X-ray and neutron scattering

Abstract:

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 Weyl nodes 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 TN = 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.

Abstract:

The description of crystal structures is given starting with some fundamental notions of crystal symmetry. The topics of lattices and space groups are briefly introduced and how these can be used with unit cell contents to describe the crystal structure. This leads to crystallographic databases where information on crystal structures is stored and can be searched. A brief discussion on refinement of diffraction information is given, together with the resulting geometric parameters.

Abstract:

The notion of periodicity in crystals is examined and how this can be varied in practice. In particular, the article discusses first of all the concept of superstructures, in which some sort of alternating motif occurs thus changing the repeat distance in a lattice. Crystals of this type are often incorrectly called in the literature superlattices: first of all they cannot be called lattices at all as they consist of atoms (a lattice must only consist of points). In any case such a superstructure is formed from a sublattice rather than a superlattice. In addition, some crystals do not have normal periodicity within a three-dimensional space, and are known as aperiodic crystals. Despite being aperiodic, they are still ordered. In mathematical terms they can by described with respect to a higher-dimension space and then projected back onto three dimensions. This generalizes our notion of what is meant by a crystal.