Chiral excitonic order from twofold van Hove singularities in kagome metals

Abstract

Recent experiments on kagome metals AV3Sb5 (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 AV3Sb5, 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 AV3Sb5. 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.

Charge-loop current order and Z3 nematicity mediated by bond order fluctuations in kagome metals

“…However, the impact of other 3 d -orbitals on the cLC order has also been studied in refs. 30 , 75 . The extension of the present theory to multi-orbital models is a very important future issue.…”

Emergent topological quantum orbits in the charge density wave phase of kagome metal CsV3Sb5

“…The recently discovered kagome superconductors AV 3 Sb 5 (A = K, Rb, Cs) 1 stimulate extensive studies for their intriguing charge density waves (CDW) [2][3][4][5][6][7][8][9][10] , possible symmetry breaking without magnetism 2,[11][12][13][14][15][16][17][18][19][20][21] , topological band structure 3,4,[22][23][24] , and exotic superconductivity 5,[25][26][27] . Aiming at deriving the Fermi surface (FS) property of CsV 3 Sb 5 , a dozen quantum oscillation experiments reveal complicated but similar oscillation frequencies [28][29][30][31][32][33][34][35][36] , bearing the Fermi energy variation in different samples.…”

Anisotropic proximity–induced superconductivity and edge supercurrent in Kagome metal, K 1− x V 3 Sb 5

“…In addition, chiral charge ordering with magnetic field tunability and charge order states breaking rotation symmetry were detected in CsV 3 Sb 5 and KV 3 Sb 5 ( 9 , 11 , 12 , 25 – 27 ), and possible unconventional superconductivity in AV 3 Sb 5 was also discussed ( 11 , 12 , 22 , 28 ). Theoretically, several ground states of AV 3 Sb 5 were proposed including a star-of-David structure, tri-hexagonal structures, and TRS breaking states such as a chiral flux state, charge/spin bond orders, spin density waves, and excitonic order ( 29 – 35 ). However, differing reports on the presence or absence of these order states and the nature of the superconductivity ( 25 , 36 – 38 ) has made understanding the magnetism and superconductivity in the AV 3 Sb 5 family difficult, and there is a lot of space to excavate the underlying physics.…”