Exact low-temperature behavior of a kagomé antiferromagnet at high fields

Low-energy degrees of freedom of a spin-$1∕2$ kagom\'e antiferromagnet in the vicinity of the saturation field are mapped to a hard-hexagon model on a triangular lattice. The latter model is exactly solvable. The presented mapping allows us to obtain the quantitative description of the magnetothermodynamics of a quantum kagom\'e antiferromagnet up to exponentially small corrections as well as predict the critical behavior for the transition into a magnon crystal state. Analogous mapping is presented for the sawtooth chain, which is mapped onto a model of classical hard dimers on a chain.