Shift spin photocurrents in two-dimensional systems

TL;DR

This paper theoretically investigates how circularly polarized light induces nonlinear spin photocurrents in two-dimensional systems, revealing symmetry-dependent behaviors and conditions for their generation, with potential implications for spintronics.

Contribution

It identifies the conditions under which shift spin photocurrents occur in 2D systems, including symmetry considerations and effects of Zeeman coupling, which were not previously detailed.

Findings

Shift spin photocurrents are supported in specific 2D systems like Rashba-Dresselhaus and Dirac surface states.

Mirror symmetry dictates the orientation of spin polarization and photocurrent direction.

Zeeman coupling induces a peak in shift spin conductivity at a specific optical frequency.

Abstract

The generation of nonlinear spin photocurrents by circularly polarized light in two-dimensional systems is theoretically investigated by calculating the shift spin conductivities. In time-reversal symmetric systems, shift spin photocurrent can be generated under the irradiation of circularly polarized light , while the shift charge photoccurrent is forbidden by symmetry. We show that the $k$-cubic Rashba-Dresselhaus system, the $k$-cubic wurtzite system and Dirac surface states can support the shift spin photocurrent. By symmetry analysis, it is found that in the Rashba type spin-orbit coupled systems, mirror symmetry requires that the spin polarization and the moving direction of the spin photocurrent be parallel, which we name longitudinal shift spin photocurrent. The Dirac surface states with warping term exhibit mirror symmetry, similar to the Rashba type system, and support longitudinal shift spin photocurrent. In contrast, in the Dresselhaus type spin-orbit coupled systems, the parity-mirror symmetry requires that the spin polarization and the moving direction of the spin photocurrent be perpendicular, which we dub transverse shift spin photocurrent. Furthermore, we find that the shift spin photocurrent always vanishes in any $k$-linear spin-orbit coupled system unless the Zeeman coupling is turned on. We find that the splitting of degenerate energy bands due to Zeeman coupling $\mu_z$ causes the van Hove singularity. The resulting shift spin conductivity has a significant peak at optical frequency $\omega=2\mu_z/\hbar$.