Hyperfine interaction in InAs/GaAs self-assembled quantum dots : dynamical nuclear polarization versus spin relaxation

TL;DR

This study investigates hyperfine interactions in InAs/GaAs quantum dots, revealing how dynamical nuclear polarization influences electron spin relaxation and creates strong, bistable nuclear magnetic fields affecting optical orientation.

Contribution

It demonstrates the role of dynamical nuclear polarization in inhibiting spin relaxation and models the hyperfine interaction effects in charged excitons within quantum dots.

Findings

Hyperfine interaction causes partial spin relaxation under modulated excitation.

Dynamical nuclear polarization can generate magnetic fields up to ~4 T.

Theoretical model explains the energy cost of electron spin flips in magnetic fields.

Abstract

We report on the influence of hyperfine interaction on the optical orientation of singly charged excitons X+ and X- in self-assembled InAs/GaAs quantum dots. All measurements were carried out on individual quantum dots studied by micro-photoluminescence at low temperature. We show that the hyperfine interaction leads to an effective partial spin relaxation, under 50kHz modulated excitation polarization, which becomes however strongly inhibited under steady optical pumping conditions because of dynamical nuclear polarization. This optically created magnetic-like nuclear field can become very strong (up to ~4 T) when it is generated in the direction opposite to a longitudinally applied field, and exhibits then a bistability regime. This effect is very well described by a theoretical model derived in a perturbative approach, which reveals the key role played by the energy cost of an electron spin flip in the total magnetic field. Eventually, we emphasize the similarities and differences between X+ and X- trions with respect to the hyperfine interaction, which turn out to be in perfect agreement with the theoretical description.