Nontrivial Berry phase in magnetic BaMnSb 2 semimetal

Significance Among topological materials, experimental study of topological semimetals that host Dirac/Weyl fermions has just begun, even though these topological concepts were proposed nearly a century ago by Dirac and Weyl. Our work shows magnetic semimetal BaMnSb 2 exhibits nearly zero-mass fermions with high mobility and a nontrivial Berry phase. What is unique is the magnetic ordering, indicating the system is Weyl type due to time-reversal symmetry breaking. Theory shows that the spin order is very fragile, so it is expected that the application of magnetic field or uniaxial pressure could drive the material to be type II Weyl semimetal. In this material, Mn mainly controls the magnetic structure, whereas Sb in only one lattice site is responsible for the Dirac behavior.