Impact of charge-density-wave pattern on the superconducting gap in V-based kagome superconductors

TL;DR

This study reveals how different charge-density-wave patterns in V-based kagome superconductors influence the superconducting gap symmetry and anisotropy, with implications for understanding their interplay and electron correlations.

Contribution

It provides the first direct comparison of superconducting gap anisotropy in different CDW-ordered V-based kagome superconductors and links these differences to specific CDW modulation patterns.

Findings

K/RbV3Sb5 exhibit fully gapped, isotropic s-wave superconductivity.

CsV3Sb5 shows strongly anisotropic superconducting gaps.

Impurity scattering minimally affects low-energy excitations and increases Tc in K/RbV3Sb5.

Abstract

Kagome metals $A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) provide a compelling platform to explore the interplay between superconductivity (SC) and charge-density-wave (CDW) orders. While distinct CDW orders have been identified in K/RbV$_3$Sb$_5$ versus CsV$_3$Sb$_5$, their influence on the SC order parameter remains unresolved. Here, we investigate low-energy quasiparticle excitations in $A$V$_3$Sb$_5$, uncovering a striking difference in SC gap anisotropy: K/RbV$_3$Sb$_5$ exhibit fully gapped, nearly isotropic $s$-wave states, in contrast to the strongly anisotropic SC gap in CsV$_3$Sb$_5$. Contrary to previous vortex-state studies suggesting nodal SC in K/RbV$_3$Sb$_5$, our Meissner-state measurements in high-quality crystals demonstrate fully gapped states with reduced anisotropy compared to CsV$_3$Sb$_5$. Impurity scattering introduced via electron irradiation in K/RbV$_3$Sb$_5$ has a minimal impact on low-energy excitations, and it induces an increase in the SC transition temperature $T_{\rm c}$, consistent with more isotropic $s$-wave SC competing with CDW order. Our theoretical analysis attributes the observed SC gap anisotropy differences to distinct CDW modulation patterns: the star-of-David structure unique to CsV$_3$Sb$_5$ preserves van Hove singularities near the Fermi level, promoting anisotropic $s$-wave SC with enhanced $T_{\rm c}$ via bond-order fluctuations. These findings establish a systematic framework for understanding the interplay between SC and CDW orders in $A$V$_3$Sb$_5$, driven by electron correlations.