Emergence of Topological insulator and Nodal line semi-metal states in XX'Bi (X=Na, K, Rb, Cs; X'=Ca, Sr)

TL;DR

This study predicts that certain XX'Bi compounds can exhibit topological insulator and nodal line semi-metal states, with their topological properties tunable via pressure, strain, or doping, confirmed through first-principles calculations.

Contribution

It introduces new topological phases in XX'Bi compounds and demonstrates methods to engineer their band topology, including pressure, strain, and doping, supported by first-principles calculations.

Findings

NaSrBi and NaCaBi are strong topological insulators under band engineering.

Some compounds exhibit nodal line semi-metal states without spin-orbit coupling.

Spin-orbit coupling opens a small band gap, transforming them into topological insulators.

Abstract

In this letter, we predict the emergence of non-trivial band topology in the family of XX'Bi compounds having $P\overline{6}2m$ (\# 189) space group. Using first principles calculations within hybrid functional framework, we demonstrate that NaSrBi and NaCaBi are strong topological insulator under controlled band engineering. Here, we propose three different ways to engineer the band topology to get a non-trivial order: (i) hydrostatic pressure, (ii) biaxial strain (due to epitaxial mismatch), and (iii) doping. Non-triviality is confirmed by investigating bulk band inversion, topological Z$_2$ invariant, surface dispersion and spin texture. Interestingly, some of these compounds also show a three dimensional topological nodal line semi-metal (NLS) state in the absence of spin orbit coupling (SOC). In these NLS phases, the closed loop of band degeneracy in the Brillouin zone lie close to the Fermi level. Moreover, a drumhead like flat surface state is observed on projecting the bulk state on the [001] surface. The inclusion of SOC opens up a small band gap making them behave like a topological insulator.