Highly anisotropic two-dimensional metal in monolayer MoOCl2

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 ${\mathrm{MoOCl}}_{2}$---and show that it possesses intriguing properties related to its high anisotropy. Monolayer ${\mathrm{MoOCl}}_{2}$ 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 ${\mathrm{MoOCl}}_{2}$. The material is a metal with highly anisotropic Fermi surfaces, giving rise to open orbits at the Fermi level, which can be probed in magnetotransport. Remarkably, the combination of high anisotropy and metallic character makes monolayer ${\mathrm{MoOCl}}_{2}$ 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 of this window is very small. The window can be further tuned by the applied strain, such as 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.