Beyond kagome: $p$-bands in kagome metals

TL;DR

This work argues that $p$-band states from non-kagome elements are essential for understanding electronic instabilities in kagome metals. By examining AV$_3$Sb$_5$, FeGe, RV$_6$Sn$_6$, and LaRu$_3$Si$_2$, it shows that $p$-bands cross the Fermi level and interact with the kagome $d$-bands to shape CDW formation and superconductivity, with pressure and chemical substitution highlighting the central role of interlayer and center-void $p$-states. The CDW and superconducting phenomena cannot be explained by $d$-band nesting alone; in some compounds the $p$-bands are primarily responsible for the instability, while in others they coexist with $d$-band effects. The findings suggest that manipulating $p$-band chemistry or driving related phonon modes offers a practical route to tailor electronic instabilities in kagome metals.

Abstract

We review recent studies on quantum materials where transition-metal atoms give rise to $d$-bands typical of kagome metals. Using examples from several material families - AV$_3$Sb$_5$, FeGe, RV$_6$Sn$_6$, and LaRu$_3$Si$_2$ - we argue that $p$-bands contributed by elements beyond the kagome network also play a crucial role in the electronic instabilities, including the charge-density-waves and superconductivity in kagome metals.

Figures (7)

• Figure 1: Left: model band structure of a nearest-neighbor kagome metal features a flat band, a Dirac crossing, and two band saddle points that give rise to van Hove singularities (VHS) in the density of states. Right: real crystal structure that combines the kagome network of a $d$-metal with $p$-elements located in the centers of the hexagonal voids ("intralayer") as well as between the kagome layers ("interlayer").
• Figure 2: Pressure-induced 2D--3D crossover in CsV$_3$Sb$_5$tsirlin2022: the shrinkage of the $c$ parameter leads to the formation of the interlayer Sb2--Sb2 bonds and a reconstruction of the band structure that mainly affects the Sb1 and Sb2 bands, whereas the kagome $d$-bands of vanadium show minor changes only. Small arrows denote the band saddle points that give rise to the van Hove singularities near the Fermi level.
• Figure 3: (a-c) Pressure evolution of CsV$_3$Sb$_5$: $T_c$ and $T_{\rm CDW}$ vs. pressure zhou2024 (a), spectral weight of the localization peak as an experimental gauge of electron-phonon coupling wenzel2023 (b), and radii of the Sb $p$-band Fermi surfaces tsirlin2023 (c). Possible CDW structures: real CDW with the modulation of the V--V bond distances (d), and imaginary CDW with the loop currents (e).
• Figure 4: Left: crystal structure of FeGe with the A-type antiferromagnetic order. The green arrows on the Ge atoms show the displacements below $T_{\rm CDW}$. Right: band structure of FeGe in the normal state (A-type AFM order) features Fe $d$-bands along with the $p$-band formed by the Ge1 and Ge2 atoms.
• Figure 5: Left: crystal structure of ScV$_6$Sn$_6$ with the arrows showing the atomic displacements in the CDW state arachchige2022. Right: band structure of ScV$_6$Sn$_6$ (normal state) near the Fermi level features multiple V $3d$ bands and a separate band with the dominant Sn1 contribution.