Tuning Spin Dynamics and Localization Near the Metal-Insulator Transition in Fe/GaAs heterostructures

TL;DR

This study investigates spin dynamics in Fe/GaAs heterostructures, revealing how localized and itinerant carriers behave near the metal-insulator transition, with implications for spintronics and quantum computing.

Contribution

It introduces a method to simultaneously monitor localized and itinerant spin states and analyzes their behavior near the metal-insulator transition.

Findings

Identified two distinct precession frequencies for localized and itinerant carriers.

Observed reentrant localization of photocarriers at high magnetic fields.

Extracted g-factors and energy levels for both localized and itinerant states.

Abstract

We present a simultaneous investigation of coherent spin dynamics in both localized and itinerant carriers in Fe/GaAs heterostructures using ultrafast and spin-resolved pump-probe spectroscopy. We find that for excitation densities that push the transient Fermi energy of photocarriers above the mobility edge there exist two distinct precession frequencies in the ob-served spin dynamics, allowing us to simultaneously monitor both localized and itinerant states. For low magnetic fields (below 3 T) the beat frequency between these two excitations evolves linearly, indicating that the nuclear polarization is saturated almost immediately and that the hyperfine coupling to these two states is comparable, despite the 100x enhancement in nuclear polarization provided by the presence of the Fe layer. At higher magnetic fields (above 3 T) the Zeeman energy drives reentrant localization of the photocarriers. Subtracting the constant hyperfine contribution from both sets of data allows us to extract the Lande g-factor for each state and estimate their energy relative to the bottom of the conduction band, yielding -2.16 meV and 17 meV for localized and itinerant states, respectively. This work advances our fundamental understanding of spin-spin interactions between electron and nuclear spin species, as well as between localized and itinerant electronics states, and therefore has implications for future work in both spintronics and quantum information/computation.