  Altermagnets constitute a novel, third fundamental class of collinear
magnetic ordered materials, alongside with ferro- and antiferromagnets. They
share with conventional antiferromagnets the feature of a vanishing net
magnetization. At the same time they show a spin-splitting of electronic bands,
just as in ferromagnets, caused by the atomic exchange interaction. On the
other hand, topology has recently revolutionized our understanding of condensed
matter physics, introducing new phases of matter classified by intrinsic
topological order. Here we connect the worlds of altermagnetism and topology,
showing that the electronic structure of the altermagnet CrSb is topological
and hosts a novel Weyl semimetallic state. Using high-resolution and spin
angleresolved photoemission spectroscopy, we observe a large momentum-dependent
spin-splitting in CrSb, reaching up to 1 eV, that induces altermagnetic Weyl
nodes with an associated magnetic quantum number. At the surface we observe
their spin-polarized topological Fermi-arcs. This establishes that in
altermagnets the large energy scale intrinsic to the spin-splitting - orders of
magnitude larger than the relativistic spin-orbit coupling - creates its own
realm of robust electronic topology.
