  Whereas electron-phonon scattering typically relaxes the electron's momentum
in metals, a perpetual exchange of momentum between phonons and electrons
conserves total momentum and can lead to a coupled electron-phonon liquid with
unique transport properties. This theoretical idea was proposed decades ago and
has been revisited recently, but the experimental signatures of an
electron-phonon liquid have been rarely reported. We present evidence of such a
behavior in a transition metal ditetrelide, NbGe$_2$, from three different
experiments. First, quantum oscillations reveal an enhanced quasiparticle mass,
which is unexpected in NbGe$_2$ due to weak electron-electron correlations,
hence pointing at electron-phonon interactions. Second, resistivity
measurements exhibit a discrepancy between the experimental data and calculated
curves within a standard Fermi liquid theory. Third, Raman scattering shows
anomalous temperature dependence of the phonon linewidths which fits an
empirical model based on phonon-electron coupling. We discuss structural
factors, such as chiral symmetry, short metallic bonds, and a low-symmetry
coordination environment as potential sources of coupled electron-phonon
liquids.
