230 research outputs found
Broken-Bond Rule for the Surface Energies of Noble Metals
Using two different full-potential ab-initio techniques we introduce a
simple, universal rule based on the number of broken first-neighbor bonds to
determine the surface energies of the three noble metals Cu, Ag and Au. When a
bond is broken, the rearrangement of the electronic charge for these metals
does not lead to a change of the remaining bonds. Thus the energy needed to
break a bond is independent of the surface orientation. This novel finding can
lead to the development of simple models to describe the energetics of a
surface like step and kink formation, crystal growth, alloy formation,
equilibrium shape of mesoscopic crystallites and surface faceting.Comment: 4 pages, 2 figure
Orbital magnetism in the half-metallic Heusler alloys
Using the fully-relativistic screened Korringa-Kohn-Rostoker method I study
the orbital magnetism in the half-metallic Heusler alloys. Orbital moments are
almost completely quenched and they are negligible with respect to the spin
moments. The change in the atomic-resolved orbital moments can be easily
explained in terms of the spin-orbit strength and hybridization effects.
Finally I discuss the orbital and spin moments derived from X-ray magnetic
circular dichroism experiments
Interface properties of the NiMnSb/InP and NiMnSb/GaAs contacts
We study the electronic and magnetic properties of the interfaces between the
half-metallic Heusler alloy NiMnSb and the binary semiconductors InP and GaAs
using two different state-of-the-art full-potential \textit{ab-initio}
electronic structure methods. Although in the case of most NiMnSb/InP(001)
contacts the half-metallicity is lost, it is possible to keep a high degree of
spin-polarization when the interface is made up by Ni and P layers. In the case
of the GaAs semiconductor the larger hybridization between the Ni- and
As- orbitals with respect to the hybridization between the Ni- and P-
orbitals destroys this polarization. The (111) interfaces present strong
interface states but also in this case there are few interfaces presenting a
high spin-polarization at the Fermi level which can reach values up to 74%.Comment: 9 pages, 9 figure
Structural and magnetic properties of the (001) and (111) surfaces of the half-metal NiMnSb
Using the full potential linearised augmented planewave method we study the
electronic and magnetic properties of the (001) and (111) surfaces of the
half-metallic Heusler alloy NiMnSb from first-principles. We take into account
all possible surface terminations including relaxations of these surfaces.
Special attention is paid to the spin-polarization at the Fermi level which
governs the spin-injection from such a metal into a semiconductor. In general,
these surfaces lose the half-metallic character of the bulk NiMnSb, but for the
(111) surfaces this loss is more pronounced. Although structural optimization
does not change these features qualitatively, specifically for the (111)
surfaces relaxations can compensate much of the spin-polarization at the Fermi
surface that has been lost upon formation of the surface.Comment: 18 pages, 8 figure
The effect of the spin-orbit interaction on the band gap of half-metals
The spin-orbit interaction can cause a nonvanishing density of states (DOS)
within the minority-spin band gap of half-metals around the Fermi level. We
examine the magnitude of the effect in Heusler alloys, zinc-blende half metals
and diluted magnetic semiconductors, using first-principles calculations. We
find that the ratio of spin-down to spin-up DOS at the Fermi level can range
from below 1% (e.g. 0.5% for NiMnSb) over several percents (4.2% for (Ga,Mn)As)
to 13% for MnBi.Comment: 5 pages, 3 figure
Towards New Half-Metallic Systems: Zinc-Blende Compounds of Transition Elements with N, P, As, Sb, S, Se, and Te
We report systematic first-principles calculations for ordered zinc-blende
compounds of the transition metal elements V, Cr, Mn with the sp elements N, P,
As, Sb, S, Se, Te, motivated by recent fabrication of zinc-blende CrAs, CrSb,
and MnAs. They show ferromagnetic half-metallic behavior for a wide range of
lattice constants. We discuss the origin and trends of half-metallicity,
present the calculated equilibrium lattice constants, and examine the
half-metallic behavior of their transition element terminated (001) surfaces.Comment: 2nd Version: lattice constants calculations added, text revise
Appearance of Half-Metallicity in the Quaternary Heusler Alloys
I report systematic first-principle calculations of the quaternary Heusler
alloys like Co[CrMn]Al, CoMn[AlSn] and
[FeCo]MnAl. I show that when the two limiting cases (x=0 or 1)
correspond to a half-metallic compound, so do the intermediate cases. Moreover
the total spin moment in scales linearly with the total number of
valence electrons (and thus with the concentration ) following the
relation , independently of the origin of the extra valence
electrons, confirming the Slater-Pauling behavior of the normal Heusler alloys.
Finally I discuss in all cases the trends in the atomic projected DOSs and in
the atomic spin moments.Comment: 4 pages, 3 figures, 2 Table
Exchange interactions and temperature dependence of the magnetization in half--metallic Heusler alloys
We study the exchange interactions in half-metallic Heusler alloys using
first-principles calculations in conjunction with the frozen-magnon
approximation. The Curie temperature is estimated within both mean-field (MF)
and random-phase-approximation (RPA) approaches. For the half-Heusler alloys
NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In
this case the MF and RPA estimations differ strongly. The RPA approach provides
better agreement with experiment. The exchange interactions are more complex in
the case of full-Heusler alloys CoMnSi and CoCrAl where the dominant
effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms
and between Co atoms at different sublattices. For these compounds we find that
both MF and RPA give very close values of the Curie temperature slightly
underestimating experimental quantities. We study the influence of the lattice
compression on the magnetic properties. The temperature dependence of the
magnetization is calculated using the RPA method within both quantum mechanical
and classical approaches.Comment: New figures and discussio
Relaxation and reconstruction on (111) surfaces of Au, Pt, and Cu
We have theoretically studied the stability and reconstruction of (111)
surfaces of Au, Pt, and Cu. We have calculated the surface energy, surface
stress, interatomic force constants, and other relevant quantities by ab initio
electronic structure calculations using the density functional theory (DFT), in
a slab geometry with periodic boundary conditions. We have estimated the
stability towards a quasi-one-dimensional reconstruction by using the
calculated quantities as parameters in a one-dimensional Frenkel-Kontorova
model. On all surfaces we have found an intrinsic tensile stress. This stress
is large enough on Au and Pt surfaces to lead to a reconstruction in which a
denser surface layer is formed, in agreement with experiment. The
experimentally observed differences between the dense reconstruction pattern on
Au(111) and a sparse structure of stripes on Pt(111) are attributed to the
details of the interaction potential between the first layer of atoms and the
substrate.Comment: 8 pages, 3 figures, submitted to Physical Review
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