Ideal Weyl semimetal induced by magnetic exchange

We report theoretical and experimental evidence that EuCd2As2 in magnetic fields greater than 1.6 T applied along the c axis is a Weyl semimetal with a single pair of Weyl nodes. Ab initio electronic structure calculations, verified at zero field by angle-resolved photoemission spectra, predict Weyl nodes with wave vectors k=(0,0,±0.03)×2π/c at the Fermi level when the Eu spins are fully aligned along the c axis. Shubnikov–de Haas oscillations measured in fields parallel to c reveal a cyclotron effective mass of mc*=0.08me and a Fermi surface of extremal area Aext=0.24nm−2, corresponding to 0.1% of the area of the Brillouin zone. The small values of mc* and Aext are consistent with quasiparticles near a Weyl node. The identification of EuCd2As2 as a model Weyl semimetal opens the door to fundamental tests of Weyl physics.