Weyl metallic state induced by helical magnetic order.*

In the rapidly expanding field of topological materials there is growing interest in systems whose topological electronic band features can be induced or controlled by magnetism. Magnetic Weyl semimetals, which contain linear band crossings near the Fermi level, are of particular interest owing to their exotic charge and spin transport properties. Up to now, the majority of magnetic Weyl semimetals have been realized in ferro or ferrimagnetically ordered compounds, but a disadvantage of these materials for practical use is their stray magnetic field which limits the minimum size of devices. Here we show that a Weyl state can be induced by a helical spin configuration, in which the magnetization is fully compensated. Using a combination of neutron diffraction and resonant elastic x-ray scattering, we find that below T_N = 14.5 K the Eu spins in EuCuAs develop a planar helical structure  which induces two quadratic Weyl nodes with Chern numbers C =  ± 2 at the A point in the Brillouin zone. In this talk we will cover maily the theoretical aspects of this project.