New perspectives on kinetics of excitons in Cu2O

Ortho-para conversion and Auger recombination of excitons in Cu2O are studied by time- and space-resolved luminescence. The interconversion between orthoexcitons and lower-lying paraexcitons is accurately measured at low excitation levels, where ortho and paraexcitons are in thermal equilibrium. Temperature- and stress-dependences of the interconversion rate D(T, Δ) imply that down-conversion occurs by TA phonon emission, which provides the needed rotation of the lattice to induce a spin flip. As the excitation level increases, excitons start to undergo a strong density-dependent Auger recombination. We report the measurements of the Auger constant A(T) over the temperature range 72-212 K. The measured Auger constant is found to depend roughly inversely with the exciton gas temperature over this range and its absolute magnitude is much larger than the present theoretical prediction. The large value of A and a 1/Tgas dependence imply that a thermodynamic BEC of excitons may be impossible to produce in this crystal.