistently resulting in a sublinear dependence on density.
Abstract The nonlinear response of a dense exciton gas in a platelet on an externally driven field is discussed using non equilibrium Green's function technique. A generalized Boltzmann‐equation of excitons containing many‐particle corrections in the diffusion‐, drift‐ and collision‐term is derived. Using a relaxed distribution (Boltzmann‐distribution) and an unrelaxed distribution (macro‐occupation) of ls‐excitons a bistability in exciton density as well as in the transmitted light intensity is obtained for a nearly resonant and stationary applied pump‐field of intensity 0.1 and 10kW/cm2, respectively. The bistability is obtained as a pure excitonic effect below the Mott‐transition.
We present an analytical and numerical study of the coherent exciton polarization including exciton-exciton correlation. The time evolution after excitation with ultrashort optical pulses can be divided into a slowly varying polarization component and novel ultrafast collective modes. The frequency and damping of the collective modes are determined by the high-frequency properties of the retarded two-exciton correlation function, which includes Coulomb effects beyond the mean-field approximation. The overall time evolution depends on the low-frequency spectral behavior. The collective mode, well separated from the slower coherent density evolution, manifests itself in the coherent emission of a resonantly excited excitonic system, as demonstrated numerically.
The Bogoliubov--de Gennes (BdG) type equations are derived for the Coulomb Bose gas describing the electrodynamics of charged bosons. The gauge invariant dielectric response function and collective excitations of a charged Bose gas at T=0 are found. We find a plasmon branch which is the same as the single-particle Bogoliubov spectrum in agreement with the textbook result. The superconducting properties of the charged Bose gas are verified. A gauge-invariant formalism for calculating the response of the supracondensate particles is proposed.