Résumé
The full point group symmetry of a crystal can be broken due to internal or external effective fields. In the study of excitons, such symmetry breaking can lead to a coupling of different exciton states and if a system is prepared in an exciton state with a defined total angular momentum (pseudospin), spin beating is obtained. Looking at the fluctuations of these fields, we use the invariant expansion of an effective Hamiltonian to investigate exciton-spin-relaxation dynamics in a model two-band bulk semiconductor and discuss the respective importance of the different spin-flip processes. We find that interaction terms leading to an electron or hole spin flip give rise to a pure transverse dephasing. Terms where the electron and hole spins are simultaneously reversed lead to transitions between the spin states, which are characterized by the longitudinal relaxation time. Similar to motional narrowing in the case of free carriers, the latter process can lead to an increase in the exciton-spin-relaxation times if extrinsic electric, magnetic, or strain fields rapidly fluctuate in the sample. This effect is shown to be due to the electron-hole exchange interaction.