Testing electron–phonon coupling for the superconductivity in kagome metal CsV₃Sb₅

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal CsV₃Sb₅, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength, λ, supporting an unconventional pairing mechanism in CsV₃Sb₅. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV₃Sb₅. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate λ=0.45–0.6 at T = 6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in CsV₃Sb₅. Remarkably, the EPC on the V 3d-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V_0.93Nb_0.07)₃Sb₅. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV₃Sb₅.