Quantum saturation and condensation of excitons inCu2O: A theoretical study

Abstract

Recent experiments on high density excitons in Cu2O 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.

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

“…This effect has been examined theoretically in Ref. [8], and has been attributed to a competition between the acoustic-phonon cooling of the exciton gas, and an Auger heating mechanism which prevents the BoseEinstein condensation of orthoexcitons. Lin and Wolfe have also reported in Ref.…”

“…The implication is that this Auger mechanism, which does not conserve the total number of excitons prevents the onset of Bose-Einstein condensation. Up to now the Auger process was thought to provide the only relatively fast channel for excitons to get destroyed [8]. However, in view of the very long intrinsic radiative lifetime of orthoexcitons reported recently in Refs.…”

Fine structure of excitons inCu2O

“…These observations are directly related with the band structure of Cu 2 O, as we have shown in Refs. [18,19].…”

Coherent manipulation of quadrupole biexcitons in cuprous oxide by 2D femtosecond spectroscopy

“…But there is a substantial discrepancy between theory and time-resolved photoluminescence (which measures the decay of exciton number following short optical pulse excitation). The calculated direct (2 Â 10 À21 cm 3 /ns at a temperature of 70 K) [7] and phonon-assisted Auger decay rates (3 Â 10 À22 cm 3 /ns) [8,9] are orders of magnitude smaller than measured decay rate (10 À16 cm 3 /ns) [3]. Moreover, conventional Auger theory predicts a linear increase of the Auger rate with temperature, whereas experiment [7] shows an inverse temperature dependence.…”