Line-Node Dirac Semimetal and Topological Insulating Phase in Noncentrosymmetric Pnictides CaAgX (X = P, As)

TL;DR

This paper investigates noncentrosymmetric pnictides CaAgP and CaAgAs, revealing their transition from topological line-node semimetals to topological insulators due to spin-orbit interaction, and introduces a new method to calculate Z2 invariants.

Contribution

It demonstrates the topological phase transition in CaAgX compounds and proposes an alternative approach to compute Z2 invariants in mirror-symmetry protected systems.

Findings

CaAgP and CaAgAs are topological line-node semimetals.

Spin-orbit interaction induces a band gap in CaAgAs, turning it into a topological insulator.

A new method for calculating Z2 invariants without inversion symmetry is proposed.

Abstract

Two noncentrosymmetric ternary pnictides, CaAgP and CaAgAs, are reported as topological line-node semimetals protected solely by mirror-reflection symmetry. The band gap vanishes on a circle in momentum space, and surface states emerge within the circle. Extending this study to spin-orbit coupled systems reveals that, compared with CaAgP, a substantial band gap is induced in CaAgAs by large spin-orbit interaction. The resulting states are a topological insulator, in which the Z2 topological invariant is given by 1; 000. To clarify the Z2 topological invariants for time-reversal-invariant systems without spatial-inversion symmetry, we introduce an alternative way to calculate the invariants characterizing a line node and topological insulator for mirror-reflection-invariant systems.