  Quantum topological materials, exemplified by topological insulators,
three-dimensional Dirac semimetals and Weyl semimetals, have attracted much
attention recently because of their unique electronic structure and physical
properties. Very lately it is proposed that the three-dimensional Weyl
semimetals can be further classified into two types. In the type I Weyl
semimetals, a topologically protected linear crossing of two bands, i.e., a
Weyl point, occurs at the Fermi level resulting in a point-like Fermi surface.
In the type II Weyl semimetals, the Weyl point emerges from a contact of an
electron and a hole pocket at the boundary resulting in a highly tilted Weyl
cone. In type II Weyl semimetals, the Lorentz invariance is violated and a
fundamentally new kind of Weyl Fermions is produced that leads to new physical
properties. WTe2 is interesting because it exhibits anomalously large
magnetoresistance. It has ignited a new excitement because it is proposed to be
the first candidate of realizing type II Weyl Fermions. Here we report our
angle-resolved photoemission (ARPES) evidence on identifying the type II Weyl
Fermion state in WTe2. By utilizing our latest generation laser-based ARPES
system with superior energy and momentum resolutions, we have revealed a full
picture on the electronic structure of WTe2. Clear surface state has been
identified and its connection with the bulk electronic states in the momentum
and energy space shows a good agreement with the calculated band structures
with the type II Weyl states. Our results provide spectroscopic evidence on the
observation of type II Weyl states in WTe2. It has laid a foundation for
further exploration of novel phenomena and physical properties in the type II
Weyl semimetals.
