Topological Semimetal-Insulator Quantum Phase Transition in Zintl Compounds Ba2X (X=Si, Ge)

TL;DR

This paper uses first-principles calculations to reveal a topological phase transition in Ba2X compounds, showing how they can switch between semimetal and insulator states with surface states and strain tuning.

Contribution

It uncovers a new class of materials exhibiting a quantum phase transition between topological semimetal and insulator phases, with tunability via strain.

Findings

Ba2X hosts a nodal ring semimetal phase protected by glide mirror symmetry.

Turning on spin-orbit coupling induces a transition to a topological insulator.

Strain can tune the surface states and induce a trivial insulating phase.

Abstract

By first-principles calculations, we find that Ba2X(X=Si, Ge) hosts a topological semimetal phase with one nodal ring in the kx=0 plane, which is protected by the glide mirror symmetry when spin-orbit coupling (SOC) is ignored. The corresponding drumheadlike surface flat band appears on the (100) surface in surface Green function calculation. Furthermore, a topological-semimetal-to-insulator transition (TSMIT) is found. The nodal line semimetal would evolve into topological insulator as SOC is turned on. The topologically protected metallic surface states emerge around the Gamma=0 point, which could be tuned into topologically-trivial insulator state by more than 3% hydrostatic strain. These results reveal a new category of materials showing quantum phase transition between topological semimetal and insulator, and tunability through elastic strain engineering.