The Two‐Dimensional Electrides XONa (X=Mg, Ca) as Novel Natural Hyperbolic Materials

In two‐dimensional electrides, anionic electrons are spatially confined in interlayer regions with high density, comparable to metals, and they are highly mobile, just as free electrons, resembling hyperbolic metamaterials with metal‐dielectric multilayered structures. In this work, two‐dimensional electride materials MgONa and CaONa are proposed as good natural hyperbolic materials. By using the first‐principles calculations based on density functional theory (DFT), the electronic structures, stabilities, and optical properties of two‐dimensional electride materials XONa (X=Mg, Ca) are investigated. Our results show that they are stable in 1‐monolayer (1‐ML) structures as well as in bulk states. They exhibit hyperbolic dispersions from visible to near infrared spectral range with high qualities up to about 700, which is two orders‐of‐magnitude larger than the preceding bulk hyperbolic materials. Numerical results reveal that they exhibit negative refraction with low losses. Their high‐quality hyperbolic responses over a wide spectral range pave the way of broad photonic applications as natural hyperbolic materials. Two‐dimensional electrides of the type XONa (X=Mg, Ca) are proposed as high‐quality natural hyperbolic materials. Using first‐principles calculations and electromagnetic simulations, their electronic structures and optical responses are investigated in detail. Their quality factors reach a value of up to about 700, which is two orders of magnitude larger than those of preceding bulk hyperbolic materials.