Chiral excitonic order from twofold van Hove singularities in kagome metals

TL;DR

This paper proposes that twofold van Hove singularities in kagome metals lead to a novel chiral excitonic order, explaining experimental phenomena like the anomalous Hall effect and enriching the understanding of correlated phases.

Contribution

It introduces a minimal model linking TvHS to chiral excitonic order and constructs a phase diagram relevant for kagome metals, highlighting a new mechanism for symmetry breaking.

Findings

Identification of TvHS as a source of chiral excitonic order

Construction of a phase diagram including excitons and charge density waves

Explanation of anomalous Hall effect in kagome metals

Abstract

Recent experiments on kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) [M. Kang et al., Nat. Phys. 18, 301-308 (2022)] 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.