Strong-coupling superconductivity in the kagome metal CsV3Sb5 revealed by soft point-contact spectroscopy

Abstract

The nature of the superconducting state in kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ is a key issue in need of experimental clarification. Here, we report on a study of the superconducting order parameter in the kagome superconductor ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ through simultaneous ``soft'' point-contact spectroscopy and resistivity measurements under both ambient and a hydrostatic pressure. Signatures of two-gap superconductivity are resolved in the soft point-contact spectra, accompanied by an asymmetric excess conductance above ${T}_{c}$. Quantitative analysis based on the two-dimensional Blonder-Tinkham-Klapwijk model reveals an ($s+s$)-wave superconducting gap with $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{B}{T}_{c}\ensuremath{\simeq}7.2$, placing ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ in the strong-coupling regime. The strong-coupling two-gap feature indicates a high electronic density of states (DOS) and possible existence of flat-band-driven multiple van Hove singularities (VHSs) at the Fermi level. The presence of asymmetric excess spectral conductance above ${T}_{c}$ hints at a modest electronic correlation in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$. Under a hydrostatic pressure of 2.1 kbar, the nodeless multigap nature of the superconducting state remains, whereas both the larger gap and the excess spectral conductance are greatly suppressed, accompanied by an enhanced ${T}_{c}$. An estimate of the spectral-weighted gap ratio reveals a weakened coupling strength, indicative of a reduced total superconducting DOS upon pressure. Our results point to key roles of both flat-band-associated VHSs and electronic correlation in the onset of kagome superconductivity and shed some light on the interplay between charge-density-wave order and superconductivity in vanadium-based kagome superconductors.