# Intrinsic persistent spin helix in two-dimensional group-IV monochalcogenide MX (M : Sn, Ge; X: S, Se, Te) monolayer

## TL;DR

This paper predicts that certain 2D group-IV monochalcogenide monolayers inherently support persistent spin helix states due to their symmetry and ferroelectric properties, enabling long spin lifetimes for energy-efficient spintronics.

## Contribution

It demonstrates the intrinsic presence of persistent spin helix in 2D MX monolayers with in-plane ferroelectricity, eliminating the need for fine-tuning spin-orbit couplings.

## Key findings

- PSH observed at valence band maximum
- Reversible out-of-plane spin orientation with ferroelectric switching
- Sizable spin splitting with small spin polarization wavelength

## Abstract

Energy-saving spintronics are believed to be implementable on the systems hosting persistent spin helix (PSH) since they support an extraordinarily long spin lifetime of carriers. However, achieving the PSH requires a unidirectional spin configuration in the momentum space, which is practically non-trivial due to the stringent conditions for fine-tuning the Rashba and Dresselhaus spin-orbit couplings. Here, we predict that the PSH can be intrinsically achieved on a two-dimensional (2D) group-IV monochalcogenide M X monolayer, a new class of the noncentrosymmetric 2D materials having in-plane ferroelctricity. Due to the C2v point group symmetry in the MX monolayer, a unidirectional spin configuration is preserved in the out-of-plane direction and thus maintains the PSH that is similar to the [110] Dresselhaus model in the [110]-oriented quantum well. Our first-principle calculations on various MX (M : Sn, Ge; X: S, Se, Te) monolayers confirmed that such typical spin configuration is observed, in particular, at near the valence band maximum where a sizable spin splitting and a substantially small wavelength of the spin polarization are achieved. Importantly, we observe reversible out-of-plane spin orientation under opposite in-plane ferroelectric polarization, indicating that an electrically controllable PSH for spintronic applications is plausible.