Chiral excitonic order from twofold van Hove singularities 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.

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• Commensurate-to-incommensurate transition of charge-density-wave order and a possible quantum critical point in pressurized kagome metal CsV${}_3$Sb${}_5$
• Intertwined van-Hove Singularities as a Mechanism for Loop Current Order in Kagome Metals
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• Drastic magnetic-field-induced chiral current order and emergent current-bond-field interplay in kagome metals
• The kagome Hubbard model from a functional renormalization group perspective
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• Kagome metals
• Emergent topological quantum orbits in the charge density wave phase of kagome metal CsV${}_3$Sb${}_5$
• Enhanced nematicity emerging from higher-order van Hove singularities
• Existence of Hebel-Slichter peak in unconventional kagome superconductors
• Condensation of preformed charge density waves in kagome metals
• Origin of anomalous magnetotransport in kagome superconductors AV${}_3$Sb${}_5$ (A=K,Rb,Cs)
• Two elementary band representation model, Fermi surface nesting, and surface topological superconductivity in $A$V${}_3$Sb${}_5$ ($A=\text{K, Rb, Cs}$)
• Perspective: imaging atomic step geometry to determine surface terminations of kagome materials and beyond
• Unified picture of superconductivity and magnetism in CeRh${}_2$As${}_2$
• Interplay between Chiral Charge Density Wave and Superconductivity in Kagome Superconductors: A Self-consistent Theoretical Analysis
• Three-dimensional higher-order saddle points induced flat bands in Co-based kagome metals
• Real-space Loop Current Pattern in Time-reversal-symmetry Breaking Phase in Kagome Metals
• Higher-Order Van Hove Singularities in Kagome Topological Bands
• Quantum-metric-induced giant and reversible nonreciprocal transport phenomena in chiral loop-current phases of kagome metals
• Superconductivity and pair density waves from nearest-neighbor interactions in frustrated lattice geometries
• Low temperature phase transitions under CDW state in kagome metals AV3Sb5 (A=Cs,Rb,K): Significance of mix-type Fermi surface electron correlations
• Electronic structure of $A$V${}_3$Sb${}_5$ kagome metals
• MPd5 kagome superconductors studied by density functional calculations
• High-order van Hove singularities and nematic instability in the kagome superconductor CsTi${}_3$Bi${}_5$
• Common sublattice-pure van Hove singularities in the kagome superconductors $\text{A}$V${}_3$Sb${}_5$ ($\text{A}$ = K, Rb, Cs)
• Constraints on the orbital flux phase in $A$V${}_3$Sb${}_5$ from polar Kerr effect
• Vestigial $d$-wave charge-$4e$ Superconductivity from Bidirectional Pair Density Waves
• Superconductivity and Electron Correlations in Kagome Metal LuOs3B2
• Origin of switchable quasiparticle-interference chirality in loop-current phase of kagome metals measured by scanning-tunneling-microscopy
• Crystallization of the transdimensional electron liquid
• Odd-parity quadrupole order and induced nonreciprocal transport in the kagome metal CsTi${}_3$Bi${}_5$ driven by quantum interference
• Loop-current order through the kagome looking glass
• Recent progress in quantum spin liquids, fractional magnetization plateaus, and unconventional superconductivity in kagome lattices
• Orbital altermagnetism on the kagome lattice and possible application to $A$V${}_3$Sb${}_5$
• Giant Impurity Effects on Charge Loop Current Order States in Kagome Metals
• Magnetic-field effect on excitonic condensation emergent in extended Falicov-Kimball model
• Kekulé order from diffuse nesting near higher-order Van Hove points
• Magnetism of kagome metals $\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)\text{Sn}$ studied by $\mu$SR
• Theory of unconventional magnetism in a Cu-based kagome metal
• Microscopic origin of period-four stripe charge-density-wave in kagome metal CsV${}_3$Sb${}_5$