"Listening" to the spin noise of conduction electrons in bulk n:GaAs

TL;DR

This study uses optical magnetometry to analyze electron spin noise in n-type GaAs, revealing how measurement conditions affect spin lifetime estimates and demonstrating the influence of probe laser parameters on observed spin fluctuations.

Contribution

It provides a comprehensive analysis of electron spin noise in bulk GaAs, comparing optical magnetometry with traditional methods, and highlights the impact of probe laser properties on spin lifetime measurements.

Findings

Spin noise spectra depend on electron density, magnetic field, temperature, and laser parameters.

Spin lifetimes inferred from noise spectra agree with Hanle measurements but are affected by laser absorption.

Spin noise power increases with temperature and decreases with probe volume size.

Abstract

We report a comprehensive study of stochastic electron spin fluctuations -- spin noise -- in lightly doped ($n$-type) bulk GaAs, which are measured using sensitive optical magnetometry based on off-resonant Faraday rotation. Frequency spectra of electron spin noise are studied as a function of electron density, magnetic field, temperature, probe laser wavelength and intensity, and interaction volume. Electron spin lifetimes $\tau_s$ are inferred from the width of the spin noise spectra, and are compared with direct measurements of $\tau_s$ using conventional Hanle effect methods. Both methods reveal a strong and similar dependence of $\tau_s$ on the wavelength and intensity of the probe laser, highlighting the undesired influence of sub-bandgap absorption effects on the nominally `non-perturbative' spin noise measurements. As a function of temperature, the spin noise power increases approximately linearly from 1.5 K to 30 K, as expected for degenerate electrons obeying Fermi-Dirac statistics, but with an additional zero-temperature offset. Finally, as the cross-sectional area of the probe laser shrinks and fewer electrons are probed, the measured Faraday rotation fluctuations due to electron spin noise are shown to increase, as expected.