Topological Type-II Nodal Line Semimetal and Dirac Semimetal State in Stable Kagome Compound Mg3Bi2

Concepts

Natural SciencesMaterials SciencePhysicsCondensed Matter PhysicsMetallic MaterialsSolid State PhysicsMetalsTopological MaterialsTopological Phases Of MatterDirac SemimetalsDirac Surface StatesExotic MaterialsNodal Line SemimetalsSurface StatesSymmetriesTopological Semimetallic PhasesTopological Semimetals

TL;DR

Topological type‑II nodal line semimetals have recently been proposed and differ from conventional type‑I NLS, yet no ambient‑condition stable material has been reported. The study proposes that Mg₃Bi₂ can host a type‑II nodal line state. This is achieved by leveraging time‑reversal and spatial‑inversion symmetries in the stable Kagome structure. Mg₃Bi₂ exhibits a type‑II nodal line with drumhead surface states, a small SOC‑induced gap, and 3D Dirac points, making it a promising platform to study both type‑II NLS and Dirac semimetal physics.

Abstract

Topological type-II nodal line semimetal (NLS) was proposed quite recently and exhibits distinct properties compared with conventional type-I NLS. To date, no ambient-condition stable candidate material has been reported. Here we propose that a stable Kagome compound Mg3Bi2 can host a type-II nodal line state with the protection of time reversal and spatial inversion symmetries. Similar to type-I NLSs, the type-II nodal line in Mg3Bi2 is characterized by the drumhead surface states, which has not been observed in the previous type-II NLSs. The nodal line in Mg3Bi2 can open a minor gap, and a pair of 3D Dirac points occurs when SOC is included. The SOC-induced gap around the nodal line is quite small, and the formation of 3D Dirac points is independent of the nodal line. Therefore, the Mg3Bi2 compound is expected to be a good candidate to investigate the exotic properties of both type-II NLS and 3D Dirac semimetal states.