system*

Whereas electron-phonon scattering relaxes the electron’s momentum in metals, a perpetual exchange of momentum between phonons and electrons may conserve total momentum and lead to a coupled electron-phonon liquid. In a previous study, we presented evidence of such an electron-phonon liquid in NbGe₂ [1], which could be a platform for observing electron hydrodynamics. Here we provide evidence on tuning the strength of electron-phonon coupling by replacing Ge with Si and Nb with Ta. We combine de Haas-van Alphen (dHvA), electron transport, Raman scattering, and first-principles calculations in this MX₂ topological semimetal family where M=Nb, Ta and X=Ge, Si [2]. Tuning electron-phonon coupling increases the transport mobilities from nearly balanced to an order magnitude larger than quantum mobilities, with crystal structure or topology unchanged, and small differences in electron Fermi surface. Simultaneously, Raman scattering and first-principles calculations demonstrate a dominant phonon-drag effect only in MGe2 compounds. Our study suggests phonon-drag as a mechanism for achieving electron-phonon hydrodynamics.