Testing Electron-phonon Coupling for the Superconductivity in Kagome Metal $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$

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 $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength,$\lambda$, supporting an unconventional pairing mechanism in $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$. However, experimental determination of $\lambda$ is still missing, hindering a microscopic understanding of the intertwined ground state of $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate $\lambda$=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 $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$. Remarkably, the EPC on the V 3d-band enhances to $\lambda$~0.75 as the superconducting transition temperature elevated to 4.4 K in $\mathrm{Cs}({\mathrm{V}}_{0.93}\mathrm{N}{\mathrm{b}}_{0.07}{)}_3\mathrm{S}{\mathrm{b}}_5$. Our results provide an important clue to understand the pairing mechanism in the Kagome superconductor $\mathrm{Cs}{\mathrm{V}}_3\mathrm{S}{\mathrm{b}}_5$.

To appear in Nature Communications

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