Type-II Dirac line node in strained Na3N

TL;DR

This paper predicts that strained Na3N becomes a type-II Dirac line node semimetal with topological properties, characterized by band velocities, Berry phase, and optical conductivity, using first-principles calculations and models.

Contribution

It introduces a criterion for classifying Dirac line nodes as type-I or type-II and predicts a new topological phase in strained Na3N with experimental signatures.

Findings

Na3N under strain hosts a type-II DLN semimetal phase.

The topological nature is confirmed by Berry phase and surface spectrum.

Optical conductivity reveals the DLN in strained Na3N.

Abstract

Dirac line node (DLN) semimetals are a class of topological semimetals that feature band-crossing lines in momentum space. We study the type-I and type-II classification of DLN semimetals by developing a criterion that determines the type using band velocities. Using first-principles calculations, we also predict that Na3N under an epitaxial tensile strain realizes a type-II DLN semimetal with vanishing spin-orbit coupling (SOC), characterized by the Berry phase that is Z2-quantized in the presence of inversion and time-reversal symmetries. The surface energy spectrum is calculated to demonstrate the topological phase, and the type-II nature is demonstrated by calculating the band velocities. We also develop a tight-binding model and a low-energy effective Hamiltonian that describe the low-energy electronic structure of strained Na3N. The occurrence of a DLN in Na3N under strain is captured in the optical conductivity, which we propose as a means to experimentally confirm the type-II class of the DLN semimetal.