Testing electron–phonon coupling for the superconductivity in kagome metal CsV ₃Sb ₅

Abstract

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 CsV ₃Sb ₅, 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 CsV ₃Sb ₅. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV ₃Sb ₅. 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 5 p and V 3 d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in CsV ₃Sb ₅. Remarkably, the EPC on the V 3 d-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V _0.93Nb _0.07) ₃Sb ₅. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV ₃Sb ₅.

Abstract

Electron-phonon coupling is thought to be too weak to be responsible for the superconducting Cooper pairing of the kagome metals AV ₃Sb ₅, but an experimental measurement is lacking. Here, the authors use ARPES measurements to find that electron-phonon coupling in CsV ₃Sb ₅ is strong enough to support the experimental superconducting transition.