  A Weyl semimetal is a new state of matter that host Weyl fermions as
quasiparticle excitations. The Weyl fermions at zero energy correspond to
points of bulk band degeneracy, Weyl nodes, which are separated in momentum
space and are connected only through the crystal's boundary by an exotic Fermi
arc surface state. We experimentally measure the spin polarization of the Fermi
arcs in the first experimentally discovered Weyl semimetal TaAs. Our spin data,
for the first time, reveal that the Fermi arcs' spin polarization magnitude is
as large as 80% and possesses a spin texture that is completely in-plane.
Moreover, we demonstrate that the chirality of the Weyl nodes in TaAs cannot be
inferred by the spin texture of the Fermi arcs. The observed non-degenerate
property of the Fermi arcs is important for the establishment of its exact
topological nature, which reveal that spins on the arc form a novel type of 2D
matter. Additionally, the nearly full spin polarization we observed (~80%) may
be useful in spintronic applications.
