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 PbHfO2 by alternately stacking Pb and 1T-HfO2 monolayers. The new compound PbHfO2 is dynamically stable and retains the properties of Pb monolayer. By applying epitaxial strains to PbHfO2, 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.
@article{arxiv.2009.01718,
title = {Designing Xenes with Two-Dimensional Triangular Lattice},
author = {Xu Duan and Zhao Liu and Brendan M. Hanrahan and Wei Zhu and Shi Liu},
journal= {arXiv preprint arXiv:2009.01718},
year = {2021}
}