36 research outputs found
C and S induces changes in the electronic and geometric structure of Pd(533) and Pd(320)
We have performed ab initio electronic structure calculations of C and S
adsorption on two vicinal surfaces of Pd with different terrace geometry and
width. We find both adsorbates to induce a significant perturbation of the
surface electronic and geometric structure of Pd(533) and Pd(320). In
particular C adsorbed at the bridge site at the edge of a Pd chain in Pd(320)
is found to penetrate the surface to form a sub-surface structure. The
adsorption energies show almost linear dependence on the number of
adsorbate-metal bonds, and lie in the ranges of 5.31eV to 8.58eV for C and
2.89eV to 5.40eV for S. A strong hybridization between adsorbate and surface
electronic states causes a large splitting of the bands leading to a drastic
decrease in the local densities of electronic states at the Fermi-level for Pd
surface atoms neighboring the adsorbate which may poison catalytic activity of
the surface. Comparison of the results for Pd(533) with those obtained earlier
for Pd(211) suggests the local character of the impact of the adsorbate on the
geometric and electronic structures of Pd surfaces.Comment: 14 pages 9 figs, Accepted J. Phys: Conden
Intrinsic hole localization mechanism in magnetic semiconductors
The interplay between clustering and exchange coupling in magnetic
semiconductors for the prototype (Ga_{1-x},Mn_x)As with manganese
concentrations x of 1/16 and 1/32 in the interesting experimental range is
investigated. For x ~ 6 %, when all possible arrangements of two atoms within a
large supercell are considered, the clustering of Mn atoms at nearest-neighbour
Ga sites is energetically preferred. As shown by spin density analysis, this
minimum energy configuration localizes further one hole and reduces the
effective charge carrier concentration. Also the exchange coupling constant
increases to a value corresponding to lower Mn concentrations with decreasing
inter Mn distance.Comment: Accepted for publication in Journal of Physics: Condensed Matte
The loss of anisotropy in MgB2 with Sc substitution and its relationship with the critical temperature
The electrical conductivity anisotropy of the sigma-bands is calculated for
the (Mg,Sc)B2 system using a virtual crystal model. Our results reveal that
anisotropy drops with relatively little scandium content (< 30%); this
behaviour coincides with the lowering of Tc and the reduction of the Kohn
anomaly. This anisotropy loss is also found in the Al and C doped systems. In
this work it is argued that the anisotropy, or 2D character, of the sigma-bands
is an important parameter for the understanding of the high Tc found in MgB2
The effect of Coulomb interaction at ferromagnetic-paramagnetic metallic perovskite junctions
We study the effect of Coulomb interactions in transition metal oxides
junctions. In this paper we analyze charge transfer at the interface of a three
layer ferromagnetic-paramagnetic-ferromagnetic metallic oxide system. We choose
a charge model considering a few atomic planes within each layer and obtain
results for the magnetic coupling between the ferromagnetic layers. For large
number of planes in the paramagnetic spacer we find that the coupling
oscillates with the same period as in RKKY but the amplitude is sensitive to
the Coulomb energy. At small spacer thickness however, large differences may
appear as function of : the number of electrons per atom in the ferromagnetics
and paramagnetics materials, the dielectric constant at each component, and the
charge defects at the interface plane emphasizing the effects of charge
transfer.Comment: tex file and 7 figure
Electronic structure of the ferromagnetic superconductor UCoGe from first principles
The superconductor UCoGe is analyzed with electronic structure calculations
using Linearized Augmented Plane Wave method based on Density Functional
Theory. Ferromagnetic and antiferromagnetic calculations with and without
correlations (via LDA+U) were done. In this compound the Fermi level is
situated in a region where the main contribution to DOS comes from the U-5f
orbital. The magnetic moment is mainly due to the Co-3d orbital with a small
contribution from the U-5f orbital. The possibility of fully non-collinear
magnetism in this compound seems to be ruled out. These results are compared
with the isostructural compound URhGe, in this case the magnetism comes mostly
from the U-5f orbital
Cancellation of probe effects in measurements of spin polarized momentum density by electron positron annihilation
Measurements of the two dimensional angular correlation of the
electron-positron annihilation radiation have been done in the past to detect
the momentum spin density and the Fermi surface. We point out that the momentum
spin density and the Fermi Surface of ferromagnetic metals can be revealed
within great detail owing to the large cancellation of the electron-positron
matrix elements which in paramagnetic multiatomic systems plague the
interpretation of the experiments. We prove our conjecture by calculating the
momentum spin density and the Fermi surface of the half metal CrO2, who has
received large attention due to its possible applications as spintronics
material
Topological insulators in the quaternary chalcogenide compounds and ternary famatinite compounds
We present first-principles calculations to predict several three dimensional
(3D) topological insulators in quaternary chalcogenide compounds which are made
of I-II-IV-VI compositions and in ternary compositions of
I-V-VI famatinite compounds. Among the large members of these two
families, we give examples of naturally occurring compounds which are mainly
Cu-based chalcogenides. We show that these materials are candidates of 3D
topological insulators or can be tuned to obtain topological phase transition
by manipulating the atomic number of the other cation and anion elements. A
band inversion can occur at a single point with considerably large
inversion strength, in addition to the opening of a bulk band gap throughout
the Brillouin zone. We also demonstrate that both of these families are related
to each other by cross-substitutions of cations in the underlying tetragonal
structure and that one can suitably tune their topological properties in a
desired manner.Comment: 7 pages, 4 figure
Surface Effects on Oxide Heterostructures
We report on surface effects on the electronic properties of interfaces in
epitaxial LaAlO/SrTiO heterostructures. Our results are based on
first-principles electronic structure calculations for well-relaxed multilayer
configurations, terminated by an ultrathin LaAlO surface layer. On varying
the thickness of this layer, we find that the interface conduction states are
subject to almost rigid band shifts due to a modified Fermi energy. Confirming
experimental data, the electronic properties of heterointerfaces therefore can
be tuned systematically by alterating the surface-interface distance. We expect
that this mechanism is very general and applies to most oxide heterostructures.Comment: 10 pages, 3 figures, accepted by Europhys. Let
Emerging nanomagnetism in 4d transition metal nanowires
Monatomic nanowires of the nonmagnetic transition metals Ru, Rh, and Pd have
been studied theoretically, using first-principles computational techniques, in
order to investigate the possible onset of magnetism in these nanosystems. Our
fully relativistic spin-polarized all-electron density functional calculations
reveal the onset of Hund's rule magnetism in nanowires of all three metals,
with mean-field moments of 1.1, 0.3, and 0.7 bohr magnetons, respectively, at
the equilibrium bond length. An analysis of the band structures indicates that
the nanocontact superparamagnetic state suggested by our calculations should
affect the ballistic conductance between tips made of Ru, Rh or Pd, leading to
possible temperature and magnetic field dependent conductance.Comment: 5 figures, 17 pages New content with respect to old arxiv versio
Substituting the main group element in cobalt - iron based Heusler alloys: CoFeAlSi
This work reports about electronic structure calculations for the Heusler
compound CoFeAlSi. Particular emphasis was put on the role of
the main group element in this compound. The substitution of Al by Si leads to
an increase of the number of valence electrons with increasing Si content and
may be seen as electron-doping. Self-consistent electronic structure
calculations were performed to investigate the consequences of the electron
doping for the magnetic properties. The series CoFeAlSi is
found to exhibit half-metallic ferromagnetism and the magnetic moment follows
the Slater-Pauling rule. It is shown that the electron-doping stabilises the
gap in the minority states for .Comment: J. Phys. D (accepted