Spin and orbital-to-charge conversion in noncentrosymmetric materials: Hall versus Rashba-Edelstein effects

TL;DR

The paper addresses how to disentangle spin-to-charge and orbital-to-charge interconversion in noncentrosymmetric materials when both spin Hall and Rashba-Edelstein mechanisms can contribute. It develops a drift-diffusion framework and microscopic Kubo-based expressions for conversion coefficients that are defined entirely through macroscopic observables, then applies the approach to ferroelectric $\alpha$-GeTe using density functional theory and Wannier-based transport calculations. A key finding is that the effective Rashba parameter $\bar{\alpha}_R$ is much smaller than prior estimates due to interband cancellations, and that the charge current is predominantly generated by the Rashba-Edelstein effect rather than by spin or orbital Hall effects. This work provides a practical route to quantify interconversion in noncentrosymmetric materials and suggests revisiting other systems lacking inversion symmetry with this unified framework, especially where ferroelectric polarization can switch the Rashba texture.

Abstract

We investigate spin- and orbital-to-charge conversion phenomena in nonmagnetic materials with broken inversion symmetry, treating the contributions from the Hall effect and the Rashba-Edelstein effect on an equal footing. We develop a general formalism for this interconversion based solely on macroscopic observables. The theory is validated through a case study of ferroelectric GeTe, where we find that the effective Rashba parameter obtained is smaller than previously reported values for the same material. Incorporating these parameters into a drift-diffusion model, we show that the generated charge current is primarily governed by the Rashba-Edelstein effect, rather than by the spin or orbital Hall effects. Our work redefines the spin- and orbit-to-charge interconversion coefficients in terms of directly measurable observables, encouraging the community to revisit these processes in other quantum materials lacking inversion symmetry.

Figures (7)

• Figure 1: (Color online) Sketch of the spin-to-charge conversion process taking place in a conductive ferroelectric/ferromagnet bilayer. The ferromagnet pumps a spin current (grey arrow) into the ferroelectric, which is then converted into a charge current (white arrows) through SHE and SRE.
• Figure 2: (Color online) Crystal structure and band structure with spin-orbit coupling of ferroelectric GeTe. The displacement of the Te atom with respect to the inversion center produces a ferroelectric polarization along (111) direction, which we aligned with the $\mathbf{z}$ direction for simplicity. The former is obtained using VESTA Vesta2011. We fix the Fermi level at the valence band maximum.
• Figure 3: (Color online) (a) Longitudinal conductivity (black line), and one of the spin Hall conductivity coefficients (red line), as a function of the energy. We also show the orbital Hall conductivity with (blue) and without (green) spin-orbit coupling. (b) Spin Hall angle (red) and orbital Hall angle with (blue) and without (green) spin-orbit coupling.
• Figure 4: (Color online) (a) The SRE (red) and ORE coefficients with (blue) and without spin-orbit coupling (green) as a function of the energy. (b) Corresponding magnetic susceptibility as a function of the energy.
• Figure 5: (Color online) (a) charge-to-spin (red) and charge-to-orbital conversion coefficients with (blue) and without spin-orbit coupling (green) as a function of the energy. (b) Spin-to-charge (red) and orbital-to-charge conversion coefficients with (blue) and without spin-orbit coupling (green) as a function of the energy.