Chiral excitonic order from twofold van Hove singularities in kagome metals

Abstract

Recent experiments on kagome metals AV3Sb5 (A=K,Rb,Cs) [M. Kang et al., arXiv:2105.01689 (2021] 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 the TvHS are responsible for the novel charge and superconducting order observed in kagome metals. We introduce a model of a quasi-two dimensional system hosting a TvHS, and investigate the interaction induced many-body instabilities via the perturbative renormalisation group technique. The TvHS generates a topological d-wave excitonic condensate, arising due to bound pairs of electrons and holes located at opposite concavity vHS. Varying the chemical potential, detuning from the TvHS, and tuning the bare interaction strength, we construct a phase diagram which features the excitonic condensate alongside d-wave superconductivity and spin density wave order. The chiral excitonic state supports a Chern band giving 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.

Van Hove tuning of A V3Sb5 kagome metals under pressure and strain

“…The recently discovered family of nonmagnetic quasi-2D AV 3 Sb 5 (A = Cs, K, Rb) kagome metals [1,2] represents an excellent example of compounds that allow us to study charge ordering [3][4][5][6][7][8][9][10][11][12][13], superconductivity [14][15][16][17][18][19][20], and electronic correlations [21], together with Dirac band crossing, Z 2 nontrivial topological bands [22], chiral symmetry breaking [23], and flat-band physics [16,24].…”

Exciton condensation in biased bilayer graphene

“…Introduction -Excitonic condensates in twodimensional (2D) materials promise novel superfluid [1][2][3][4][5] or topological [6][7][8][9][10][11] properties, and have thereby attracted considerable theoretical and experimental attention. Moreover, due to these novel transport behaviours, excitonic condensates promise a route to future technological advancements.…”