First-Principles Study on Thermal and Electrical Transport Properties of NbGe₂ and NbSi₂: The Role of Electron-Phonon Coupling

We investigate coupled electron and phonon transport in NbX₂ with X=Ge, Si, where experimental evidence of strong electron-phonon coupling and hydrodynamic transport has been reported. Based on first-principles density functional theory calculations, we measured the thermal and electrical transport properties of the compounds. We found that phonon-electron scattering strongly affects phonon thermal conductivity (κ_ph) and leads to a weak temperature dependence of κ_ph instead of a normal inverse temperature dependence when anharmonic three-phonon scattering dominates. In addition, κ_ph contributes to a quarter of the total thermal conductivity, which differs from typical metals in which the total thermal conductivity is predominantly derived from electrons. In contrast to previous numerical research, our electrical resistivity results agree well with the experimental measurements. The anisotropic properties of the transport coefficients are attributed to the electron-phonon dispersion relation. In addition, we found a negligible effect of electron-phonon drag on the transport properties, contrary to the expectation from a strongly coupled electron-phonon fluid.