Bulk evidence of anisotropic $s$-wave pairing with no sign change in the kagome superconductor CsV$_3$Sb$_5$

TL;DR

This study provides bulk evidence that CsV$_3$Sb$_5$ is an anisotropic $s$-wave superconductor with no sign change, challenging theories predicting unconventional or sign-changing pairing symmetries in kagome superconductors.

Contribution

It demonstrates that CsV$_3$Sb$_5$ exhibits a fully-gapped, anisotropic $s$-wave pairing symmetry with no sign change, supported by impurity and pressure experiments.

Findings

Impurity doping transforms the gap from anisotropic to isotropic without creating nodes.

The double superconducting dome persists under high pressure despite impurities.

CsV$_3$Sb$_5$ is a non-chiral, anisotropic $s$-wave superconductor with no sign change.

Abstract

The recently discovered kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) possess a unique band structure with van Hove singularities and Dirac dispersions, in which unusual charge-density-wave (CDW) orders with time-reversal and rotational symmetry breaking have been reported. One of the most crucial unresolved issues is identifying the symmetry of the superconductivity that develops inside the CDW phase. Theory predicts a variety of unconventional superconducting symmetries, including exotic states with chiral and topological properties accompanied by a sign-changing superconducting gap. Experimentally, however, the phase information on the superconducting gap in $A$V$_3$Sb$_5$ is still lacking. Here we report the electron irradiation effects in CsV$_3$Sb$_5$ using introduced impurities as a phase-sensitive probe of superconductivity. Our magnetic penetration depth measurements reveal that with increasing impurities, a highly anisotropic fully-gapped state changes gradually to an isotropic full-gap state without passing through a nodal state. Furthermore, transport measurements under high pressure show that the double superconducting dome in the pressure-temperature phase diagram survives against sufficient impurities. These results are strong bulk evidence that CsV$_3$Sb$_5$ is a non-chiral, anisotropic $s$-wave superconductor with no sign change both at ambient and high pressure, which provides a clue to understanding the relationship between CDW and superconductivity in kagome superconductors.