Ab initio electronic-structure calculations on the Nb/Cu multilayer system

Ab initio electronic-structure calculations are reported for coherent and incoherent Nb/Cu multilayem. An incoherent unit cell describing three Nb BCC (110) layers and three Cu FCC (111) layers is constructed for the layers in the Nishiyama- Wasserman orientation and with relaxed atomic positions at the interface. It is found that the total density of states is a combination of the broadened DOS curves of the parent metals and that at the interface Nb has a decreased, and Cu has an increased, density of states at the Fermi energy. These results are in agreement with experimetital results and for Nb can be explained by a broadening of the density of states. Possible coherency for small modulation wavelengths is investigated by calculating the total energies for overall BCC [110] and overall FCC [111] Nb/Cu multilayers consisting of one layer of each metal and comparing these with the total energy results of the incoherent structure. The positive interface energy found for the Nb/Cu system favours the incoherent multilayer over the coherent multilayers, where a positive structural energy is also involved

Sputum Induction in Children Is Feasible and Useful in a Bustling General Hospital Practice

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The Jeroen Bosch Hospital funded this study.Peer reviewedPublisher PD

Ab initio electronic-structure calculations on the Nb/Ta multilayer system

Ab initio electronic-structure calculations are performed for Nb/Ta multilayers with small modulation wavelengths in the [100], [110] and [111] BCC modulation directions. The [110] modulated Nb/Ta multilayer with the smallest modulation wavelength was also calculated with an 8% increased modulation wavelength on the one hand, with an undisturbed structure but containing 50% interstitial hydrogen on the other hand, and with a combination of both. The ease with which Nb/Ta multilayers can be grown in different modulation directions is understood from total energy calculations which show energy differences of less than 1 mRyd between the various directions. Further, it is found that interstitial hydrogen lowers the density of states at the Fermi level and that, but to a lesser extent, the lattice expansion on hydrogenation raises the density of states at the Fermi level. The net effect of a lowering of the density of states at the Fermi level is in agreement with the measured reduction of the superconducting critical temperature on hydrogenation

Ab initio electronic-structure calculations on the Au/Ag multilayer system and Au epitaxy on the Ag(110) surface

Ab initio electronic-structure calculations are performed for Au/Ag multilayers with small modulation wavelengths in the [100], [110], and [111] fcc modulation directions and for Ag [110] slabs covered with none, one, two, or three Au(110) layers. The total energies of the multilayers can be ordered according to the number of Au-Ag nearest-neighbor pairs in these systems, from which the bonding at the interface can be estimated. The multilayer and slab results taken together allow for a decomposition of the energetics of Au adlayer behavior on the Ag(110) surface. It is found that a bare Ag(110) surface is energetically more favorable than an Ag(110) surface covered by Au monolayers. This is in agreement with the difference in surface energy of Au and Ag surfaces, and it implies that the bonding energy gained at the Au-Ag interface is insufficient to overcome this difference. Therefore Au will tend to grow in islands on an Ag(110) surface rather than wetting it. Our results, based on Au-Ag bonding and the open geometry of the Ag(110) surface, support the bilayer-growth model that has recently been reported in the literature, but this support is only for the first two layers in the islands. Whereas the density-of-states (DOS) curves of the multilayers show hardly any variation for the different systems, the layer-resolved densities of states for the slabs show pronounced changes on going from the surface layers inward. For all slabs, with or without Au atop, an interface state can be observed, localized between the first and second surface layer. Furthermore, a shift in the d-state density to higher energies at the interface and to lower energies for the layer below the interface occurs as compared with the elemental DOS curves

ELECTRIC-FIELD-GRADIENT CALCULATIONS ON CADMIUM IN CADMIUM-HELIUM VACANCY CLUSTERS IN TUNGSTEN

Electric-field gradients (EFG) at the position of the cadmium atom in low-symmetry CdVnHem clusters in tungsten were calculated with the augmented spherical wave method. The results agreed within 70% with experimental values. It is observed that lattice relaxation has a large effect on both the quadrupole frequency omega(0) and the asymmetry parameter eta. The value of eta for the unrelaxed dusters CdV2 and CdV3He2 are not equal to 1.0, as predicted by the point charge model. The cluster CdV2He2 has a temperature-dependent EFG with a transition temperature of about 100 K. The same holds for CdV3He4, but in this case there are two transitions, at abut 170 and 250 K, respectively

Minimal charge gap in the ionic Hubbard model

We study the ionic Hubbard model at temperature T=0 within the mean-field approximation, and show that the charge gap does not close completely at the ionic-band insulator to antiferromagnetic insulator transition, contrary to previous expectations. Furthermore, we find an intermediate phase for on-site repulsions $U>U_c$ for different lattices, and calculate the phase diagram for the ionic Hubbard model with alternating U, corresponding to a Cu-O lattice.Comment: 5 pages with 7 figures; minor correction

Transport, optical and electronic properties of the half metal CrO2

The electronic structure of CrO_2 is critically discussed in terms of the relation of existing experimental data and well converged LSDA and GGA calculations of the electronic structure and transport properties of this half metal magnet, with a particular emphasis on optical properties. We find only moderate manifestations of many body effects. Renormalization of the density of states is not large and is in the typical for transition metals range. We find substantial deviations from Drude behavior in the far-infrared optical conductivity. These appear because of the unusually low energy of interband optical transitions. The calculated mass renormalization is found to be rather sensitive to the exchange-correlation functional used and varies from 10% (LSDA) to 90% (GGA), using the latest specific heat data. We also find that dressing of the electrons by spin fluctuations, because of their high energy, renormalizes the interband optical transition at as high as 4 eV by about 20%. Although we find no clear indications of strong correlations of the Hubbard type, strong electron-magnon scattering related to the half metallic band structure is present and this leads to a nontrivial temperature dependence of the resistivity and some renormalization of the electron spectra.Comment: 9 Revtex 2 column pages, including 8 postscript figures. Two more figures are included in the submission that are not embedded in the paper, representing DOS and bandstructure of the paramagnetic CrO

Half-metallic Antiferromagnet BaCrFeAs2

First-principles calculations and a tight-binding analysis predict that the iron-pnictide BaCrFeAs2 is a promising candidate for half-metallic material with fully-compensated magnetization. The transition-metal ions Cr and Fe prefer the three-dimensional intervening lattice, which yields the antiferromagnetic order of spin orientations. Due to the difference between Cr and Fe in the electronegativity, a band gap is opened at the Fermi level in the spin channel in which Fe provides the majority carriers. The selective hybridization between 3d orbitals of Cr and As:4p states due to the peculiar lattice structure of the iron-pnictide is shown to be crucial for the novel properties.Comment: added reference

Spin-dependent transport in metal/semiconductor tunnel junctions

This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface. Measurements are presented of the circular-polarization-dependent photocurrent (the so-called helicity asymmetry) in thin-film tunnel junctions of Co/Al2O3/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/τ-MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of τ-MnAl along the tunnelling direction; the latter is corroborated by bandstructure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.