Magnetic higher-order nodal lines

TL;DR

This paper introduces magnetic higher-order nodal lines with higher-order energy splitting in magnetic systems, providing symmetry conditions, models, and exploring their topological states and surface phenomena.

Contribution

It proposes the concept of magnetic higher-order nodal lines, establishes symmetry conditions for their stabilization, and demonstrates their existence and topological properties through lattice models.

Findings

Magnetic quadratic nodal lines can be the sole band degeneracy at the Fermi level.

Surface states form a torus-shaped band spanning the surface Brillouin zone.

Transformations into topological states like quantum anomalous Hall insulators are possible.

Abstract

Nodal lines, as one-dimensional band degeneracies in momentum space, usually feature a linear energy splitting. Here, we propose the concept of magnetic higher-order nodal lines, which are nodal lines with higher-order energy splitting and realized in magnetic systems with broken time reversal symmetry. We provide sufficient symmetry conditions for stabilizing magnetic quadratic and cubic nodal lines, based on which concrete lattice models are constructed to demonstrate their existence. Unlike its counterpart in nonmagnetic systems, the magnetic quadratic nodal line can exist as the only band degeneracy at the Fermi level. We show that these nodal lines can be accompanied by torus surface states, which form a surface band that span over the whole surface Brillouin zone. Under symmetry breaking, these magnetic nodal lines can be transformed into a variety of interesting topological states, such as three-dimensional quantum anomalous Hall insulator, multiple linear nodal lines, and magnetic triple-Weyl semimetal. The three-dimensional quantum anomalous Hall insulator features a Hall conductivity $\sigma_{xy}$ quantized in unit of $e^2/(hd)$ where $d$ is the lattice constant normal to the $x$-$y$ plane. Our work reveals previously unknown topological states, and offers guidance to search for them in realistic material systems.