Designing Xenes with Two-Dimensional Triangular Lattice

TL;DR

This study explores the electronic properties of 2D triangular lattice monolayers of group-IV elements like Pb, Ge, and Sn, revealing their topological features and potential for designing new Xenes with tunable electronic phases.

Contribution

It introduces the concept of 2D triangular lattice Xenes, analyzes their topological properties, and proposes a new quasi-3D material, PbHfO$_2$, with tunable electronic and ferroelectric phases.

Findings

Flat Pb monolayer supports a mirror-symmetry-protected spinless nodal line.

Buckling creates a glide mirror protecting an anisotropic Dirac nodal loop.

PbHfO$_2$ can undergo insulator-metal and anti-ferroelectric-paraelectric transitions.

Abstract

Xenes, graphene-like two-dimensional (2D) monoelemental crystals with a honeycomb symmetry, have been the focus of numerous experimental and theoretical studies. In comparison, single-element 2D materials with a triangular lattice symmetry have not received due attention. Here, taking Pb as an example, we investigate the triangular-lattice monolayer made of group-IV atoms employing first-principles density functional theory calculations. The flat Pb monolayer supports a mirror-symmetry-protected spinless nodal line in the absence spin-orbit coupling (SOC). The introduction of an out-of-plane buckling creates a glide mirror, protecting an anisotropic Dirac nodal loop. Both flat and buckled Pb monolayers become topologically trivial after including SOC. A large buckling will make the Pb sheet a 2D semiconductor with symmetry-protected Dirac points below the Fermi level. The electronic structures of other group-IV triangular lattices such as Ge and Sn demonstrate strong similarity to Pb. We further design a quasi-3D crystal PbHfO$_2$ by alternately stacking Pb and 1T-HfO$_2$ monolayers. The new compound PbHfO$_2$ is dynamically stable and retains the properties of Pb monolayer. By applying epitaxial strains to PbHfO$_2$, it is possible to drive an insulator-to-metal transition coupled with an anti-ferroelectric-to-paraelectric phase transition. Our results suggest the potential of the 2D triangular lattice as a complimentary platform to design new type of broadly-defined Xenes.