Bi

We measured the band dispersions of topological Dirac semimetal Na₃Bi using Fourier-transform scanning tunnelling spectroscopy to image quasiparticle interference (QPI) on the (001) surface of molecular beam epitaxy-grown Na₃Bi thin films. We find that the velocities for the lowest-lying conduction and valence bands are significantly higher than previous theoretical predictions. We compare the experimental band dispersions to the theoretical band structures calculated using an increasing hierarchy of approximations of self-energy corrections due to interactions: GGA, meta-GGA, 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 GW method. The results indicate that exchange-correlation effects are important in determining the electronic structure of this Na₃Bi, and are likely the origin of the high velocity. The electron velocity is consistent with recent experiments on ultrathin Na₃Bi and also may explain the ultra-high carrier mobility observed in heavily electron-doped Na₃Bi.