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Spin current, the flow of spin angular momentum, is a central element in spintronics for future technological applications. Thus, elucidating various mechanisms to generate spin currents is an important topic. Since the discovery of the gyromagnetic effect more than 100 years ago by Einstein, de Haas, and Barnett, spin angular momentum has been known to be mutually converted with the mechanical angular momentum associated with rotational motion of materials. This suggests that spin currents can be generated mechanically. Present-day experiments have shown that spin currents are generated by shear flows in liquid metals and by surface acoustic waves in solids.

The electron spin also interacts with its orbital motion through relativistic effects, that is, the spin-orbit interaction (SOI). The SOI is responsible for various spin-current generation methods because it bends the electron orbits in spin-dependent directions. In particular, the Rashba SOI appears in systems with broken spatial inversion symmetries, such as at the surfaces and interfaces of materials. When generating spin currents using surface acoustic waves, the effects of Rashba SOI may be utilized.

In this study, we investigated spin-current generation from dynamic lattice distortions in systems with Rashba SOI. Unlike prior theoretical studies, we started from a multiorbital tight-binding model to derive a Rashba model perturbed by lattice distortions. This method enabled us to treat the lattice distortion effects microscopically through the modulation of hopping integrals and local rotation of the crystal axes. By calculating the linear response to the effective perturbations, we observed that surface acoustic waves can generate a variety of spin currents through the Rashba SOI, including unconventional spin currents, such as the quadrupolar spin current, perpendicular spin current, and helicity current.

Written by Y. Ogawa on behalf of all authors.