Kadanoff-Baym Equations with Initial Correlations

The Kadanoff-Baym equations (KBE) are usually derived under the assumption of the weakening of initial correlations (Bogolyubov's condition) and, therefore, fail to correctly describe the short time behavior. We demonstrate that this assumption is not necessary. Using functional derivatives techniques, we present a straightforward generalization of the KBE which allows to include arbitrary initial correlations and which is more general than previous derivations. As a result, an additional collision integral is obtained which is being damped out after a few collisions. Our results are complemented with numerical investigations showing the effect of initial correlations.Comment: 10 pages, 3 figures, to be published in ``Progress in Nonequilibrium Green's Functions'', M. Bonitz (Ed.), World Scientific, Singapore 2000, uses sprocl.st

Interacting multi-component exciton gases in a potential trap: phase separation and Bose-Einstein condensation

The system under consideration is a multi-component gas of interacting para- and orthoexcitons confined in a three dimensional potential trap. We calculate the spatially resolved optical emission spectrum due to interband transitions involving weak direct and phonon mediated exciton-photon interactions. For each component, the occurrence of a Bose-Einstein condensate changes the spectrum in a characteristic way so that it directly reflects the constant chemical potential of the excitons and the renormalization of the quasiparticle excitation spectrum. Moreover, the interaction between the components leads, in dependence on temperature and particle number, to modifications of the spectra indicating phase separation of the subsystems. Typical examples of density profiles and luminescence spectra of ground-state para- and orthoexcitons in cuprous oxide are given.Comment: 7 pages, 6 figure

Kadanoff-Baym equations and non-Markovian Boltzmann equation in generalized T-matrix approximation

A recently developed method for incorporating initial binary correlations into the Kadanoff-Baym equations (KBE) is used to derive a generalized T-matrix approximation for the self-energies. It is shown that the T-matrix obtains additional contributions arising from initial correlations. Using these results and taking the time-diagonal limit of the KBE, a generalized quantum kinetic equation in binary collision approximation is derived. This equation is a far-reaching generalization of Boltzmann-type kinetic equations: it selfconsistently includes memory effects (retardation, off-shell T-matrices) as well as many-particle effects (damping, in-medium T-Matrices) and spin-statistics effects (Pauli-blocking).Comment: 9 pages, 7 figures, corrected misprints in eqs. 48-5

An exciton-polariton laser based on biologically produced fluorescent protein

Under adequate conditions, cavity-polaritons form a macroscopic coherent quantum state, known as Bose-Einstein condensate (BEC). Compared to Wannier-Mott excitons in inorganic semiconductors, the localized Frenkel excitons in organic emitter materials show weaker interaction but stronger coupling, which recently enabled the first realization of BEC at room temperature. However, this required ultrafast optical pumping which limits the applications of organic BECs. Here, we demonstrate room-temperature BEC of cavity-polaritons in simple laminated microcavities filled with the biologically produced enhanced green fluorescent protein (eGFP). The unique molecular structure of eGFP prevents exciton annihilation even at high excitation densities, thus facilitating BEC under conventional nanosecond pumping. BEC is clearly evidenced by a distinct threshold, an interaction-induced blueshift of the condensate, long-range coherence and the presence of a second threshold at higher excitation density which is associated with the onset of photon lasing and results from thermalization of the exciton reservoir.Comment: 13(+8) pages, 4(+7) figure

Condensation of Excitons in Cu2O at Ultracold Temperatures: Experiment and Theory

We present experiments on the luminescence of excitons confined in a potential trap at milli-Kelvin bath temperatures under cw-excitation. They reveal several distinct features like a kink in the dependence of the total integrated luminescence intensity on excitation laser power and a bimodal distribution of the spatially resolved luminescence. Furthermore, we discuss the present state of the theoretical description of Bose-Einstein condensation of excitons with respect to signatures of a condensate in the luminescence. The comparison of the experimental data with theoretical results with respect to the spatially resolved as well as the integrated luminescence intensity shows the necessity of taking into account a Bose-Einstein condensed excitonic phase in order to understand the behaviour of the trapped excitons.Comment: 41 pages, 23 figure