RESUMEN
Magnons in ferromagnets have one chirality, and typically are in the GHz range and have a quadratic dispersion near the zero wave vector. In contrast, magnons in antiferromagnets are commonly considered to have bands with both chiralities that are degenerate across the entire Brillouin zone, and to be in the THz range and to have a linear dispersion near the center of the Brillouin zone. Here we theoretically demonstrate a new class of magnons on a prototypical d-wave altermagnet RuO_{2} with the compensated antiparallel magnetic order in the ground state. Based on density-functional-theory calculations we observe that the THz-range magnon bands in RuO_{2} have an alternating chirality splitting, similar to the alternating spin splitting of the electronic bands, and a linear magnon dispersion near the zero wave vector. We also show that, overall, the Landau damping of this metallic altermagnet is suppressed due to the spin-split electronic structure, as compared to an artificial antiferromagnetic phase of the same RuO_{2} crystal with spin-degenerate electronic bands and chirality-degenerate magnon bands.
RESUMEN
Ca and S K-edge spectra of CaS are calculated by the full-potential Green's function multiple-scattering method, by the FLAPW method and by the finite-difference method. All three techniques lead to similar spectra. Some differences remain close to the edge, both when comparing different calculations with each other and when comparing the calculations with earlier experimental data. Here it is found that using the full potential does not lead to significant improvement over the atomic spheres approximation and that the effect of the core hole can be limited to the photoabsorbing atom alone. Doping CaS with Eu will not affect the Ca and S K-edge XANES of CaS significantly but may give rise to a pre-edge structure not present for clean CaS.
RESUMEN
X-ray absorption spectra calculated within an effective one-electron approach have to be broadened to account for the finite lifetime of the core hole. For methods based on Green's function this can be achieved either by adding a small imaginary part to the energy or by convoluting the spectra on the real axis with a Lorentzian. By analyzing the Fe K- and L2,3-edge spectra it is demonstrated that these procedures lead to identical results only for energies higher than a few core-level widths above the absorption edge. For energies close to the edge, spurious spectral features may appear if too much weight is put on broadening via the imaginary energy component. Special care should be taken for dichroic spectra at edges which comprise several exchange-split core levels, such as the L3-edge of 3d transition metals.
RESUMEN
Polarization-dependent damping of the fine structure in the Cu K-edge spectrum of creatinium tetrachlorocuprate [(creat)2CuCl4] in the X-ray absorption near-edge structure (XANES) region is shown to be due to atomic vibrations. These vibrations can be separated into two groups, depending on whether the respective atoms belong to the same molecular block; individual molecular blocks can be treated as semi-rigid entities while the mutual positions of these blocks are subject to large mean relative displacements. The effect of vibrations can be efficiently included in XANES calculations by using the same formula as for static systems but with a modified free-electron propagator which accounts for fluctuations in interatomic distances.
RESUMEN
In order to assess the usability of X-ray absorption near-edge structure (XANES) for studying the structure of BO(n)-containing materials, the dependence of theoretical XANES at the B K-edge on the way the scattering potential is constructed is investigated. Real-space multiple-scattering calculations are performed for self-consistent and non-self-consistent potentials and for different ways of dealing with the core hole. It is found that in order to reproduce the principal XANES features it is sufficient to use a non-self-consistent potential with a relaxed and screened core hole. Employing theoretical modelling of XANES for studying the structure of boron-containing glasses is thus possible. The core hole affects the spectrum significantly, especially in the pre-edge region. In contrast to minerals, B K-edge XANES of BPO(4) can be reproduced only if a self-consistent potential is employed.
RESUMEN
The arrangement of B atoms in a doped Si(1 1 1)-[Formula: see text]:B system was studied using a near-edge x-ray absorption fine structure (NEXAFS). Boron atoms were deposited via segregation from the bulk by flashing the sample repeatedly. The positions of B atoms are determined by comparing measured polarized (angle-dependent) NEXAFS spectra with spectra calculated for various structural models based on ab initio total energy calculations. It is found that most of boron atoms are located in sub-surface L[Formula: see text] positions, beneath a Si atom. However, depending on the preparation method a significant portion of B atoms may be located elsewhere. A possible location of these non-L[Formula: see text]-atoms is at the surface, next to those Si atoms which form the [Formula: see text] reconstruction.
RESUMEN
Methodological advances in multiple scattering theory (MST) in both wave and Green's function versions are reported for the calculation of electronic ground and excited state properties of condensed matter systems with an emphasis on core-level photoemission and absorption spectra. Full-potential MST is reviewed and extended to non-local potentials. Multichannel MST is reformulated in terms of the multichannel density matrix whereby strong electron correlation of atomic multiplet type can be accounted for in both ground and excited states.
RESUMEN
Cu doped ZnO polycrystalline pellets were synthesized with Cu concentrations varying from 2 to 10 wt% by a solid state reaction route (mixing of ZnO and CuO powders). Global magnetization measurements showed that all the samples were paramagnetic. Fitting the temperature-dependence of the magnetization to the Curie-Weiss law revealed the presence of an antiferromagnetic interaction between magnetic moments. Structural characterizations were carried out by x-ray diffraction and x-ray absorption spectroscopy (XAS) at the Cu K-edge. By analyzing the XAS data, we found that at low Cu content most of the Cu atoms substitute for Zn inside the ZnO wurtzite lattice, while for higher Cu concentrations some unreacted CuO remains segregated from the Zn(1-x)Cu(x)O solid solution. Element-specific magnetic measurements were carried out by x-ray magnetic circular dichroism (XMCD) and compared to the results of ab initio calculations. The XMCD signal at the Cu K-edge originates from magnetic moments localized at Cu sites and, by monitoring the magnetic field dependence, we concur that these moments are associated with a paramagnetic state.