  The close approach of the Fermi energy EF of a Dirac semimetal to the Dirac
point ED uncovers new physics such as velocity renormalization,1,2,3 and the
Dirac plasma 4,5 at |EF -ED| < kBT, where kBT is the thermal energy. In
graphene, substrate disorder drives fluctuations in EF. Three-dimensional
topological Dirac semimetals (TDS)6,7 obviate the substrate, and should show
reduced EF fluctuations due to better metallic screening and higher dielectric
constants. Here we map the potential fluctuations in TDS Na3Bi using a scanning
tunneling microscope. The rms potential fluctuations are significantly smaller
than room temperature ({\Delta}EF,rms = 4-6 meV = 40-70 K) and comparable to
the highest quality graphene on h-BN;8 far smaller than graphene on SiO2,9,10
or the Dirac surface state of a topological insulator.11 Surface Na vacancies
produce a novel resonance close to the Dirac point with surprisingly large
spatial extent and provides a unique way to tune the surface density of states
in a TDS thin-film material.
