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Symmetry or topology protected Dirac fermion states in two and threedimensions constitute novel quantum systems that exhibit exotic physicalphenomena. However, none of the studied spin-orbit materials are suitable forrealizing bulk multiplet Dirac states for the exploration of interacting Diracphysics. Here we present experimental evidence, for the first time, that thecompound Na3Bi hosts a bulk spin-orbit Dirac multiplet and their interaction oroverlap leads to a Lifshitz transition in momentum space - a condition forrealizing interactions involving Dirac states. By carefully preparing thesamples at a non-natural-cleavage (100) crystalline surface, we uncover manynovel electronic and spin properties in Na3Bi by utilizing high resolutionangle- and spin-resolved photoemission spectroscopy measurements. We observetwo bulk 3D Dirac nodes that locate on the opposite sides of the bulk zonecenter point $\Gamma$, which exhibit a Fermi surface Lifshitz transition and asaddle point singularity. Furthermore, our data shows evidence for the possibleexistence of theoretically predicted weak 2D nontrivial spin-orbit surfacestate with helical spin polarization that are nestled between the two bulkDirac cones, consistent with the theoretically calculated (100)surface-arc-modes. Our main experimental observation of a rich multiplet ofDirac structure and the Lifshitz transition opens the door for inducingelectronic instabilities and correlated physical phenomena in Na3Bi, and pavesthe way for the engineering of novel topological states using Na3Bi predictedin recent theory.

Observation of a bulk 3D Dirac multiplet, Lifshitz transition, and nestled spin states in Na3Bi