Trend reversal in the magnetic-field dependence of exciton spin-transfer rates in diluted magnetic semiconductors due to non-Markovian dynamics

TL;DR

This paper theoretically explores how non-Markovian quantum effects influence exciton spin-transfer rates in diluted magnetic semiconductors under magnetic fields, revealing a reversal in trend for in-plane fields due to carrier-dopant correlations.

Contribution

It introduces a quantum kinetic theory beyond the Markov approximation to accurately describe exciton spin dynamics in magnetic fields, highlighting non-Markovian effects that alter expected behaviors.

Findings

Markovian rate decreases monotonically with magnetic field when aligned parallel.

Quantum kinetic effects cause a reversal of the rate trend for in-plane magnetic fields.

Carrier-dopant correlations lead to increased long-time spin polarization.

Abstract

We investigate theoretically the influence of an external magnetic field on the spin dynamics of excitons in diluted magnetic semiconductor quantum wells. To this end, we apply a quantum kinetic theory beyond the Markov approximation which reveals that non-Markovian effects can significantly influence the exciton spin dynamics. If the magnetic field is oriented parallel to the growth direction of the well, the Markovian spin-transfer rate decreases monotonically with increasing field as predicted by Fermi's golden rule. The quantum kinetic theory follows this result qualitatively but predicts pronounced quantitative differences in the spin-transfer rate as well as in the long-time spin polarization. However, for an in-plane magnetic field, where the Markovian spin-transfer rate first drops and then increases again, quantum kinetic effects become so pronounced that the Markovian trend is completely reversed. This is made evident by a distinct maximum of the rate followed by a monotonic decrease. The deviations can be traced back to a redistribution of carriers in energy space caused by correlations between excitons and magnetic dopants. The same effect leads to a finite electron-spin polarization at long times in longitudinal as well as transverse fields which is much larger than the corresponding Markovian prediction.