Three-dimensional higher-order saddle points induced flat bands in Co-based kagome metals

TL;DR

This study discovers higher-order saddle points in 3D kagome metals YbCo$_6$Ge$_6$ and MgCo$_6$Ge$_6$, leading to flat bands that could enhance electronic correlations and instabilities.

Contribution

It reports the first observation of 3D higher-order saddle points in kagome metals, revealing their impact on flat band formation and electronic properties.

Findings

Enhanced density of states at HOSPs

Formation of large-area flat bands near Fermi energy

Distinct robustness of HOSPs under substitutions and distortions

Abstract

The saddle point (van Hove singularity) exhibits a divergent density of states in 2D systems, leading to fascinating phenomena like strong correlations and unconventional superconductivity, yet it is seldom observed in 3D systems. In this work, we have found two types of 3D higher-order saddle points (HOSPs) in emerging 3D kagome metals, YbCo$_6$Ge$_6$ and MgCo$_6$Ge$_6$. Both HOSPs exhibit a singularity in their density of states, which is significantly enhanced compared to the ordinary saddle point. The HOSP near the Fermi energy generates a flat band extending a large area in the Brillouin zone, potentially amplifying the correlation effect and fostering electronic instabilities. Two types of HOSPs exhibit distinct robustness upon element substitution and lattice distortions in these kagome compounds. Our work paves the way for engineering exotic band structures, such as saddle points and flat bands, and exploring interesting phenomena in Co-based kagome materials.