  Constrained by the Nielsen-Ninomiya no-go theorem, in all so-far
experimentally determined Weyl semimetals (WSMs) the Weyl points (WPs) always
appear in pairs in the momentum space with no exception. As a consequence,
Fermi arcs occur on surfaces which connect the projections of the WPs with
opposite chiral charges. However, this situation can be circumvented in the
case of unpaired WP, without relevant surface Fermi arc connecting its surface
projection, appearing singularly, while its Berry curvature field is absorbed
by nontrivial charged nodal walls. Here, combining angle-resolved photoemission
spectroscopy with density functional theory calculations, we show
experimentally that a singular Weyl point emerges in PtGa at the center of the
Brillouin zone (BZ), which is surrounded by closed Weyl nodal walls located at
the BZ boundaries and there is no Fermi arc connecting its surface projection.
Our results reveal that nontrivial band crossings of different dimensionalities
can emerge concomitantly in condensed matter, while their coexistence ensures
the net topological charge of different dimensional topological objects to be
zero. Our observation extends the applicable range of the original
Nielsen-Ninomiya no-go theorem which was derived from zero dimensional paired
WPs with opposite chirality.
