P. Strange

Understanding the valency of rare earths from first-principles theory

The rare-earth metals have high magnetic moments and a diverse range of magnetic structures. Their magnetic properties are determined by the occupancy of the strongly localized 4f electronic shells, while the outer s–d electrons determine the bonding and other electronic properties. Most of the rare-earth atoms are divalent, but generally become trivalent in the metallic state. In some materials, the energy difference between these valence states is small and, by changing some external parameter (such as pressure), a transition from one to the other occurs. But the mechanism underlying this transition and the reason for the differing valence states are not well understood. Here we report first-principles electronic-structure calculations that enable us to determine both the valency and the lattice size as a function of atomic number, and hence understand the valence transitions. We find that there are two types of f electrons: localized core-like f electrons that determine the valency, and delocalized band-like f electrons that are formed through hybridization with the s–d bands and which participate in bonding. The latter are found only in the trivalent systems; if their number exceeds a certain threshold, it becomes energetically favourable for these electrons to localize, causing a transition to a divalent ground state.

Half-metallic to insulating behavior of rare-earth nitrides

The electronic structure of the rare earth nitrides is studied systematically using the {\it ab-initio} self-interaction corrected local-spin-density approximation (SIC-LSD). This approach allows both a localised description of the rare earth

Stability of gold atoms and dimers adsorbed on graphene

f-

The electronic structure of europium chalcogenides and pnictides

electrons and an itinerant description of the valence electrons. Localising different numbers of

Theory of circularly polarized x‐ray absorption by ferromagnetic Fe

f

A relativistic spin-polarised multiple-scattering theory, with applications to the calculation of the electronic structure of condensed matter

-electrons on the rare earth atom corresponds to different valencies, and the total energies can be compared, providing a first-principles description of valence. CeN is found to be tetravalent while the remaining rare earth nitrides are found to be trivalent. We show that these materials have a broad range of electronic properties including forming a new class of half-metallic magnets with high magnetic moments and are strong candidates for applications in spintronic and spin-filtering devices.

Pressure‐Induced Valence Transitions in Rare Earth Chalcogenides and Pnictides

We report density functional theory (DFT) calculations for gold atoms and dimers on the surface of graphene. The calculations were performed using the plane wave pseudopotential method. Calculations were performed for a variety of geometries, and both the graphene surface and gold atoms were allowed to fully relax. In agreement with experiment, our results show that the gold‐gold interaction is considerably stronger than the gold‐graphene interaction, implying that uniform coverage could not be attained. The minimum energy configuration for a single gold atom is found to be directly above a carbon atom, while for the dimer it is perpendicular to the surface and directly above a carbon‐carbon bond. Our results are consistent with previous similar calculations. (Some figures in this article are in colour only in the electronic version)

Experimental and theoretical investigation of the crystal structure of CuS

The electronic structure of some europium chalcogenides and pnictides is calculated using the ab initio self-interaction corrected local-spin-density approximation (SIC-LSD). This approach allows both a localized description of the rare earth f-electrons and an itinerant description of s-, p-, and d-electrons. Localizing different numbers of f-electrons on the rare earth atom corresponds to different nominal valencies, and the total energies can be compared, providing a first-principles description of valency. All the chalcogenides are found to be insulators in the ferromagnetic state and to have a divalent configuration. For the pnictides we find that EuN is half-metallic and the rest are normal metals. However, a valence change occurs as we go down the pnictide column of the periodic table. EuN and EuP are trivalent, EuAs is only just trivalent, and EuSb is found to be divalent. Our results suggest that these materials may find applications in spintronic and spin filtering devices.

Electronic structure of samarium monopnictides and monochalcogenides

A description of the absorption of circularly polarized x rays based on a spin‐polarized version of relativistic multiple scattering theory is presented. The approach treats all relativistic effects and spin polarization on equal footing, permitting us to study the difference in absorption of left and right circularly polarized x rays. Results obtained for ferromagnetic Fe are presented and compared with experiment.

A relativistic theory of X-ray absorption by spin-polarized targets

A fully relativistic spin-polarised multiple-scattering theory is described for solving the fundamental Kohn-Sham equations of the relativistic spin-density functional theory. Particular attention is paid to calculating the Green functions for the Dirac-like equations occurring in the theory. Practical ways of evaluating the appropriate formulae are discussed and illustrated by explicit calculations of energy bands, density of states, and spin contribution to the magnetic moment in ferromagnetic iron. The difference between this fully relativistic theory and previous treatments of relativistic effects in magnetic materials is emphasised. Several physical applications of the method are suggested.