Fermi-surface-sheet dependent electron-phonon coupling in a borocarbide superconductor YNi$_2$B$_2$C

We performed de Haas-van Alphen (dHvA) oscillation measurements and band-structure calculations for YNi$_2$B$_2$C. Our improved band structure successfully explained the origins of the large dHvA frequencies $β$ and $ζ$, which were inexplicable in previous works. By comparing experimental effective masses with band masses, we determined the electron-phonon coupling for each orbit. The results showed a clear Fermi-surface-sheet dependence of the electron-phonon coupling strength, especially highlighting that the coupling for the band-28 sheet is very weak, almost absent for the orbit with $B \parallel c$. This finding is consistent with previous observations of dHvA oscillations from this orbit in the mixed state down to very low fields. Amidst growing interest in high-temperature superconductivity driven by electron-phonon coupling in hydrides under high pressure, this study provides foundational data pivotal to precisely understanding electron-phonon coupling.

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