Theory of chirality-induced spin Seebeck effect

In recent years, the application of chiral materials to spintronics has gained attention, and various applications utilizing the coupling between electron spins and phonon excitations in chiral materials have been discussed. Moreover, a spin current strongly dependent on the chirality of the chiral material has been observed in response to the temperature gradient [1]. Recently, the interaction between spin degrees of freedom and macroscopic mechanical rotation, known as gyromagnetic effect, has been extended to the microscopic rotation of the lattice in chiral materials for describing phonon-spin conversion [2].

In this study, we consider a chiral insulator-normal metal junction system, where the junction interface is set perpendicular to the chiral axis, and a temperature gradient is applied between both ends of the sample. This creates a temperature gradient in the bulk of the chiral insulator, leading to a difference in the distribution of phonons with rightward and leftward velocities. As a result, phonons are converted into a spin current at the junction interface. We calculated the spin current using the spin-phonon interaction based on spin-rotation coupling.

[1] K. Kim et al., Nat. Mater. 22, 322 (2023).

[2] T. Funato et al., Phys. Rev. Lett. 132, 236201 (2024).