Type-III Weyl Semi-Half-Metal in an Ultralight Monolayer Li$_2$N

TL;DR

This paper predicts a novel magnetic topological state, the type-III Weyl semi-half-metal, in monolayer Li$_2$N, characterized by fully spin-polarized Weyl fermions with strong transport anisotropy, robust under strain.

Contribution

It introduces the first theoretical prediction of a type-III Weyl semi-half-metal state in monolayer Li$_2$N, combining magnetic half-metallicity with topological Weyl features.

Findings

Identifies a fully spin-polarized, critically tilted Weyl cone in Li$_2$N

Demonstrates robustness of the Weyl SHM phase under biaxial strain

Uncovers topologically nontrivial edge states associated with the Weyl phase

Abstract

The interplay between magnetic ordering and band topology has emerged as a fertile ground for discovering novel quantum states with profound implications for fundamental physics and next-generation electronics. Here, we theoretically predict a new type-III Weyl semi-half-metal (SHM) state in monolayer Li$_2$N, uniquely combining magnetic half-metallicity and type-III Weyl semimetal characteristics. First-principles calculations reveal a fully spin-polarized and critically tilted Weyl cone around the Fermi level in monolayer Li$_2$N, driven by $p$-orbital ferromagnetism. This arises from the symmetry-protected band crossing between a flat valence band and a highly dispersive conduction band, leading to type-III Weyl fermions with strong transport anisotropy. A low-energy $k{\cdot}p$ Hamiltonian is constructed and corresponding nontrivial edge states are uncovered to capture the topological nature of Li$_2$N. Notably, this Weyl SHM phase remains robust under biaxial strain ranging from -2$\%$ to $4\%$, with an ideal type-III Weyl fermion emerging alongside a line-like ergodic surface emerging at 3.7$\%$ strain, offering a promising platform for exploring correlated electronic phenomena. Our results establish Li$_2$N as a viable candidate for realizing exotic type-III Weyl SHM states and open a new avenue for exploring the intricate interplay among magnetism, topology, and flat-band physics.