Exciton-bound electron-spin relaxation

Exciton luminescence polarization studies in semiconductor quantum wells have revealed the coexistence of two main mechanisms of exciton-spin relaxation: a well-known direct relaxation with simultaneous electron and hole spin flip due to the electron-hole exchange interaction and an indirect one with sequential spin flips of the single particles. The rate of exciton spin relaxation in this indirect channel is limited by the slower single-particle spin-flip rate, which is typically the electron one. In this work a theory of exciton-bound electron-spin dynamics driven by the spin-orbit splitting in the conduction band is presented. It is shown that the off-diagonal matrix element between optical active and inactive exciton states that differ only with regard to the electron spin direction represents an effective magnetic field that changes randomly as the exciton is elastically scattered and relaxes its spin. The exchange splitting between the optical active and inactive states acts as a constant external magnetic field, reducing the relaxation. The estimated rate of the bound electron spin flip agrees well with values obtained from previous fittings of the experimental data. Semiconductor heterostructures with real-space indirect excitons, for which the sequential spin-flip relaxation channel becomes the dominant one, are also briefly discussed together with the dependence of-the relaxation time on the well width.