Evidence for strong localization of orbital polarization

TL;DR

This paper provides experimental evidence that orbital polarization is strongly localized and does not propagate significantly, challenging the idea of non-local orbital effects in orbitronics.

Contribution

It demonstrates that orbital polarization relaxes quickly and does not contribute to non-local orbital torque, clarifying a key debate in orbitronics.

Findings

Orbital polarization undergoes rapid relaxation.

No detectable orbital torque from Al bulk or interface.

Orbital polarization does not participate in non-local transport.

Abstract

Whether orbital polarization propagates has become the most essential question of the blooming orbitronics that aims to generate non-local orbital torque and orbital pumping. Recent theories have suggested a strong orbital Hall effect within the light metal Al and a strong orbital Rashba effect at Co/Al interfaces, providing ideal platforms for experimental verification of possible orbital transport effects. Here, we report robust experimental evidence for the strong localization of orbital polarization. We demonstrate that neither the bulk nor the interface of the Al contributes a detectable orbital torque on adjacent magnetic layer with strong bulk and interfacial spin-orbit coupling necessary for potential orbital-spin conversion. These results have clarified that orbital polarization undergoes much faster relaxation than spin polarization and hardly participates in non-local accumulation, transport, or torque generation. The experimental evidence for strong localization of orbital polarization represents a groundbreaking advance towards solving the essential orbital torque debate.