Transverse spin photocurrents in ultrathin topological insulator films

TL;DR

This paper theoretically investigates how linear and circular polarized light generate transverse charge and spin photocurrents in ultrathin topological insulator films, revealing symmetry-breaking effects that produce helical spin currents.

Contribution

It introduces a theoretical framework for understanding spin and charge photocurrents in ultrathin topological insulators with broken surface symmetry, highlighting the generation of helical transverse spin currents.

Findings

Helical transverse spin currents are generated when surface symmetry is broken.

Spin currents exhibit s-wave and d-wave components under rotation transformations.

Potential applications include spin current photo-transistors and amplifiers.

Abstract

Nonlinear helicity-dependent photocurrents have been reported in 3D topological materials lacking inversion symmetry. Here, we theoretically study the charge and spin photocurrents generated by linear and circularly polarized radiation in ultrathin topological insulator films. Using time-dependent perturbation theory and detailed balance equations, we find that helical transverse spin currents are generated when the symmetry between the top and bottom film surfaces is disturbed. Such spin currents are invariant under in-plane mirror transformations but have $s$-wave and $d$-wave components in regard to transformations under $\pi/2$ rotations. Spin current photo-transistors and amplifiers can be based on these effects.