Monolayer Kagome Metals AV$_3$Sb$_5$

Recently, layered kagome metals AV$_3$Sb$_5$ (A = K, Rb, and Cs) have emerged as a fertile platform for exploring frustrated geometry, correlations, and topology. Here, using first-principles and mean-field calculations, we demonstrate that AV$_3$Sb$_5$ can crystallize in a mono-layered form, revealing a range of properties that render the system unique. Most importantly, the two-dimensional monolayer preserves intrinsically different symmetries from the three-dimensional layered bulk, enforced by stoichiometry. Consequently, the van Hove singularities, logarithmic divergences of electronic density of states, are enriched, leading to a variety of competing instabilities such as doublets of charge density waves and s-and d-wave superconductivity. We show that the competition between orders can be fine-tuned in the monolayer via electron-filling of the van Hove singularities. Thus, our results suggest the monolayer kagome metal AV$_3$Sb$_5$ as a promising platform for designer quantum phases.