YNi2B2C is a superconductor with a relatively high Tc of 15.6 K mediated by electron-phonon (e-ph) interaction, making it an excellent system for studying the electron-lattice coupling. However, its established Fermi surface model had notable ambiguities, as the origins of the large de Haas-van Alphen (dHvA) frequencies beta and zeta were unexplained.

We performed precise dHvA measurements and improved band-structure calculations. Our new calculations successfully explain the experimental Fermi surface, revealing that the beta and zeta orbits arise from a multiply-connected "flower-petal" sheet (band-26).

This definitive model enabled us to determine the e-ph coupling strength lambda by comparing experimental and calculated masses. The results show a clear sheet-dependent anisotropy: lambda is largest for band-26, intermediate for band-27, and remarkably small for band-28 (lambda ~ 0 for B // c). This strongly suggests a highly anisotropic gap, consistent with dHvA oscillations observed from the band-28 sheet deep in the mixed state.

In light of recent discoveries of e-ph driven high-Tc superconductivity in hydrides, our experimental determination of anisotropic e-ph coupling is highly relevant. It provides crucial insights into how this interaction varies across the Fermi surface and relates to the gap anisotropy.