High temperature ideal Weyl semimetal phase and quantum anomalous Hall phase in ferromagnetic BaEuNiOsO6 and its (111) (BaEuNiOsO6)/(BaTiO3)10 superlattice

Weyl semimetals (WSMs) have recently stimulated intensive interest because they exhibit fascinating physical properties and also promise exciting technological applications. So far, however, the few confirmed magnetic WSMs generally have a large number of Weyl points either located away from the Fermi level (EF) or shrouded by nontopological Fermi surface pockets. Based on first-principles calculations, we establish double perovskite BaEuNiOsO6 to be a high Curie temperature (Tc) ferromagnetic WSM with magnetization along the [111] direction, just two pairs of Weyl points at EF and Tc = 325 K. The strong ferromagnetism is attribued to the strong ferromagnetic Ni 3d - Eu 4f - Os 5d coupling induced by the substitution of half of Ba atoms with Eu atoms in Ba2NiOsO6. Moreover, the momentum separation of one Weyl point pair is large, thus giving rise to not only a long (001) surface Fermi arc but also large anomalous Hall conductivity. Intriguingly, as a unique physical result of a ferromagnetic WSM, the (111) BaEuNiOsO6 monolayer (ML) superlattice (BaEuNiOsO6)/(BaTiO3)10}, being its (111) quantum-well structure, is found to be a high temperature (Tc = 210 K) Chern insulator with a large band gap of 90 meV. Thus, double perovskite BaEuNiOsO6 will provide a superior material plotform for exploring fundamental physics of Weyl fermions and its (111) ML superlattices will offer a high temperature magnetic topological insulator for studying exotic quantum phenomena.