Starfruit-like nodal semimetal to Dirac and Weyl semimetal state in CaAuAs

TL;DR

This paper predicts that CaAuAs hosts a starfruit-like nodal-link structure that transitions into Dirac and Weyl semimetal states upon inclusion of spin-orbit coupling, revealing rich topological phases.

Contribution

It introduces a new topological phase in CaAuAs with a unique nodal-link structure and details the evolution into Dirac and Weyl semimetals with symmetry analysis.

Findings

CaAuAs exhibits a starfruit-like nodal-link structure without SOC.

Turning on SOC gaps the nodal loops, leading to a Dirac semimetal.

Symmetry breaking can produce Weyl semimetal and topological insulator phases.

Abstract

Band-crossings occurring on a mirror plane are compelled to form a nodal loop in the momentum space without spin-orbit coupling (SOC). In the presence of other equivalent mirror planes, multiple such nodal loops can combine to form interesting nodal-link structures. Here, based on first-principles calculations and an effective $\mathbf{k.p}$ model analysis, we show that CaAuAs hosts a unique starfruit-like nodal-link structure in the bulk electronic dispersion in the absence of SOC. This nodal-link is comprised of three nodal loops, which cross each other at the time-reversal-invariant momentum point $A$. When the SOC is turned on, the nodal loops are gapped out, resulting in a stable Dirac semimetal state with a pair of Dirac points along the $\mathrm{\Gamma-A}$ direction in the Brillouin zone. The Dirac points are protected by the combination of time reversal, inversion, and $C_3$ rotation symmetries. We show how a systematic elimination of the symmetry constraints yields a Weyl semimetal and eventually a topological insulator state.