  The three-dimensional topological semimetals represent a new quantum state of
matter. Distinct from the surface state in the topological insulators that
exhibits linear dispersion in two-dimensional momentum plane, the
three-dimensional semimetals host bulk band dispersions linearly along all
directions, forming discrete Dirac cones in three-dimensional momentum space.
In addition to the gapless points (Weyl/Dirac nodes) in the bulk, the
three-dimensional Weyl/Dirac semimetals are also characterized by
"topologically protected" surface state with Fermi arcs on their specific
surface. The Weyl/Dirac semimetals have attracted much attention recently they
provide a venue not only to explore unique quantum phenomena but also to show
potential applications. While Cd3As2 is proposed to be a viable candidate of a
Dirac semimetal, more experimental evidence and theoretical investigation are
necessary to pin down its nature. In particular, the topological surface state,
the hallmark of the three-dimensional semimetal, has not been observed in
Cd3As2. Here we report the electronic structure of Cd3As2 investigated by
angle-resolved photoemission measurements on the (112) crystal surface and
detailed band structure calculations. The measured Fermi surface and band
structure show a good agreement with the band structure calculations with two
bulk Dirac-like bands approaching the Fermi level and forming Dirac points near
the Brillouin zone center. Moreover, the topological surface state with a
linear dispersion approaching the Fermi level is identified for the first time.
These results provide strong experimental evidence on the nature of
topologically non-trivial three-dimensional Dirac cones in Cd3As2.
