Spin Dynamics Driven by the Electron-Hole Exchange Interaction in Quantum Wells

In general, control over the degree of spin orientation of photocreated charge carriers in semiconductor structures is achieved by polarized-light excitation of optical transitions in conformity with the angular momentum selection rules. In semiconductor quantum wells (QWs) the spin dynamics results from the individual dynamics of the electron spin (components 1/2, in units of ~) or heavy hole spin (angular momentum projections 3/2), or from the spin dynamics of the bound electron-hole pair (exciton). Individual electron and hole spin ips follow from band spin mixing in addition to momentum scattering, and they have little eÆciency on cold carriers at the band edges of narrow QWs due to vanishing spin mixing and reduced available phase space for scattering. In intrinsic QWs the band-edge optical properties are dominated by the exciton state for which the correlation between the electron and hole spins is set by the electron-hole (e-h) exchange Coulomb interaction. Although small, the exchange contribution to the spin dynamics becomes important in the aforementioned situation of strongly con ned carriers at the band edges. The long-range part of the e-h exchange interaction (LRX) depends on the exciton center-of-mass (c.m.) momentum, such that scattering leads to an exciton spin relaxation according to a motional narrowing process, for which shorter scattering time gives longer exciton spin relaxation time s (which is valid for s). However, such mechanism cannot be directly applied for samples that exhibit lateral localization of exciton in sites created by QW interface imperfections. Localized excitons do not scatter as frequently as free excitons, such that the motional narrowing process is inappropriate if loc s. Also, the localization of the exciton in large sites quantizes the c.m. motion and as a result the LRX coupling for exciton spin states j+1i ! j 1i vanishes in symmetric sites.[1] The inhomogeneity of sites and it e ects on the exciton spin dynamics via the e-h exchange have been addressed experimentally and theoretically by Nickolaus et al.[2] for (Zn,Cd)Se/ZnSe QWs. They have found that the dominant mechanism in the spin decay was not a true relaxation process, but instead a dephasing of the macroscopic spin polarization resulting from the LRX matrix elements, which are subjected to the same inhomogeneity of the sites. Further evidences of the combined role of the exciton localization and e-h exchange interaction are provided by experiments in QWs with transverse magnetic eld (Voigt con guration) where Larmor precessions of the electron spin are observed in the case that the exchange interaction lacks strength to correlate the electron and hole spins. Otherwise, the precessing electron spin would have to drag the heavy-hole spin that, in rst approximation, is pinned along the growth axis. In GaAs/AlAsGa QWs[3] electron spin precessions were observed, however, in (Zn,Cd)Se/ZnSe QWs,[4] the precessions were seen only at high temperatures ( 100 K). In this paper, we present a brief account of the theoretical study[5] of the spin dynamics in ZnSe/(Zn,Cd)Se QWs including the e ects of exciton localization, eh exchange interaction and spin precessions about a transverse magnetic eld.