Spin Excitations and Flat Electronic Bands in a Cr-based Kagome Superconductor

TL;DR

This study demonstrates that a Cr-based kagome superconductor hosts flat bands at the Fermi level, with magnetic excitations linked to low-temperature order, highlighting the role of flat bands in emergent quantum states.

Contribution

It provides direct experimental evidence of flat bands at the Fermi level in a bulk kagome superconductor and links them to magnetic order emergence.

Findings

Flat bands are at the Fermi level in { ext{Cr}}

Low-energy magnetic excitations evolve with temperature

Kagome flat bands may influence emergent order

Abstract

In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction. Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their role in emergent orders in bulk materials remains lacking. Using angle-resolved photoemission spectroscopy, resonant inelastic X-ray scattering, and density functional theory, we show that the low-energy structure of the Cr-based kagome metal superconductor {\Cr} is dominated by FBs at the Fermi level. We also observe low-energy magnetic excitations evolving across the low-temperature transition, largely consistent with the FB shift. Our results suggest that the low-temperature order contains a magnetic origin and that the kagome FBs may play a role in the emergence of this order.