A comprehensive theory of second-order spin photocurrents

TL;DR

This paper develops a comprehensive theory for second-order spin photocurrents in materials, revealing new phenomena like pure spin rectification in insulators and demonstrating their tunability and potential for spintronics applications.

Contribution

It introduces a general classification of second-order spin photocurrents, including a novel prediction of pure spin rectification in insulators induced by subgap photons.

Findings

Pure spin rectification current predicted in insulators with subgap photons.

Large, observable spin photocurrents in BiTeI induced by subgap light.

Spin photocurrents are highly tunable with light polarization.

Abstract

The spin photocurrents, direct currents induced by light, hold great promise for introducing new elements to spintronics. However, a general theory for spin photocurrents in real materials which is applicable to systems with spin-orbit coupling or noncollinear magnetism is absent. Here, we develop such a general theory of second-order spin photocurrents. We find that the secondorder spin photocurrents can be classified into Drude, Berry curvature dipole, shift, injection, and rectification currents, which have different physical origins and symmetry properties. Surprisingly, our theory predicts a direct pure spin rectification current in an insulator induced by photons with energies lower than the material band gap. This phenomenon is absent in the case of the charge photocurrent. We find that the pure spin current of BiTeI induced by subgap light is large enough to be observable in experiments. Moreover, the subgap pure spin photocurrent is highly tunable with the polarization of light and the flowing direction of the spin photocurrent. This study lays the groundwork for the study of nonlinear spin photocurrents in real materials and provides a route to engineer light-controlled spin currents.