Giant Orbital Rashba-Edelstein Effect in Crystalline Cu₂O/Cu Heterostructures

Orbital current-induced torque (OT) has emerged as a promising alternative to spin-orbit torque owing to the typically larger orbital Hall conductivity. A substantial OT can be generated at metal/oxide interfaces via the orbital Rashba-Edelstein effect (OREE), which has been predominantly reported in surface-oxidized Cu structures. However, the lack of well-defined crystalline and interfacial structures in oxidized Cu has hindered a clear understanding of the underlying mechanism and limited further enhancement of OT. Here, we demonstrate a significantly enhanced OREE in crystalline CuO_x/Cu heterostructures with well-defined interfaces. By employing controlled oxidation of single-crystalline Cu, we fabricate crystalline CuO_x layers with distinct chemical phases and quantify the resulting OT using harmonic Hall measurements. We find that the crystalline Cu₂O/Cu heterostructure exhibits a damping-like OT efficiency approximately seven times larger than that of naturally oxidized Cu, highlighting the crucial role of structural ordering and interface sharpness. Notably, this large OT efficiency, combined with the high electrical conductivity of Cu, yields a spin torque conductivity of 1.9 × 10⁶ (ℏ/2e)Ω^-1 m^-1, exceeding that of Pt. These results establish crystalline Cu₂O/Cu heterostructures as a promising platform for ultralow-power spin-orbitronic devices.