  We experimentally measure the band dispersions of topological Dirac semimetal
Na3Bi using Fourier-transform scanning tunneling spectroscopy to image
quasiparticle interference on the (001) surface of molecular-beam epitaxy-grown
Na3Bi thin films. We find that the velocities for the lowest-lying conduction
and valencebands are 1.6x10^6 m/s and 4.2x10^5 m/s respectively, significantly
higher than previous theoreticalpredictions. We compare the experimental band
dispersions to the theoretical band structures calculated usingan increasing
hierarchy of approximations of self-energy corrections due to interactions:
generalized gradientapproximation (GGA), meta-GGA, Heyd-Scuseria-Ernzerhof
exchange-correlation functional (HSE06), and GW methods. We find that density
functional theory methods generally underestimate the electron velocities.
However, we find significantly improved agreement with an increasingly
sophisticated description of the exchange and interaction potential,
culminating in reasonable agreement with experiments obtained by the GW method.
The results indicate that exchange-correlation effects are important in
determining the electronicstructure of this Na3Bi, and are likely the origin of
the high velocity. The electron velocity is consistent withrecent experiments
on ultrathin Na3Bi and also may explain the ultrahigh carrier mobility observed
in heavilyelectron-doped Na3Bi.
