  Experimental identification of three-dimensional (3D) Dirac semimetals in
solid state systems is critical for realizing exotic topological phenomena and
quantum transport such as the Weyl phases, high temperature linear quantum
magnetoresistance and topological magnetic phases. Using high resolution
angle-resolved photoemission spectroscopy, we performed systematic electronic
structure studies on well-known compound Cd3As2. For the first time, we observe
a highly linear bulk Dirac cone located at the Brillouin zone center projected
onto the (001) surface which is consistent with a 3D Dirac semimetal phase in
Cd3As2. Remarkably, an unusually high Dirac Fermion velocity up to 10.2
\textrm{\AA}{\cdot}$eV (1.5 \times 10^{6} ms^-1) is seen in samples where the
mobility far exceeds 40,000 cm^2/V.s suggesting that Cd3As2 can be a promising
candidate as a hypercone analog of graphene in many device-applications which
can also incorporate topological quantum phenomena in a large gap setting. Our
experimental identification of this novel topological 3D Dirac semimetal phase,
distinct from a 3D topological insulator phase discovered previously, paves the
way for exploring higher dimensional relativistic physics in bulk transport and
for realizing novel Fermionic matter such as a Fermi arc nodal metal.
