  Symmetry or topology protected Dirac fermion states in two and three
dimensions constitute novel quantum systems that exhibit exotic physical
phenomena. However, none of the studied spin-orbit materials are suitable for
realizing bulk multiplet Dirac states for the exploration of interacting Dirac
physics. Here we present experimental evidence, for the first time, that the
compound Na3Bi hosts a bulk spin-orbit Dirac multiplet and their interaction or
overlap leads to a Lifshitz transition in momentum space - a condition for
realizing interactions involving Dirac states. By carefully preparing the
samples at a non-natural-cleavage (100) crystalline surface, we uncover many
novel electronic and spin properties in Na3Bi by utilizing high resolution
angle- and spin-resolved photoemission spectroscopy measurements. We observe
two bulk 3D Dirac nodes that locate on the opposite sides of the bulk zone
center point $\Gamma$, which exhibit a Fermi surface Lifshitz transition and a
saddle point singularity. Furthermore, our data shows evidence for the possible
existence of theoretically predicted weak 2D nontrivial spin-orbit surface
state with helical spin polarization that are nestled between the two bulk
Dirac cones, consistent with the theoretically calculated (100)
surface-arc-modes. Our main experimental observation of a rich multiplet of
Dirac structure and the Lifshitz transition opens the door for inducing
electronic instabilities and correlated physical phenomena in Na3Bi, and paves
the way for the engineering of novel topological states using Na3Bi predicted
in recent theory.
