Exciton spin dynamics in quantum wells

TLDR

Exciton spin relaxation follows motional narrowing, inversely proportional to the exciton center‑of‑mass scattering time, and theoretical electric and magnetic field dependences remain untested experimentally. The study presents a theory of exciton spin dynamics based on an exchange spin‑flip mechanism that fully accounts for quantum‑well confinement. The authors analyze time‑dependent optical intensities of both circular polarizations, modeling competing relaxation mechanisms from exciton exchange and single‑particle spin flips into optically inactive states, to reveal their relative contributions. The analysis yields characteristic intensity shapes that reflect the relative importance of the competing mechanisms, and the calculated well‑width dependence of exciton spin relaxation agrees well with experimental polarization measurements.

Abstract

A theory of exciton spin dynamics is given in terms of the exchange spin-flip mechanism taking full account of the confinement of the quantum well. Exciton spin relaxation belongs to the motional narrowing class, with a characteristic inverse proportionality to the momentum scattering time of the exciton center of mass. Analysis of the time dependence of optical intensities of both circular polarizations, including competing relaxation mechanics from exciton exchange and from single-particle spin flip into optically inactive states, leads to characteristic shapes reflecting their relative importance. The calculated well-width dependence from our theory of the exciton spin relaxation leads to polarization intensities in good agreement with measurement. Theoretical electric and magnetic field dependences have yet to be tested against experiment.