Kekul\'e order from diffuse nesting near higher-order Van Hove points

TL;DR

This paper introduces a novel mechanism called diffuse nesting, where anisotropic band flattening near higher-order Van Hove singularities causes Fermi surface nesting at specific wavevectors, leading to Kekulé order.

Contribution

It demonstrates how diffuse nesting induces Kekulé density wave formation near higher-order Van Hove points, a new concept in Fermi surface instability studies.

Findings

Diffuse nesting leads to Kekulé density wave formation.

Anisotropic band flattening is key to the mechanism.

Fermi surface becomes approximately nested at specific wavevectors.

Abstract

Translation symmetry-breaking order is assumed to be suppressed by the lack of Fermi surface nesting near certain higher-order Van Hove singularities (HOVHS). We show the anisotropic band-flattening inherent to such HOVHS, combined with broadening of the Fermi surface due to elevated critical temperatures, results in the Fermi surface becoming approximately nested at a wavevector unrelated to the precise shape of the Fermi surface - leading to a $\sqrt{3}\times\sqrt{3}$ Kekul\'e density wave formation. The effect is demonstrated using unbiased renormalization group calculations for a model of the breathing kagome lattice. Our mechanism - termed diffuse nesting - represents an entirely new notion in the study of Fermi surface instabilities.