# Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment

## TL;DR

This study investigates exciton spin dynamics and magnetic-field-induced polarization in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II alignment, combining experiments and a kinetic model for detailed understanding.

## Contribution

The paper introduces a kinetic master equation model that accurately describes exciton spin dynamics and polarization in these quantum wells, providing quantitative parameters like g-factors and spin relaxation times.

## Key findings

- Nonmonotonic behavior of polarization with magnetic field explained by bright and dark exciton interplay.
- Quantitative agreement between model and experiment for polarization and spin relaxation.
- Determined heavy-hole g factor and spin relaxation times for electrons and holes.

## Abstract

The exciton spin dynamics are investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The magnetic-field-induced circular polarization of photoluminescence, $P_c$, is studied as function of the magnetic field strength and direction as well as sample temperature. The observed nonmonotonic behaviour of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. To interpret the experiment, we have developed a kinetic master equation model which accounts for the dynamics of the spin states in this exciton quartet, radiative and nonradiative recombination processes, and redistribution of excitons between these states as result of spin relaxation. The model offers quantitative agreement with experiment and allows us to evaluate, for the studied structure, the heavy-hole $g$ factor, $g_{hh}=+3.5$, and the spin relaxation times of electron, $\tau_{se} = 33~\mu$s, and hole, $\tau_{sh} = 3~\mu$s, bound in the exciton.