Excitons In Cu Research Articles

Auger decay, spin exchange, and their connection to Bose-Einstein condensation of excitons inCu2O

In view of the recent experiments of O'Hara, et al. on excitons in Cu_2O, we examine the interconversion between the angular-momentum triplet-state excitons and the angular-momentum singlet-state excitons by a spin-exchange process which has been overlooked in the past. We estimate the rate of this particle-conserving mechanism and find a substantially higher value than the Auger process considered so far. Based on this idea, we give a possible explanation of the recent experimental observations, and make certain predictions, with the most important being that the singlet-state excitons in Cu_2O is a very serious candidate for exhibiting the phenomenon of Bose-Einstein condensation.

Auger decay of degenerate and Bose-condensed excitons in Cu2O.

We study the non-radiative Auger decay of excitons in Cu$_2$O, in which two excitons scatter to an excited electron and hole. The exciton decay rate for the direct and the phonon-assisted processes is calculated from first principles; incorporating the band structure of the material leads to a relatively shorter lifetime of the triplet state ortho excitons. We compare our results with the Auger decay rate extracted from data on highly degenerate triplet excitons and Bose-condensed singlet excitons in Cu$_2$O.

Quantum saturation and condensation of excitons in Cu2O: A theoretical study.

Recent experiments on high density excitons in Cu$_2$O provide evidence for degenerate quantum statistics and Bose-Einstein condensation of this nearly ideal gas. We model the time dependence of this bosonic system including exciton decay mechanisms, energy exchange with phonons, and interconversion between ortho (triplet-state) and para (singlet-state) excitons, using parameters for the excitonic decay, the coupling to acoustic and low-lying optical phonons, Auger recombination, and ortho-para interconversion derived from experiment. The single adjustable parameter in our model is the optical-phonon cooling rate for Auger and laser-produced hot excitons. We show that the orthoexcitons move along the phase boundary without crossing it (i.e., exhibit a ``quantum saturation''), as a consequence of the balance of entropy changes due to cooling of excitons by phonons and heating by the non-radiative Auger two-exciton recombination process. The Auger annihilation rate for para-para collisions is much smaller than that for ortho-para and ortho-ortho collisions, explaining why, under the given experimental conditions, the paraexcitons condense while the orthoexcitons fail to do so.

The Para‐ and Ortho‐Phases of High‐Density Excitons in Cu2O

AbstractThe quasiequilibrium state of the two‐component non‐linear ortho‐para‐exciton gas is investigated. The concentration shifts of the exciton levels are taken into account at high densities of quasiparticles. Three exciton phases appear at any value of the chemical potential, due to the effect of optical bistability: the para‐phase, the ortho‐phase and the mixed ortho‐para‐phase. It is pointed out that Bose‐Einstein condensation (BEC) in the para‐ and ortho‐phase is possible, in spite of the fact that the initial level of ortho‐excitons is situated higher on the energetic scale than the initial level of para‐excitons.

Study of Absorption Spectra of Excitons in Cu2O by Wavelength Modulation Technique

The wavelength derivative absorption spectra of Cu 2 O have been measured in the energy regions of the yellow and green exciton series over a wide temperature range. Three phonon assisted indirect transition edges are newly found at 4.2 K, and the structures associated with direct transitions to the 1s state and the higher exciton states are observable up to room temperature. By analysing the impurity associated transition and using the Rydberg constants for both the yellow and the green exciton series, we determine the effective mass of the electron in the conduction band and those of the holes in the valence bands to be m e * ≃0.99 m 0 , m l h * ≃0.64 m 0 (light hole) and m h h * ≃1.40 m 0 (heavy hole).