Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi$_2$B$_2$C

TL;DR

This study reveals that in YNi$_2$B$_2$C, strong electron-momentum-dependent electron-phonon coupling causes significant phonon renormalization without Fermi surface nesting or lattice anharmonicity, challenging traditional understanding.

Contribution

It demonstrates that electron-momentum dependence of electron-phonon coupling can induce phonon renormalization independently of Fermi surface nesting or anharmonic effects.

Findings

Strong phonon broadening occurs without Fermi surface nesting.

Electron-momentum dependence enhances electron-phonon coupling.

Experimental and theoretical methods confirm the scenario.

Abstract

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. Strong coupling of phonons to electrons near the Fermi surface, which reduces the phonon lifetimes and broadens the phonon peaks in scattering experiments, is often associated with Fermi surface nesting. 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 YNi$_2$B$_2$C. This new scenario likely applies to a wide range of compounds.