Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi$_{2}$B$_{2}$C

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa,
is ubiquitous in solids and can lead to emergent ground states such as superconductivity and
charge-density wave order. A broad spectral phonon line shape is often interpreted as a
marker of strong electron-phonon coupling associated with Fermi surface nesting, i.e., parallel
sections of the Fermi surface connected by the phonon momentum. Alternatively broad
phonons are known to arise from strong atomic lattice anharmonicity. Here, we show that
strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice
anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-
momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission
spectroscopy measurements and ab-initio lattice dynamical and electronic band
structure calculations to demonstrate this scenario in the highly anisotropic tetragonal
electron-phonon superconductor YNi2B2C. This new scenario likely applies to a wide range of
compounds.