  A new type of Weyl semimetal state, in which the energy values of Weyl nodes
are not the local extrema, has been theoretically proposed recently, namely
type II Weyl semimetal. Distinguished from type I semimetal (e.g. TaAs), the
Fermi surfaces in a type II Weyl semimetal consist of a pair of electron and
hole pockets touching at the Weyl node. In addition, Weyl fermions in type II
Weyl semimetals violate Lorentz invariance. Due to these qualitative
differences distinct spectroscopy and magnetotransport properties are expected
in type II Weyl semimetals. Here, we present the direct observation of the
Fermi arc states in MoTe2 by using angle resolved photoemission spectroscopy.
Two arc states are identified for each pair of Weyl nodes whoes surface
projections of them possess single topological charge, which is a unique
property for type II Weyl semimetals. The experimentally determined Fermi arcs
are consistent with our first principle calculations. Our results unambiguously
establish that MoTe2 is a type II Weyl semimetal, which serves as a great test
bed to investigate the phenomena of new type of Weyl fermions with Lorentz
invariance violated.
