From Node-Line Semimetals to Large Gap QSH States in New Family of Pentagonal Group-IVA Chalcogenide

TL;DR

This paper predicts a new family of 2D pentagonal IVX2 materials that can host node-line semimetals and large-gap quantum spin Hall states, with tunable properties and potential for device applications.

Contribution

It introduces a new class of 2D pentagonal IVX2 materials exhibiting node-line semimetal phases and large-gap topological insulator states, expanding the landscape of 2D topological materials.

Findings

Node-line semimetals in p-IVX2 are protected by mirror symmetry.

Large bulk gaps up to 0.715 eV are induced by spin-orbit coupling.

Nontrivial topology persists under external strain and in quantum well structures.

Abstract

Two-dimensional (2D) topological insulators (TIs) have attracted tremendous research interest from both theoretical and experimental fields in recent years. However, it is much less investigated in realizing node line (NL) semimetals in 2D materials.Combining first-principles calculations and $k \cdot p$ model, we find that NL phases emerge in p-CS$_2$ and p-SiS$_2$, as well as other pentagonal IVX$_2$ films, i.e. p-IVX$_2$ (IV= C, Si, Ge, Sn, Pb; X=S, Se, Te) in the absence of spin-orbital coupling (SOC). The NLs in p-IVX$_2$ form symbolic Fermi loops centered around the $\Gamma$ point and are protected by mirror reflection symmetry. As the atomic number is downward shifted, the NL semimetals are driven into 2D TIs with the large bulk gap up to 0.715 eV induced by the remarkable SOC effect.The nontrivial bulk gap can be tunable under external biaxial and uniaxial strain. Moreover, we also propose a quantum well by sandwiching p-PbTe$_2$ crystal between two NaI sheets, in which p-PbTe$_2$ still keeps its nontrivial topology with a sizable band gap ($\sim$ 0.5 eV). These findings provide a new 2D materials family for future design and fabrication of NL semimetals and TIs.