Ferromagnetic hybrid nodal loop and switchable type-I and type-II Weyl fermions in two-dimension

TL;DR

This paper predicts a ferromagnetic 2D CrN monolayer hosting a hybrid nodal loop with both type-I and type-II Weyl fermions, which can be manipulated via strain and magnetization direction, opening new avenues in 2D topological materials.

Contribution

First proposal of a ferromagnetic 2D material with a hybrid nodal loop and switchable Weyl fermions, demonstrating robustness and tunability of topological features.

Findings

High Curie temperature (>600 K) FM ground state.

Presence of a fully spin-polarized hybrid nodal loop.

Switchable Weyl nodes by changing magnetization orientation.

Abstract

As a novel type of fermionic state, hybrid nodal loop with the coexistence of both type-I and type- II band crossings has attracted intense research interest. However, it remains a challenge to realize hybrid nodal loop in both two-dimensional (2D) materials and in ferromagnetic (FM) materials. Here, we propose the first FM hybrid nodal loop in 2D CrN monolayer. We show that the material has a high Curie temperature (> 600 K) FM ground state, with the out-of-plane [001] magnetization. It shows a half-metallic band structure with two bands in the spin-up channel crossing each other near the Fermi level. These bands produce both type-I and type-II band crossings, which form a fully spin-polarized hybrid nodal loop. We find the nodal loop is protected by the mirror symmetry and robust against spin-orbit coupling (SOC). An effective Hamiltonian characterizing the hybrid nodal loop is established. We further find the configuration of nodal loop can be shifted under external perturbations such as strain. Most remarkably, we demonstrate that both type-I and type-II Weyl nodes can be realized from such FM hybrid nodal loop by simply shifting the magnetization from out-of-plane to in-plane. Our work provides an excellent candidate to realize FM hybrid nodal loop and Weyl fermions in 2D material, and is also promising for related topological applications with their intriguing properties.