  The Fermi arcs are signatures of exotic states in solids because they defy
conventional concept of Fermi surfaces as closed contours in momentum space.
Fermi arcs were first discovered in cuprates, and caused by the pseudogap. Weyl
semimetals provided another way to generate Fermi arcs by breaking either the
time reversal symmetry (TRS) or inversion symmetry of a 3D Dirac semimetal,
which can result in a Weyl semimetal with pairs of Weyl nodes that have
opposite chirality. The bulk-boundary correspondence associated with the Chern
number leads to the emergence of Fermi arcs on the boundary. Here, we present
experimental evidence that pairs of magnetically split hole- and electron-like
Fermi arcs emerge below the Neel temperature, in the antiferromagnetic (AFM)
state of cubic NdBi due to a novel band splitting effect. Whereas TRS is broken
by the AFM order, both inversion and nonsymmorphic TRS are preserved in the
bulk, precluding the possibility of a Weyl semimetal. The observed magnetic
splitting is highly unusual, as it creates bands of opposing curvature, that
changes with temperature and follows the antiferromagnetic order parameter.
This is completely different from previously reported cases of magnetic
splittings such as traditional Zeeman and Rashba, where the curvature of the
bands is preserved. Therefore, our finding represents a new Fermionic state
created by new type of magnetic band splitting in the presence of a long-range
AFM order that are not readily explained by existing theoretical ideas.
