Chiral excitonic order from twofold van Hove singularities in kagome metals

Recent experiments on kagome metals AV(3)Sb(5) (A=K,Rb,Cs) identify twofold van Hove singularities (TvHS) with opposite concavity near the Fermi energy, generating two approximately hexagonal Fermi surfaces - one electron-like and the other hole-like. Here we propose that a TvHS generates a novel time-reversal symmetry breaking excitonic order - arising due to bound pairs of electrons and holes located at opposite concavity van Hove singularities. We introduce a minimal model for the TvHS and investigate interaction induced many-body instabilities via the perturbative renormalisation group technique and a free energy analysis. Specialising to parameters appropriate for the kagome metals AV(3)Sb(5), we construct a phase diagram comprising chiral excitons, charge density wave and a region of coexistence. We propose this as an explanation of a diverse range of experimental observations in AV(3)Sb(5). Notably, the chiral excitonic state gives rise to a quantum anomalous Hall conductance, providing an appealing interpretation of the observed anomalous Hall effect in kagome metals. Possible alternative realisations of the TvHS mechanism in bilayer materials are also discussed. We suggest that TvHS open up interesting possibilities for correlated phases, enriching the set of competing ground states to include excitonic order.

Automated summary

Recent experiments on kagome metals AV(3)Sb(5) reveal the presence of twofold van Hove singularities (TvHS) near the Fermi energy, leading to the formation of two approximately hexagonal Fermi surfaces - one electron-like and the other hole-like. A TvHS is suggested to give rise to a novel time-reversal symmetry breaking excitonic order, caused by bound pairs of electrons and holes located at opposite concavity van Hove singularities. A minimal model for TvHS is introduced, and many-body instabilities induced by interactions are investigated using perturbative renormalisation group technique and free energy analysis. A phase diagram consisting of chiral excitons, charge density wave, and a coexistence region is constructed for the kagome metals AV(3)Sb(5), explaining various experimental observations. The chiral excitonic state leads to a quantum anomalous Hall conductance, providing an explanation for the observed anomalous Hall effect in kagome metals. Possible alternative realisations of the TvHS mechanism in bilayer materials are also discussed, suggesting that TvHS offer interesting possibilities for correlated phases, expanding the range of competing ground states to include excitonic order.