  A Weyl semimetal possesses spin-polarized band-crossings, called Weyl nodes,
connected by topological surface arcs. The low-energy excitations near the
crossing points behave the same as massless Weyl fermions, leading to exotic
properties like chiral anomaly. To have the transport properties dominated by
Weyl fermions, Weyl nodes need to locate nearly at the chemical potential and
enclosed by pairs of individual Fermi surfaces with nonzero Fermi Chern
numbers. Combining angle-resolved photoemission spectroscopy and
first-principles calculation, here we show that TaP is a Weyl semimetal with
only single type of Weyl fermions, topologically distinguished from TaAs where
two types of Weyl fermions contribute to the low-energy physical properties.
The simple Weyl fermions in TaP are not only of fundamental interests but also
of great potential for future applications. Fermi arcs on the Ta-terminated
surface are observed, which appear in a different pattern from that on the
As-termination in TaAs and NbAs.
