  Bulk Dirac semimetals (DSMs) exhibit unconventional transport properties and
phase transitions due to their peculiar low-energy band structure. Yet the
electronic interactions governing nonequilibrium phenomena in DSMs are not
fully understood. Here we show that electron-phonon (e-ph) interactions in a
prototypical bulk DSM, Na3Bi, are predominantly two-dimensional (2D). Our
first-principles calculations discover a 2D optical phonon with strong e-ph
interactions associated with in-plane vibrations of Na atoms. We show that this
2D mode governs e-ph scattering and charge transport in Na3Bi, and induces a
dynamical phase transition to a Weyl semimetal. Our work advances quantitative
analysis of electron interactions in topological semimetals and reveals
dominant low-dimensional interactions in bulk quantum materials.
