Fermion–boson many-body interplay in a frustrated kagome paramagnet

Abstract

Kagome-nets, appearing in electronic, photonic and cold-atom systems, host frustrated fermionic and bosonic excitations. However, it is rare to find a system to study their fermion–boson many-body interplay. Here we use state-of-the-art scanning tunneling microscopy/spectroscopy to discover unusual electronic coupling to flat-band phonons in a layered kagome paramagnet, CoSn. We image the kagome structure with unprecedented atomic resolution and observe the striking bosonic mode interacting with dispersive kagome electrons near the Fermi surface. At this mode energy, the fermionic quasi-particle dispersion exhibits a pronounced renormalization, signaling a giant coupling to bosons. Through the self-energy analysis, first-principles calculation, and a lattice vibration model, we present evidence that this mode arises from the geometrically frustrated phonon flat-band, which is the lattice bosonic analog of the kagome electron flat-band. Our findings provide the first example of kagome bosonic mode (flat-band phonon) in electronic excitations and its strong interaction with fermionic degrees of freedom in kagome-net materials.

Intrinsic nature of chiral charge order in the kagome superconductor RbV3Sb5

“…For instance, insulating kagome magnets have been investigated for decades in the hopes of realizing quantum spin liquids [2]. Recently, focused STM research on correlated kagome magnets has revealed many topological and many-body phenomena [3], including Chern gapped phases [4,5], tunable electronic nematicity [4], orbital magnetism [6][7][8], and many-body interplay [9,10]. These observations are all closely related to the emergent physics arising from the fundamental kagome band structure, which includes Dirac cones, flat bands, and Van Hove singularities.…”

“…These observations are all closely related to the emergent physics arising from the fundamental kagome band structure, which includes Dirac cones, flat bands, and Van Hove singularities. Notably, the many-body fermion-boson interplay [10] observed in certain kagome paramagnets leads us to conjecture from the spectroscopic point of view that there can be superconductivity instability that competes with magnetism. Then, we realize that kagome superconductors * These authors contributed equally to this work.…”

Flat bands and Z2 topological phases in a non-Abelian kagome lattice

“…The model shows interesting flat bands and topological phases. Flat bands have attracted a great deal of attention recently due to a variety of interesting phenomena they can provide [30][31][32][33][34][35][36][37][38][39][40][41][42] and have been studied experimentally in the kagome magnet Co 3 Sn 2 S 2 [43] and the kagome metal CoSn [44][45][46]. In our model, the energy bands and conditions for the presence of flat bands could be obtained analytically, allowing an apparent understanding about the flat band physics.…”

Observation of a chiral wave function in the twofold-degenerate quadruple Weyl system BaPtGe

“…A fascinating property of BaPtGe is that it is predicted to host multiple chiral topological states, including a new and undiscovered twofold-degenerate quadruple Weyl node (TQW) [32] that can only be realized in bosonic excitations, like phonons, or electronic structures without spin-orbit coupling. Unlike mechanical metamaterials [19,23,25], phonons in crystalline materials are true quantum objects and correlate with electronic and magnetic excitations [7,33]. Past work has tended to assume that a Weyl node with large C requires a large band degeneracy, n [6].…”