Effect of the Pauli principle on photoelectron spin transport in $p^+$ GaAs

TL;DR

This study investigates how the Pauli principle influences spin transport in degenerate p+ GaAs thin films, revealing a polarization dip caused by spin-dependent diffusion and confirming the weak impact of mobility changes.

Contribution

It provides a combined theoretical and experimental analysis of spin transport under degeneracy, highlighting the role of the Pauli principle and ambipolar effects in GaAs.

Findings

Polarization dip appears at about 2 μm from excitation spot.

Degeneracy causes spin-dependent diffusion, affecting polarization profiles.

Ambipolar coupling enhances the polarization dip.

Abstract

In p+ GaAs thin films, the effect of photoelectron degeneracy on spin transport is investigated theoretically and experimentally by imaging the spin polarization profile as a function of distance from a tightly-focussed light excitation spot. Under degeneracy of the electron gas (high concentration, low temperature), a dip at the center of the polarization profile appears with a polarization maximum at a distance of about $2 \; \mu m$ from the center. This counterintuitive result reveals that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle. This causes a concentration dependence of the spin stiffness while the spin dependence of the mobility is found to be weak in doped material. The various effects which can modify spin transport in a degenerate electron gas under local laser excitation are considered. A comparison of the data with a numerical solution of the coupled diffusion equations reveals that ambipolar coupling with holes increases the steady-state photo-electron density at the excitation spot and therefore the amplitude of the degeneracy-induced polarization dip. Thermoelectric currrents are predicted to depend on spin under degeneracy (spin Soret currents), but these currents are negligible except at very high excitation power where they play a relatively small role. Coulomb spin drag and bandgap renormalization are negligible due to electrostatic screening by the hole gas.