Substrate-induced antiferromagnetism of an Fe monolayer on the Ir(001) surface

We present detailed ab initio study of structural and magnetic stability of a Fe-monolayer on the fcc(001) surface of iridium. The Fe-monolayer has a strong tendency to order antiferromagnetically for the true relaxed geometry. On the contrary an unrelaxed Fe/Ir(001) sample has a ferromagnetic ground state. The antiferromagnetism is thus stabilized by the decreased Fe-Ir layer spacing in striking contrast to the recently experimentally observed antiferromagnetism of the Fe/W(001) system which exists also for an ideal bulk-truncated, unrelaxed geometry. The calculated layer relaxations for Fe/Ir(001) agree reasonably well with recent experimental LEED data. The present study centers around the evaluation of pair exchange interactions between Fe-atoms in the Fe-overlayer as a function of the Fe/Ir interlayer distance which allows for a detailed understanding of the antiferromagnetism of a Fe/Ir(001) overlayer. Furthermore, our calculations indicate that the nature of the true ground state could be more complex and display a spin spiral-like rather than a c(2x2)-antiferromagnetic order. Finally, the magnetic stability of the Fe monolayer on the Ir(001) surface is compared to the closely related Fe/Rh(001) system.Comment: 8 pages, 4 figure

Magnetism without magnetic impurities in oxides ZrO2 and TiO2

We perform a theoretical study of the magnetism induced in transition metal dioxides ZrO2 and TiO2 by substitution of the cation by a vacancy or an impurity from the groups 1A or 2A of the periodic table, where the impurity is either K or Ca. In the present study both supercell and embedded cluster methods are used. It is demonstrated that the vacancy and the K-impurity leads to a robust induced magnetic moment on the surrounding O-atoms for both the cubic ZrO2 and rutile TiO2 host crystals. On the other hand it is shown that Ca-impurity leads to a non magnetic state. The native O-vacancy does not induce a magnetic moment in the host dioxide crystal.Comment: 7 pages, 5 figures, submitte

Anomalous Ferromagnetism of Monatomic Co Wire at the Pt(111) Surface Step Edge

A first-principles investigation of the anomalous ferromagnetism of a quasi-one-dimensional Co chain at the Pt(111) step edge is reported. Our calculations show that the symmetry breaking at the step leads to an easy magnetization axis at an odd angle of $\sim20^{\circ}$ {\em towards} the Pt step, in agreement with experiment [P. Gambardella {\em et al.}, {\em Nature} {\bf 416}, 301 (2002)]. Also, the Co spin and orbital moments become noncollinear, even in the case of a collinear ferromagnetic spin arrangement. A significant enhancement of the Co orbital magnetic moment is achieved when modest electron correlations are treated within LSDA+$U$ calculations.Comment: Presented at MRS Meeting in Boston, Dec. 2003; 4 pages including 3 figure

Effect of dexamethasone in patients with ARDS and COVID-19 (REMED trial)—study protocol for a prospective, multi-centre, open-label, parallel-group, randomized controlled trial

BACKGROUND: Since December 2019, SARS-CoV-2 virus has infected millions of people worldwide. In patients with COVID-19 pneumonia in need of oxygen therapy or mechanical ventilation, dexamethasone 6 mg per day is currently recommended. However, the dose of 6 mg of dexamethasone is currently being reappraised and may miss important therapeutic potential or may prevent potential deleterious effects of higher doses of corticosteroids. METHODS: REMED is a prospective, open-label, randomised controlled trial testing the superiority of dexamethasone 20 mg (dexamethasone 20 mg on days 1-5, followed by dexamethasone 10 mg on days 6-10) vs 6 mg administered once daily intravenously for 10 days in adult patients with moderate or severe ARDS due to confirmed COVID-19. Three hundred participants will be enrolled and followed up for 360 days after randomization. Patients will be randomised in a 1:1 ratio into one of the two treatment arms. The following stratification factors will be applied: age, Charlson Comorbidity Index, CRP levels and trial centre. The primary endpoint is the number of ventilator-free days (VFDs) at 28 days after randomisation. The secondary endpoints are mortality from any cause at 60 days after randomisation; dynamics of the inflammatory marker, change in WHO Clinical Progression Scale at day 14; and adverse events related to corticosteroids and independence at 90 days after randomisation assessed by the Barthel Index. The long-term outcomes of this study are to assess long-term consequences on mortality and quality of life at 180 and 360 days. The study will be conducted in the intensive care units (ICUs) of ten university hospitals in the Czech Republic. DISCUSSION: We aim to compare two different doses of dexamethasone in patients with moderate to severe ARDS undergoing mechanical ventilation regarding efficacy and safety. TRIAL REGISTRATION: EudraCT No. 2020-005887-70. ClinicalTrials.gov NCT04663555. Registered on December 11, 2020

Tetragonal CuMnAs alloy: Role of defects

The antiferromagnetic (AFM) CuMnAs alloy with tetragonal structure is a promising material for the AFM spintronics. The resistivity measurements indicate the presence of defects. We confirmed vacancies on Mn or Cu sublattices and Mncu and Cum antisites as most probable defects in CuMnAs by our new ab initio total energy calculations. We have estimated resistivities of possible defect types as well as resistivities of samples for which the X-ray structural analysis is available. In the latter case we have found that samples with Cu- and Mn-vacancies with low formation energies have also resistivities which agree well with the experiment. Finally, we have also calculated exchange interactions and estimated the Neel temperatures by using the Monte Carlo approach. A good agreement with experiment was obtained.Web of Science47447146

Effect of Twinning on Angle-Resolved Photoemission Spectroscopy Analysis of Ni49.7Mn29.1Ga21.2(100) Heusler Alloy

To explain the observed features of k-space photoelectron images taken on off-stoichiometric Heusler Ni49.7Mn29.1Ga21.2 single-crystals in the cubic austenitic and pseudotetragonal martensitic phases, the images were simulated theoretically. Despite the moderate structural difference of both phases, there is large difference in photoemission spectra. Analysis of the final states’ structure, matrix elements, and interface barrier scattering was performed to interpret discrepancies between the external photoemission of the austenite and martensite. The missing signal at the surface-normal emission of the martensitic phase is, ultimately, explained by repeated scatterings of escaping electrons on the interfaces between nanotwins

Vyšetření Bi2Te3 dopovaného prvky z VIII B skupiny periodické tabulky

Previous studies have demonstrated that the fabricated topological insulators (TIs) are always heavily doped by intrinsic defects, vacancies and antisites. These defects pin the Fermi level to the bulk band edges and make it difficult to characterize the transport properties of topological states. Bi2Te3 behaves as an n-type semiconductor due to these defects and the Dirac point is buried deep below the EF. Thus, Fermi level tuning has become a technologically important issue in TI research. Single crystal samples with the nominal composition Bi2-xMxTe3 (M = Fe, Ru, Os, x = 0, 0.02, 0.04, 0.06) were grown by heating stoichiometric mixtures of elements followed by cooling in a horizontal furnace from 1073 K to 823 K at a rate of 6 K per hour. The crystal was subsequently annealed at 823 K for 350 h and quenched in air. Samples were examined with X-ray Diffraction and Electron Spectroscopy for Chemical Analysis. Electronic structure of samples was measured with k-space microscope via equienergetic cuts through the first Brillouin zone (BZ). The images were taken in the binding energy interval (+1, -2) eV. Electronic states in the vicinity of gap with an aspect of the Fermi edge location in the bulk were theoretically mapped using ab initio VASP code. The calculations were done by including relaxation, spin-orbit interaction and with the doped atoms in the substitutional (Bi) or interstitial positions.Předchozí studie ukázaly, že připravené topologické izolátory (TIs) jsou vždy silně dopovány intrinzickými defekty, vakancemi a antistrukturními. Tyto defekty připoutají Fermiho hladinu k okrajům objemového pásu a ztěžují charakterizaci transportních vlastností topologických stavů. Bi2Te3 se kvůli těmto defektům chová jako polovodič n-typu a Diracův bod se nachází hluboko pod EF. Ladění Fermiho hladiny se tak stalo technologicky důležitým tématem ve výzkumu TI. Monokrystalické vzorky s nominálním složením Bi2-xMxTe3 (M = Fe, Ru, Os, x = 0, 0.02, 0.04, 0.06) byly pěstovány zahříváním stechiometrických směsí prvků následovaným chlazením v horizontální peci z 1073 K na 823 K rychlostí 6 K za hodinu. Krystal byl následně temperován 350 h při 823 K a zakalen na vzduchu. Vzorky byly charakterizovány RTG difrakcí a elektronovou spektroskopií pro chemickou analýzu. Elektronická struktura vzorků byla měřena mikroskopem k-prostoru pomocí ekvienergetických řezů skrze první Brillouinovu zónu (BZ). Obrázky byly pořízeny v intervalu vazebné energie (+1, -2) eV. Elektronické stavy v okolí zakázaného pásu s ohledem na polohu Fermiho okraje v objemu byly teoreticky mapovány za použití ab initio VASP kódu. Výpočty byly provedeny zahrnutím relaxace, spin-orbitální interakce a s dopujícími atomy v substitučních (Bi) nebo intersticiálních polohách