The Knight field and the local nuclear dipole-dipole field in an (In,Ga)As quantum dot ensemble

TL;DR

This study investigates how weak magnetic fields influence electron-nuclear interactions in InGaAs quantum dots, revealing unique behaviors and enabling measurement of internal fields affecting electron spin dynamics.

Contribution

It provides new insights into the effects of milli-Tesla fields on nuclear and electron spins in quantum dots, and introduces methods to measure internal magnetic fields like the Knight and Overhauser fields.

Findings

Weak magnetic fields significantly affect electron spin dynamics.

The maximum Knight and Overhauser fields in the system are quantified.

The angle of the Overhauser field can be precisely tuned.

Abstract

We present a comprehensive investigation of the electron-nuclear system of negatively charged InGaAs/GaAs self-assembled quantum dots under the influence of weak external magnetic fields (up to 2 mT). We demonstrate that, in contrast to conventional semiconductor systems, these small fields have a profound influence on the electron spin dynamics, via the hyperfine interaction. Quantum dots, with their comparatively limited number of nuclei, present electron-nuclear behavior that is unique to low-dimensional systems. We show that the conventional Hanle effect used to measure electron spin relaxation times, for example, cannot be used in these systems when the spin lifetimes are long. An individual nucleus in the QD is subject to milli-Tesla effective fields, arising from the interaction with its nearest-neighbors and with the electronic Knight field. The alignment of each nucleus is influenced by application of external fields of the same magnitude. A polarized nuclear system, which may have an effective field strength of several Tesla, may easily be influenced by these milli-Tesla fields. This in turn has a dramatic effect on the electron spin dynamics, and we use this technique to gain a measure of both the dipole-dipole field and the maximum Knight field in our system, thus allowing us to estimate the maximum Overhauser field that may be generated at zero external magnetic field. We also show that one may fine-tune the angle which the Overhauser field makes with the optical axis.