Charge density waves and electronic properties of superconducting kagome metals

TL;DR

This study uses first-principles calculations to analyze the charge density wave phase and electronic properties of superconducting kagome metals, revealing an inverse Star of David structure, the role of electrons in CDW formation, and implications for unconventional superconductivity.

Contribution

It uncovers the inverse Star of David CDW ground state, links structural instability to electronic factors, and shows electron-phonon coupling is insufficient for superconductivity, suggesting unconventional pairing mechanisms.

Findings

Inverse Star of David CDW structure matches experiments

Structural instability indicated by soft phonon modes

Weak electron-phonon coupling suggests unconventional superconductivity

Abstract

Kagome metals $A$V$_3$Sb$_5$ ($A=$ K, Rb, and Cs) exhibit intriguing superconductivity below $0.9 \sim 2.5 $ K, a charge density wave (CDW) transition around $80\sim 100 $ K, and $\mathbb{Z}_{2}$ topological surface states. The nature of the CDW phase and its relation to superconductivity remains elusive. In this work, we investigate the electronic and structural properties of CDW by first-principles calculations. We reveal an inverse Star of David deformation as the $2\times2\times2$ CDW ground state of the kagome lattice. The kagome lattice shows softening breathing-phonon modes, indicating the structural instability. However, electrons play an essential role in the CDW transition via Fermi surface nesting and van Hove singularity. The inverse Star of David structure agrees with recent experiments by scanning tunneling microscopy (STM). The CDW phase inherits the nontrivial $\mathbb{Z}_{2}$-type topological band structure. Further, we find that the electron-phonon coupling is too weak to account for the superconductivity $T_c$ in all three materials. It implies the existence of unconventional pairing of these kagome metals. Our results provide essential knowledge toward understanding the superconductivity and topology in kagome metals.