2,024 research outputs found
Structurally-driven magnetic state transition of biatomic Fe chains on Ir(001)
Using first-principles calculations, we demonstrate that the magnetic
exchange interaction and the magnetocrystalline anisotropy of biatomic Fe
chains grown in the trenches of the 5x1 reconstructed Ir(001) surface depend
sensitively on the atomic arrangement of the Fe atoms. Two structural
configurations have been considered which are suggested from recent
experiments. They differ by the local symmetry and the spacing between the two
strands of the biatomic Fe chain. Since both configurations are very close in
total energy they may coexist in experiment. We have investigated collinear
ferro- and antiferromagnetic solutions as well as a collinear state with two
moments in one direction and one in the opposite direction (up-down-up-state).
For the structure with a small interchain spacing, there is a strong exchange
interaction between the strands and the ferromagnetic state is energetically
favorable. In the structure with larger spacing, the two strands are
magnetically nearly decoupled and exhibit antiferromagnetic order along the
chain. In both cases, due to hybridization with the Ir substrate the exchange
interaction along the chain axis is relatively small compared to freestanding
biatomic iron chains. The easy magnetization axis of the Fe chains also
switches with the structural configuration and is out-of-plane for the
ferromagnetic chains with small spacing and along the chain axis for the
antiferromagnetic chains with large spacing between the two strands. Calculated
scanning tunneling microscopy images and spectra suggest the possibility to
experimentally distinguish between the two structural and magnetic
configurations.Comment: Accepted for publication in Physical Review
Surface state scattering by adatoms on noble metals
When surface state electrons scatter at perturbations, such as magnetic or
nonmagnetic adatoms or clusters on surfaces, an electronic resonance, localized
at the adatom site, can develop below the bottom of the surface state band for
both spin channels. In the case of adatoms, these states have been found very
recently in scanning tunneling spectroscopy experiments\cite{limot,olsson} for
the Cu(111) and Ag(111) surfaces. Motivated by these experiments, we carried
out a systematic theoretical investigation of the electronic structure of these
surface states in the presence of magnetic and non-magnetic atoms on Cu(111).
We found that Ca and all 3 adatoms lead to a split-off state at the bottom
of the surface band which is, however, not seen for the elements Ga and
Ge. The situation is completely reversed if the impurities are embedded in the
surface: Ga and Ge are able to produce a split-off state whereas the 3
impurities do not. The resonance arises from the s-state of the impurities and
is explained in terms of strength and interaction nature (attraction or
repulsion) of the perturbing potential.Comment: 6 pages, 5 figure
Quantum interference in deformed carbon nanotube waveguides
Quantum interference (QI) in two types of deformed carbon nanotubes (CNTs),
i.e., axially stretched and AFM tip-deformed CNTs, has been investigated by the
pi-electron only and four-orbital tight-binding (TB) method. It is found that
the rapid conductance oscillation (RCO) period is very sensitive to the applied
strains, and decreases in an inverse proportion to the deformation degree,
which could be used as a powerful experimental tool to detect precisely the
deformation degree of the deformed CNTs. Also, the sigma-pi coupling effect is
found to be negligible under axially stretched strain, while it works on the
transport properties of the tip-deformed CNTs.Comment: 14 pages and 5 figure
Non-collinear Korringa-Kohn-Rostoker Green function method: Application to 3d nanostructures on Ni(001)
Magnetic nanostructures on non-magnetic or magnetic substrates have attracted
strong attention due to the development of new experimental methods with atomic
resolution. Motivated by this progress we have extended the full-potential
Korringa-Kohn-Rostoker (KKR) Green function method to treat non-collinear
magnetic nanostructures on surfaces. We focus on magnetic 3d impurity
nanoclusters, sitting as adatoms on or in the first surface layer on Ni(001),
and investigate the size and orientation of the local moments and moreover the
stabilization of non-collinear magnetic solutions. While clusters of Fe, Co, Ni
atoms are magnetically collinear, non-collinear magnetic coupling is expected
for Cr and Mn clusters on surfaces of elemental ferromagnets. The origin of
frustration is the competition of the antiferromagnetic exchange coupling among
the Cr or Mn atoms with the antiferromagnetic (for Cr) or ferromagnetic (for
Mn) exchange coupling between the impurities and the substrate. We find that Cr
and Mn first-neighbouring dimers and a Mn trimer on Ni(001) show non-collinear
behavior nearly degenerate with the most stable collinear configuration.
Increasing the distance between the dimer atoms leads to a collinear behavior,
similar to the one of the single impurities. Finally, we compare some of the
non-collinear {\it ab-initio} results to those obtained within a classical
Heisenberg model, where the exchange constants are fitted to total energies of
the collinear states; the agreement is surprisingly good.Comment: 11 page
Ferromagnetism in Nitrogen-doped MgO
The magnetic state of Nitrogen-doped MgO, with N substituting O at
concentrations between 1% and the concentrated limit, is calculated with
density-functional methods. The N atoms are found to be magnetic with a moment
of 1 Bohr magneton per Nitrogen atom and to interact ferromagnetically via the
double exchange mechanism. The long-range magnetic order is established above a
finite concentration of about 1.5% when the percolation threshold is reached.
The Curie temperature increases linearly with the concentration, and is found
to be about 30 K for 10% concentration. Besides the substitution of single
Nitrogen atoms, also interstitial Nitrogen atoms, clusters of Nitrogen atoms
and their structural relaxation on the magnetism are discussed. Possible
scenarios of engineering a higher Curie temperature are analyzed, with the
conclusion that an increase of the Curie temperature is difficult to achieve,
requiring a particular attention to the choice of chemistry
Hysteretic resistance spikes in quantum Hall ferromagnets without domains
We use spin-density-functional theory to study recently reported hysteretic
magnetoresistance \rho_{xx} spikes in Mn-based 2D electron gases
[Jaroszy\'{n}ski et al. Phys. Rev. Lett. (2002)]. We find hysteresis loops in
our calculated Landau fan diagrams and total energies signaling
quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation
effects are crucial to stabilize the relevant magnetic phases arising from
distinct symmetry-broken excited- and ground-state solutions of the Kohn-Sham
equations. Besides hysteretic spikes in \rho_{xx}, we predict hysteretic dips
in the Hall resistance \rho_{xy}. Our theory, without domain walls,
satisfactorily explains the recent data.Comment: 4 pages, 4 figures, published version (some changes to the text; same
figures as in v1
Cd-vacancy and Cd-interstitial complexes in Si and Ge
The electrical field gradient (EFG), measured e.g. in perturbed angular
correlation (PAC) experiments, gives particularly useful information about the
interaction of probe atoms like 111In / 111Cd with other defects. The
interpretation of the EFG is, however, a difficult task. This paper aims at
understanding the interaction of Cd impurities with vacancies and interstitials
in Si and Ge, which represents a controversial issue. We apply two
complementary ab initio methods in the framework of density functional theory
(DFT), (i) the all electron Korringa-Kohn-Rostoker (KKR) Greenfunction method
and (ii) the Pseudopotential-Plane-Wave (PPW) method, to search for the correct
local geometry. Surprisingly we find that both in Si and Ge the substitutional
Cd-vacancy complex is unstable and relaxes to a split-vacancy complex with the
Cd on the bond-center site. This complex has a very small EFG, allowing a
unique assignment of the small measured EFGs of 54MHz in Ge and 28MHz in Si.
Also, for the Cd-selfinterstitial complex we obtain a highly symmetrical split
configuration with large EFGs, being in reasonable agreement with experiments
Intrinsic spin Hall effect in platinum metal
Spin Hall effect in metallic Pt is studied with first-principles relativistic
band calculations. It is found that intrinsic spin Hall conductivity (SHC) is
as large as at low temperature, and
decreases down to at room
temperature. It is due to the resonant contribution from the spin-orbit
splitting of the doubly degenerated -bands at high-symmetry and
points near the Fermi level. By modeling these near degeneracies by effective
Hamiltonian, we show that SHC has a peak near the Fermi energy and that the
vertex correction due to impurity scattering vanishes. We therefore argue that
the large spin Hall effect observed experimentally in platinum is of intrinsic
nature.Comment: Accepted for publication in Phys. Rev. Let
Analytical infrared intensities for periodic systems with local basis sets
We report a method for the efficient evaluation of analytic infrared (IR)
intensities within generalized Kohn-Sham density functional theory using
Gaussian orbitals and periodic boundary conditions. A discretized form of the
Berry phase is used to evaluate a periodic dipole moment and its derivatives
with respect to in-phase nuclear coordinate displacements. Benchmark
calculations are presented for one-dimensional chains of water molecules and
poly(paraphenylenevinylene).Comment: 16 pages, 3 figures, 4 tables, submitted to Phys. Rev.
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
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