Evidence of a coupled electron-phonon liquid in NbGe₂

Hydrodynamic electron flow can be realized when the momentum-conserving scatterings of electrons in a material are more frequent than the momentum-relaxing ones. In principle, momentum-conserving electron-phonon scatterings can be enhanced in a coupled electron-phonon liquid to fulfill the above condition, but the candidates remain rare. Here, we present NbGe₂ as a new candidate of such a liquid with strong electron-phonon interactions, phonon-drag behavior, and pronounced phonon-electron couplings as evidenced by experiments of quantum oscillations, electrical and thermal transport, and Raman scattering. Possible design principles of a coupled electron-phonon liquid will be discussed.