Topological semimetals and topological insulators in rare earth monopnictides

TL;DR

This study uses first principles calculations to explore the topological electronic properties of rare earth monopnictides, revealing linked nodal rings, Dirac semimetals, and topological insulators depending on the element and spin-orbit coupling effects.

Contribution

It uncovers a new type of topological band crossing called linked nodal rings and demonstrates how different rare earth monopnictides exhibit diverse topological phases.

Findings

Linked nodal rings in LaN when SOC is neglected

LaN becomes a 3D Dirac semimetal with SOC

Other compounds are topological insulators with surface Dirac cones

Abstract

We use first principles calculations to study the electronic properties of rock salt rare earth monopnictides La$X$ ($X=$N, P, As, Sb, Bi). A new type of topological band crossing termed `linked nodal rings' is found in LaN when the small spin-orbital coupling (SOC) on nitrogen orbitals is neglected. Turning on SOC gaps the nodal rings at all but two points, which remain gapless due to $C_4$-symmetry and leads to a 3D Dirac semimetal. Interestingly, unlike LaN, compounds with other elements in the pnictogen group are found to be topological insulators (TIs), as a result of band reordering due to the increased lattice constant as well as the enhanced SOC on the pnictogen atom. These TI compounds exhibit multi-valley surface Dirac cones at three $\bar{M}$-points on the $(111)$-surface.