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

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.

Fully spin-polarized double-Weyl fermions with type-III dispersion in quasi-one dimensional materials X2RhF6 (X=K, Rb, Cs)

“…Second, the double Weyl points proposed here are solely formed by the bands from the spin-down channel; thus, they are fully spin-polarized. Fully spinpolarized fermions have also been proposed in several other half-metal phases, such as the nodal-loop fermions in compounds Li 3 (FeO 3 ) 2 [69], β-V 2 OPO 4 [70], MnN and CrN monolayers [71,72], nodal-chain fermions in half-metallic LiV 2 O 4 [73], Dirac/Weyl fermions in PtCl 3 monolayer [74], YN 2 monolayer [75,76], and some spinel compounds [77]. Such fully spin-polarized fermions are believed for special spintronics applications [69,71].…”