# Fully spin-polarized nodal loop semimetals in alkaline-metal monochalcogenide monolayers

## TL;DR

This paper predicts a new family of ferromagnetic monolayers that host fully spin-polarized nodal loops, which are robust against spin-orbit coupling and can exhibit various topological phases, promising for spintronics applications.

## Contribution

The study introduces alkaline-metal monochalcogenide monolayers as a new platform for fully spin-polarized nodal loop semimetals with robust topological features.

## Key findings

- Presence of a spin-polarized nodal loop at the Fermi energy.
- Nodal loop stability against spin-orbit coupling due to symmetry protection.
- Ability to induce quantum anomalous Hall and Weyl-like phases by spin orientation.

## Abstract

Topological semimetals in ferromagnetic materials have attracted enormous attention due to the potential applications in spintronics. Using the first-principles density functional theory together with an effective lattice model, here we present a new family of topological semimetals with a fully spin-polarized nodal loop in alkaline-metal monochalcogenide $MX$ ($M$ = Li, Na, K, Rb, Cs; $X$ = S, Se, Te) monolayers. The half-metallic ferromagnetism can be established in $MX$ monolayers, in which one nodal loop formed by two crossing bands with the same spin components is found at the Fermi energy. This nodal loop half-metal survives even when considering the spin-orbit coupling owing to the symmetry protection provided by the $\mathcal{M}_{z}$ mirror plane. The quantum anomalous Hall state and Weyl-like semimetal in this system can be also achieved by rotating the spin from the out-of-plane to the in-plane direction. The $MX$ monolayers hosting rich topological phases thus offer an excellent materials platform for realizing the advanced spintronics concepts.