  The subject of topological materials has attracted immense attention in
condensed-matter physics, because they host new quantum states of matter
containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can
only exist on the surface of topological superconductors, Dirac and Weyl
fermions can be realized in both 2D and 3D materials. The latter are semimetals
with Dirac/Weyl cones either not tilted (type I) or tilted (type II). Although
both Dirac and Weyl fermions have massless nature with the nontrivial Berry
phase, the formation of Weyl fermions in 3D semimetals require either
time-reversal or inversion symmetry breaking to lift degeneracy at Dirac
points. Here, we demonstrate experimentally that canted antiferromagnetic
BaMnSb2 is a 3D Weyl semimetal with a 2D electronic structure. The Shubnikov-de
Hass oscillations of the magnetoresistance give nearly zero effective mass with
high mobility and the nontrivial Berry phase. The ordered magnetic arrangement
(ferromagnetic ordering in the ab plane and antiferromagnetic ordering along
the c axis below 286 K) breaks the time-reversal symmetry, thus offering us an
ideal platform to study magnetic Weyl fermions in a centrosymmetric material.
