Orbital Rashba effect in surface oxidized Cu film

TL;DR

This study reveals that surface oxidation of copper significantly enhances the orbital Rashba effect, leading to strong orbital textures and large orbital Hall currents, which could impact spin-orbit torque applications.

Contribution

First-principles investigation showing surface oxidation induces a giant orbital Rashba effect and large orbital Hall currents in Cu films.

Findings

Gigantic enhancement of the orbital Rashba effect in oxidized Cu

Resonant hybridization causes the emergence of the orbital Rashba effect

External electric fields generate large orbital Hall currents, surpassing spin Hall currents in heavy metals

Abstract

Recent experimental observation of unexpectedly large current-induced spin-orbit torque in surface oxidized Cu on top of a ferromagnet suggested a possible role of the orbital Rashba effect (ORE). With this motivation, we investigate the ORE from first principles by considering an oxygen monolayer on top of a Cu(111) film. We show that surface oxidization of Cu film leads to gigantic enhancement of the ORE for states near the Fermi surface. The resulting chiral orbital texture in the momentum space is exceptionally strong, reaching $\sim 0.5\hbar$ in magnitude. We find that resonant hybridization between O $p$-states and Cu $d$-states is responsible for the emergence of the ORE. We demonstrate that application of an external electric field generates huge orbital Hall current, which is an order of magnitude larger than the spin Hall current found in heavy metals. This implies that "orbital torque" mechanism may be significant in surface oxidized Cu/ferromagnet structures. It also encourages experimental verification of the orbital texture in surface oxidized Cu through optical measurements such as angle-resolved photoemission spectroscopy.