Thermodynamics of strain-confined paraexcitons in Cu2O.

Abstract

The thermodynamic behavior of long-lived paraexcitons confined to a parabolic potential well is examined. The potential well is produced by a Hertzian contact stress. A wavelength-tunable dye laser is used to create excitons directly in the potential well or at any other localized point inside the crystal. Spectral and spatial distributions of the exciton recombination luminescence are measured for cw and pulsed excitation. The possibility of Bose-Einstein condensation of these long-lived excitons is examined both theoretically and experimentally. We calculate the spectral and spatial distribution of luminescence from a gas of noninteracting particles in a three-dimensional harmonic-oscillator well. The results are markedly different for direct (no-phonon) and indirect (phonon-assisted) recombination. The calculated spectra are compared to the data for moderate cw excitation at ${T}_{\mathrm{bath}=2--}$4.2 K. Taken alone, the no-phonon spectra suggest that the excitonic gas is in the quantum regime; however, this conclusion is shown to be inconsistent with the estimated density of the gas. A consistent interpretation of all spectral and spatial distributions is possible, however, if one assumes Maxwell-Boltzmann statistics and takes into account the rapidly changing paraexciton intensity with applied stress. From time- and space-resolved studies of the orthoexciton and paraexciton luminescence, plausible causes for a saturation of paraexciton density in the strain well are deduced. First, an Auger-like recombination of colliding paraexcitons seems to limit their density---an idea supported by the observed power dependence of orthoexciton and paraexciton signals. Secondly, an anomalously slow thermalization of strain-confined paraexcitons is observed.