Highly anisotropic two-dimensional metal in monolayer MoOCl$_2$

TL;DR

This paper predicts a stable, highly anisotropic 2D metallic material, monolayer MoOCl$_2$, with unique optical and electronic properties, including wide hyperbolic frequency windows and tunability via strain, promising for advanced applications.

Contribution

It introduces monolayer MoOCl$_2$ as a new 2D metal with exceptional anisotropic properties and tunable hyperbolic optical windows, based on first-principles calculations and analysis.

Findings

Monolayer MoOCl$_2$ is a stable, exfoliable 2D metal.

It exhibits two wide hyperbolic frequency windows, including part of the visible spectrum.

Strain can tune the transition between elliptic and hyperbolic optical behavior.

Abstract

Anisotropy is a general feature in materials. Strong anisotropy could lead to interesting physical properties and useful applications. Here, based on first-principles calculations and theoretical analysis, we predict a stable two-dimensional (2D) material---the monolayer MoOCl$_2$, and show that it possesses intriguing properties related to its high anisotropy. Monolayer MoOCl$_2$ can be readily exfoliated from the van der Waals layered bulk, which has already been synthesized. We show that a high in-plane anisotropy manifests in the structural, phononic, mechanical, electronic, and optical properties of monolayer MoOCl$_2$. The material is a metal with highly anisotropic Fermi surfaces, giving rise to open orbits at the Fermi level, which can be probed in magneto-transport. Remarkably, the combination of high anisotropy and metallic character makes monolayer MoOCl$_2$ an almost ideal hyperbolic material. It has two very wide hyperbolic frequency windows from 0.41 eV (99 THz) to 2.90 eV (701 THz), and from 3.63 eV (878 THz) to 5.54 eV (1340 THz). The former window has a large overlap with the visible spectrum, and the dissipation for most part of this window is very small. The window can be further tuned by the applied strain, such that at a chosen frequency, a transition between elliptic and hyperbolic character can be induced by strain. Our work discovers a highly anisotropic 2D metal with extraordinary properties, which holds great potential for electronic and optical applications.