Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi2B2C

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 YNi$_2$B$_2$C. This new scenario likely applies to a wide range of compounds.

电子-声子耦合（electron-phonon coupling）即电子与晶格振动之间的相互散射，反之亦然，在固体中普遍存在，并可导致超导性、电荷密度波序等涌现基态的形成。宽谱声子线型常被视作与费米面嵌套（Fermi surface nesting）相关的强电子-声子耦合的标志——费米面嵌套指由声子动量连接的费米面平行区段。另外，已知宽幅声子信号也可源于强原子晶格非简谐性。本文证明，若电子-动量k取特定值时电子-声子耦合被显著增强，则即使不存在费米面嵌套与晶格非简谐性，也可出现显著的声子展宽现象。我们通过非弹性中子散射、软X射线角分辨光电子能谱（soft x-ray angle-resolved photoemission spectroscopy）测量，以及从头算晶格动力学与电子能带结构计算，在高各向异性四方相电子-声子超导体YNi₂B₂C中验证了这一机制。该全新机制有望适用于众多化合物体系。