  Topological semimetals have recently attracted extensive research interests
as host materials to condensed matter physics counterparts of Dirac and Weyl
fermions originally proposed in high energy physics. These fermions with linear
dispersions near the Dirac or Weyl points obey Lorentz invariance, and the
chiral anomaly leads to novel quantum phenomena such as negative
magnetoresistance. The Lorentz invariance is, however, not necessarily
respected in condensed matter physics, and thus Lorentz-violating type-II Dirac
fermions with strongly tilted cones can be realized in topological semimetals.
Here, we report the first experimental evidence of type-II Dirac fermions in
bulk stoichiometric PtTe$_2$ single crystal. Angle-resolved photoemission
spectroscopy (ARPES) measurements and first-principles calculations reveal a
pair of strongly tilted Dirac cones along the $\Gamma$-A direction under the
symmetry protection, confirming PtTe$_2$ as a type-II Dirac semimetal. The
realization of type-II Dirac fermions opens a new door for exotic physical
properties distinguished from type-I Dirac fermions in condensed matter
materials.
