Transmission Line Model for Materials with Spin-Momentum Locking

Abstract

We provide a transmission line representation for channels exhibiting spin-momentum locking (SML) which can be used for both time-dependent and steady-state transport analysis on a wide variety of materials with spin-orbit coupling such as topological insulators, heavy metals, oxide interfaces, and narrow bandgap semiconductors. This model is based on a time-dependent four-component diffusion equation obtained from the Boltzmann transport equation assuming linear response and elastic scattering in the channel. We classify all electronic states in the channel into four groups ($U^{+}$, $D^{+}$, $U^{-}$, and $D^{-}$) depending on the spin index (up ($U$), down ($D$)) and the sign of the $x$-component of the group velocity ($+,-$) and assign an average electrochemical potential to each of the four groups to obtain the four-component diffusion equation. For normal metal channels, the model decouples into the well-known transmission line model for charge and a time-dependent version of Valet-Fert equation for spin. We first show that in the steady-state limit our model leads to simple expressions for charge-spin interconversion in SML channels in good agreement with existing experimental data on diverse materials. We then use the full time-dependent model to study spin-charge separation in the presence of SML, a subject that has been controversial in the past. Our model shows that the charge and spin signals travel with two distinct velocities resulting in well-known spin-charge separation which is expected to persist even in the presence of SML. However, our model predicts that the lower velocity signal is purely spin while the higher velocity signal is largely charge with an additional spin component which has not been noted before. Finally, we note that our model can be used within standard circuit simulators like SPICE to obtain numerical results for complex geometries.

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Cited by in corpus (9)

• Active topolectrical circuits
• Spin-orbit magnetic state readout in scaled ferromagnetic/heavy metal nanostructures
• Multifunctional Spin Logic Gates In Graphene Spin Circuits
• The Non-Equilibrium Green Function (NEGF) Method
• Unified Framework for Charge-Spin Interconversion in Spin-Orbit Materials
• Edelstein and inverse Edelstein effects caused by the pristine surface states of topological insulators
• Electrical detection of the inverse Edelstein effect on the surface of SmB${}_6$
• Connecting physics to systems with modular spin-circuits
• Rectification in Spin-Orbit Materials Using Low Energy Barrier Magnets