Visualizing the Localized Electrons of a Kagome Flat Band

TL;DR

This study visualized the localized electronic flat band states in a kagome metal, CoSn, confirming theoretical predictions and revealing extremely localized states with potential for strong many-body interactions.

Contribution

First direct visualization of kagome flat band Wannier states in a real material, demonstrating their extreme localization and implications for many-body physics.

Findings

Flat band states are localized at kagome lattice centers.

Localization length is only two to three angstroms.

Strongly renormalized quasiparticle velocity suggests enhanced interaction effects.

Abstract

Destructive interference between electron wavefunctions on the two-dimensional (2D) kagome lattice induces an electronic flat band, which could host a variety of interesting many-body quantum states. Key to realize these proposals is to demonstrate the real space localization of kagome flat band electrons. In particular, the extent to which the often more complex lattice structure and orbital composition of realistic materials counteract the localizing effect of destructive interference, described by the 2D kagome lattice model, is hitherto unknown. We used scanning tunneling microscopy (STM) to visualize the non-trivial Wannier states of a kagome flat band at the surface of CoSn, a kagome metal. We find that the local density of states associated with the flat bands of CoSn is localized at the center of the kagome lattice, consistent with theoretical expectations for their corresponding Wannier states. Our results show that these states exhibit an extremely small localization length of two to three angstroms concomitant with a strongly renormalized quasiparticle velocity, which is comparable to that of moir\'e superlattices. Hence, interaction effects in the flat bands of CoSn could be much more significant than previously thought. Our findings provide fundamental insight into the electronic properties of kagome metals and are a key step for future research on emergent many-body states in transition metal based kagome materials.