Evidence of a coupled electron-phonon liquid in NbGe$_2$

TL;DR

This paper provides experimental evidence for a coupled electron-phonon liquid in NbGe$_2$, revealing unique transport properties and anomalous behaviors through quantum oscillations, resistivity, and Raman scattering.

Contribution

It presents the first multi-experimental evidence of a coupled electron-phonon liquid in NbGe$_2$, highlighting its distinct transport and spectroscopic signatures.

Findings

Enhanced quasiparticle mass indicating electron-phonon interactions

Discrepancy between experimental and theoretical resistivity data

Anomalous temperature dependence of phonon linewidths in Raman spectra

Abstract

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.