Hyperfine interaction mediated exciton spin relaxation in (In,Ga)As quantum dots

TL;DR

This study investigates how hyperfine interactions influence exciton spin relaxation in (In,Ga)As quantum dots, revealing that magnetic field-induced resonances significantly shorten dark exciton lifetimes through spin flips, with electron flips being notably faster.

Contribution

It provides new insights into hyperfine-mediated spin flip mechanisms and quantifies the differing relaxation times for electron and hole spins in quantum dots.

Findings

Dark excitons decay in about 10 ns without magnetic resonance.

Magnetic field resonances drastically shorten exciton lifetimes due to spin flips.

Electron spin flips are approximately 25 times faster than hole spin flips.

Abstract

The population dynamics of dark and bright excitons in (In,Ga)As/GaAs quantum dots is studied by two-color pump-probe spectroscopy in an external magnetic field. With the field applied in Faraday geometry and at T<20 K, the dark excitons decay on a ten nanoseconds time scale unless the magnetic field induces a resonance with a bright exciton state. At these crossings their effective lifetime is drastically shortened due to spin flips of either electron or hole by which the dark excitons are converted into bright ones. Due to the quasielastic character we attribute the origin of these flips to the hyperfine interaction with the lattice nuclei. We compare the exciton spin relaxation times in the two resonances and find that the spin flip involving an electron is approximately 25 times faster than the one of the hole. A temperature increase leads to a considerable, nonmonotonic decrease of the dark exciton lifetime. Here phonon-mediated spin flips due to the spin-orbit interaction gradually become more important.