Time evolution of two ground-state excitons
Résumé
This work essentially deals with the time evolution of two ground-state excitons when these excitons have the same center-of-mass momentum. Since all coupled states have a higher energy, scatterings toward same energy states are formally impossible. As a result, previous works on the time evolution of two-exciton states should not apply. This led us to carefully reconsider the requirement of ``energy conservation in the large time limit'' through the Fermi golden rule, when dealing with composite excitons and to enlighten the key role of coherence time in the time evolution of excitons which is experimentally observed.