Direct Observation of the Fermi Arc Surface State in the Three-Dimensional Dirac Semimetal Na$_{3}$Bi

The three dimensional (3D) Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction and valence bands connect to each other. Here we report the direct observation of the linearly dispersive 3D bulk Dirac points at the natural (001) cleaving surface of Na$_{3}$Bi single crystal by high resolution ARPES. In addition, we have directly observed two separated 3D bulk Dirac nodes by elaborately cleaving Na$_{3}$Bi samples at a non-natural-cleavage (100) crystalline surface. We further unveil the unusual Fermi-arc surface states connecting the two 3D Dirac nodes. At this unique (100) crystalline surface, the identification of the 3D Dirac semimetal state in Na$_{3}$Bi paves the way for systematically exploring rich exotic topological physics such as topological insulator and Weyl semimetal state.