The role of pressure-induced stacking faults on the magnetic properties of gadolinium

Experimental data show that under pressure, Gd goes through a series of structural transitions hcp to Sm-type (close-packed rhombohedral) to dhcp that is accompanied by a gradual decrease of the Curie temperature and magnetization till the collapse of a finite magnetization close to the dhcp structure. We explore theoretically the pressure-induced changes of the magnetic properties, by describing these structural transitions as the formation of fcc stackings faults. Using this approach, we are able to describe correctly the variation of the Curie temperature with pressure, in contrast to a static structural model using the hcp structure.Comment: Preprint (no peer-reviewed

Chiral magnons in altermagnetic RuO2

Magnons in ferromagnets have one chirality, and typically are in the GHz range and have a quadratic dispersion near the zero wavevector. 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 wavevector. 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.Comment: 6 pages, 4 figure

Influence of spin-orbit coupling on the magnetic dipole term Tα

Vliv spin-orbitální spojky (SOC) na magnetického dipólu termínu Tα je studována v celé řadě systému s cílem zkontrolovat, zda je Tα termín může být vyřazena z analýzy x-ray magnetického kruhového dichroismu spektra provádí přes pravidlo spin momentu částky. Plně relativistický Korringa-Kohn-Rostoker Greena Funkce pro výpočty Co monovrstvy a adatoms na Cu, Pd, Ag, Pt a Au (111) povrchy byly provedeny K ověření, zda je součet přes magnetický dipól termíny Tx + Ty + Tz je nula a úhlová závislost Ať z Tα termín platí jako 3 cos2 θ - 1. Z toho vyplývá, thatthere jsou okolnosti, kdy vliv na SOC Tα nelze opomenout ani na 3d atomu, kde SOC je nominálně malý. Rozhodujícím faktorem se jeví rozměrnosti systému: pro 3D adatoms, vliv na SOC Tα mohou být významné, zatímco u monovrstvy je vždy prakticky zanedbatelný. Kromě rozměrnost, hybridizace mezi adatom a substráty státy jsou tak důležité: Malý hybridizace Zvyšuje význam SOC a naopak.The influence of the spin-orbit coupling (SOC) on the magnetic dipole term Tα is studied across a range of systems in order to check whether the Tα term can be eliminated from analysis of x-ray magnetic circular dichroism spectra performed via the spin moment sum rule. Fully relativistic Korringa-Kohn-Rostoker Green’s function calculations for Co monolayers and adatoms on Cu, Pd, Ag, Pt, and Au (111) surfaces were performed to verify whether the sum over magnetic dipole terms Tx + Ty + Tz is zero and whether the angular dependence of the Tα term goes as 3 cos2 θ − 1. It follows that there are circumstances when the influence of the SOC on Tα cannot be neglected even for 3d atoms where the SOC is nominally small. The crucial factor appears to be the dimensionality of the system: For 3d adatoms, the influence of SOC on Tα can be significant whereas for monolayers it is always practically negligible. Apart from the dimensionality, hybridization between adatom and substrate states is also important: Small hybridization enhances the importance of the SOC and vice versa

Adaptive Anderson mixing for electronic structure calculations

Convergence rates of iterative algorithms for solving non-linear fixed-point (or root-finding) problems depend on the quality of the solution guess done in each iteration, which is used as the starting value in the next step. To avoid instabilities and oscillations, that guess is usually constructed (mixed) as a linear combination of the newly calculated value with values from the previous iterations. The mixing algorithm constitutes a crucial component for electronic structure calculation methods based on iterative seeking for a self-consistent state. This paper reviews several mixing algorithms for electronic structure calculations. The most important numerically confirmed finding is that the calculation speed depends more on the choice of the so-called “mixing coefficient” than on the choice of a particular algorithm.Then a new method allowing an automatic adaptation of this coefficient is proposed, implemented, and tested on various solid-state structures within three electronic structure calculation codes. In our testing cases, the newly designed Adaptive Anderson Algorithm exhibits better convergence for a broader range of initial mixing coefficients, and similar or better robustness, in comparison to the standard Anderson method. The Fortran implementation of the new algorithm and its Python wrapper are briefly described in the paper and made available for public use

Teoretické štúdium XANES Ca a S viacerýmy metódami: vlyv plného potenciálu, korovej diery a Eu.

Ca a S K-edge spektra CaS jsou vypočítány pomocí plného-potenciálu metodou vícenásobného rozptylu, metodou FLAPW a metodou finite-difference. Všechny tři techniky vedou k podobným spekterám. Zde se zjistilo, že využití plného potenciálu nevede k významnému zlepšení nad aproximací atomových koulí . Doping CaS s Eu neovlivní významně Ca a S K-hranu XANES CaS.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

Dependence of the Electronic Structure of β-Si6 - zAlzOzN8 - z on the (Al,O) Concentration z and on the Temperature

beta-Si6-zAlzOzN8-z is a prominent example of systems suitable as hosts for creating materials for light-emitting diodes (LEDs). In this work, the electronic structure of a series of semiordered and disordered beta-Si6-zAlzOzN8-z systems is investigated by means of ab initio calculations, using the FLAPW and Green function KKR methods. Finite temperature effects are included by averaging over thermodynamic configurations within the alloy analogy model. We found that the dependence of the electronic structure on the (Al,O) concentration z is similar for semiordered and disordered structures. The electronic band gap decreases with increasing z by about 1.5 eV when going from z=0 to z=2. States at the top of the valence band are mostly associated with N atoms whereas the states at the bottom of the conduction band are mostly derived from O atoms. Increasing the temperature leads to a shift of the bottom of the conduction band to lower energies. The amount of this shift increases with increasing z

Dependence of the Electronic Structure of β-Si6 - zAlzOzN8 - z on the (Al,O) Concentration z and on the Temperature

beta-Si6-zAlzOzN8-z is a prominent example of systems suitable as hosts for creating materials for light-emitting diodes (LEDs). In this work, the electronic structure of a series of semiordered and disordered beta-Si6-zAlzOzN8-z systems is investigated by means of ab initio calculations, using the FLAPW and Green function KKR methods. Finite temperature effects are included by averaging over thermodynamic configurations within the alloy analogy model. We found that the dependence of the electronic structure on the (Al,O) concentration z is similar for semiordered and disordered structures. The electronic band gap decreases with increasing z by about 1.5 eV when going from z=0 to z=2. States at the top of the valence band are mostly associated with N atoms whereas the states at the bottom of the conduction band are mostly derived from O atoms. Increasing the temperature leads to a shift of the bottom of the conduction band to lower energies. The amount of this shift increases with increasing z

Electric field control of magnons in magnetic thin films: Ab initio predictions for two-dimensional metallic heterostructures

We explore possibilities for control of magnons in two-dimensional heterostructures by an external electric field acting across a dielectric barrier. By performing ab initio calculations for a Fe monolayer and Fe bilayer, both suspended in vacuum and deposited on Cu(001), we demonstrate that external electric field can significantly modify magnon lifetimes and that these changes can be related to field-induced changes in layer-resolved Bloch spectral functions. For systems with more magnon dispersion branches, the gap between high- and low-energy eigenmodes varies with the external field. These effects are strongly influenced by the substrate. Considerable variability in how the magnon spectra are sensitive to the external electric field can be expected, depending on the substrate and on the thickness of the magnetic layer