Optimal spin-charge interconversion in graphene through spin-pseudospin entanglement control

TL;DR

This work addresses maximizing spin-charge interconversion in graphene by exploiting spin-pseudospin entanglement. Using a minimal graphene model with Rashba and Kane-Mele SOC, the authors identify a conserved quantity $\mathcal{Q}=\sigma_k s_\varphi$ and show that tuning $\lambda_{KM}$ relative to $\lambda_R$ controls entanglement between spin and pseudospin textures, enabling near-100% Rashba-Edelstein efficiency with $\Theta_{\text{REE}}$ reaching $\pm 1$ when $\lambda_{KM}=\pm\lambda_R$. Real-space, disorder-robust simulations reveal persistent high efficiency and, additionally, a Kane-Mele–driven, disorder-resilient spin Hall effect arising from inter-band transitions between cones of opposite winding $\chi$. Overall, the results propose graphene-based platforms for maximally efficient spintronic transduction and highlight spin-pseudospin correlations as a tunable mechanism for device design.

Abstract

The electrical generation of spin signals is of central interest for spintronics, where graphene stands as a relevant platform as its spin-orbit coupling (SOC) is tuned by proximity effects. Here, we propose an enhancement of spin-charge interconversion in graphene by controlling the intraparticle entanglement between the spin and pseudospin degrees of freedom. We demonstrate that, although the spin alone is not conserved in Rashba-Dirac systems, a combined spin-pseudospin operator is conserved. This conserved quantity represents the interconversion between pure spin and pseudospin textures to a spin-pseudospin entangled structure, where Kane-Mele SOC tunes this balance. By these means, we achieve spin-charge interconversion of 100\% efficiency via the Rashba-Edelstein effect. Quantum transport simulations in disordered micron-size systems demonstrate the robustness of this effect, and also reveal a disorder resilient spin Hall effect generated by the interplay between Rashba and Kane-Mele SOC. Our findings propose a platform for maximally efficient spin-charge interconversion, and establish spin-pseudospin correlations as a mechanism to tailor spintronic devices.

Figures (3)

• Figure 1: Depiction of the REE originating from helical spin textures in graphene. (a) Depiction of the REE effect, where the blue and red arrows represent the spin texture at their respective Fermi contours. Applying an electric field $\mathcal{E}$ generates a net spin density $\bm{S}_\text{REE} \propto \hat{\bm{z}} \times \bm{\mathcal{E}}$ (yellow arrow). (b, c) Band structure for (b) $\lambda_\text{KM} = 0$ and (c) $\lambda_\text{KM} = \lambda_\text{R} = 10 \,\text{meV}$. The Rashba pseudogap (RPG) is shaded in gray, while the light and dark red (blue) curves correspond to the $\chi=+$ ($\chi=-$) subspace. (d) Spin helicity (bottom axis) and spin-pseudospin entanglement (top axis) of the bands. Kane-Mele SOC reduces the effective mass in the blue cone, suppressing entanglement and enhancing the spin helicity throughout the pseudogap, leading to a larger REE.
• Figure 2: REE results. Panels (a) and (b) respectively show $\Theta_\text{REE}$ as a function of the Fermi energy in the clean limit and disordered system. The REE is optimised up to maximal efficiency $\Theta_\text{REE} = \pm 1$ for $\lambda_\text{KM} \!=\! \pm \lambda_\text{R}$ in the clean limit, remaining robust against disorder. Panel (c) shows the maximum $|\Theta_\text{REE}|$ as a function of the disorder strength $W$. [$\lambda_\text{R} = 10 \,\text{meV}$]
• Figure 3: SHE conductivity, $\sigma_\text{SHE}$, for $\lambda_\text{KM}=\lambda_\text{R}$ (main panel) and $\lambda_\text{KM} = 0$ (inset), without disorder and with disorder of $W=2.7 \,\text{eV}$, respectively shown in dashed and solid lines. [$\lambda_\text{R} = 10 \,\text{meV}$]