Hanle effect in (In,Ga)As quantum dots: Role of nuclear spin fluctuations

TL;DR

This study investigates how nuclear spin fluctuations affect the polarization of nuclear spins in (In,Ga)As quantum dots, revealing that considering these fluctuations is essential to accurately model the Hanle effect and nuclear spin cooling.

Contribution

The paper introduces a model incorporating nuclear spin fluctuations to accurately describe the Hanle effect in quantum dots, surpassing the limitations of mean-field theories.

Findings

Nuclear spin fluctuations significantly influence the Hanle curves.

The proposed model accurately fits experimental data.

Parameters of the electron-nuclear spin system are determined.

Abstract

The role of nuclear spin fluctuations in the dynamic polarization of nuclear spins by electrons is investigated in (In,Ga)As quantum dots. The photoluminescence polarization under circularly polarized optical pumping in transverse magnetic fields (Hanle effect) is studied. A weak additional magnetic field parallel to the optical axis is used to control the efficiency of nuclear spin cooling and the sign of nuclear spin temperature. The shape of the Hanle curve is drastically modified with changing this control field, as observed earlier in bulk semiconductors and quantum wells. However, the standard nuclear spin cooling theory, operating with the mean nuclear magnetic field (Overhauser field), fails to describe the experimental Hanle curves in a certain range of control fields. This controversy is resolved by taking into account the nuclear spin fluctuations owed to the finite number of nuclei in the quantum dot. We propose a model describing cooling of the nuclear spin system by electron spins experiencing fast vector precession in the random Overhauser fields of nuclear spin fluctuations. The model allows us to accurately describe the measured Hanle curves and to determine the parameters of the electron-nuclear spin system of the studied quantum dots.