# Nonlinear spin current generation in noncentrosymmetric spin-orbit coupled systems

## TL;DR

This paper introduces a theoretical mechanism for generating nonlinear spin currents in noncentrosymmetric spin-orbit coupled systems using electric fields, with potential for enhanced efficiency and control in spintronics applications.

## Contribution

It proposes a new nonlinear spin current generation mechanism based on electric field application, supported by Boltzmann transport theory, applicable to topological insulators and Rashba/Dresselhaus systems.

## Key findings

- Nonlinear spin current proportional to E^2 induced by electric fields.
- Enhanced spin current magnitude compared to previous methods.
- Rectification of spin current with AC electric fields like terahertz light.

## Abstract

Spin current plays a central role in spintronics. In particular, finding more efficient ways to generate spin current has been an important issue and studied actively. For example, representative methods of spin current generation include spin polarized current injections from ferromagnetic metals, spin Hall effect, and spin battery. Here we theoretically propose a new mechanism of spin current generation based on nonlinear phenomena. By using Boltzmann transport theory, we show that a simple application of the electric field $\bf{E}$ induces spin current proportional to $\bf{E^2}$ in noncentrosymmetric spin-orbit coupled systems. We demonstrate that the nonlinear spin current of the proposed mechanism is supported in the surface state of three-dimensional topological insulators and two-dimensional semiconductors with the Rashba and/or Dresselhaus interaction. In the latter case, the angular dependence of the nonlinear spin current can be manipulated by the direction of the electric field and by the ratio of the Rashba and Dresselhaus interactions. We find that the magnitude of the spin current largely exceeds those in the previous methods for a reasonable magnitude of the electric field. Furthermore, we show that application of AC electric fields (e.g. terahertz light) leads to the rectifying effect of the spin current where DC spin current is generated. These findings will pave a new route to manipulate the spin current in noncentrosymmetric crystals.