Two-dimensional (2D) materials with nodal-loop band crossing have been attracting great research interest. However, it remains a challenge to find 2D nodal loops that are robust against spin-orbit coupling (SOC) and realized in magnetic states. Here, based on first-principles calculations and theoretical analysis, we predict that monolayer MnN is a 2D nodal-loop half metal with fully spin polarized nodal loops. We show that monolayer MnN has a ferromagnetic ground state with out-of-plane magnetization. Its band structure shows half metallicity with three low-energy bands belonging to the same spin channel. The crossing between these bands forms two concentric nodal loops centered around the Γ point near the Fermi level. Remarkably, the nodal loops and their spin polarization are robust under SOC, due to the protection of a mirror symmetry. We construct an effective model to characterize the fully polarized emergent nodal-loop fermions. We also find that a uniaxial strain can induce a loop transformation from a localized single loop circling around Γ to a pair of extended loops penetrating the Brillouin zone.
@article{arxiv.1903.04705,
title = {Two-dimensional nodal-loop half metal in monolayer MnN},
author = {Shan-Shan Wang and Zhi-Ming Yu and Ying Liu and Yalong Jiao and Shan Guan and Xian-Lei Sheng and Shengyuan A. Yang},
journal= {arXiv preprint arXiv:1903.04705},
year = {2019}
}