Spin-exchange kinetics of excitons in Cu₂O: Transverse acoustic phonon mechanism

The interconversion between orthoexcitons and lower-lying paraexcitons in Cu₂O is studied by time resolving the orthoexciton and paraexciton luminescence following picosecond photoexcitation. The buildup of paraexciton numbers and the orthoexciton decay are observed over the temperature range 2–20 K. The low-temperature limit of the conversion rate is accurately measured. The temperature dependence of the interconversion rate suggests that the dominant mechanism for ortho-para interconversion is the emission or absorption of a single transverse acoustic (TA) phonon. We propose a microscopic model in which rotation of the lattice induces the spin flip through spin-orbit coupling. The energy splitting between orthoexcitons and paraexcitons is modified by applying stress to the crystal, and the the interconversion rate as a function of this splitting is found to be consistent with the single TA phonon mechanism.