1,520 research outputs found
Effects of strain on the electronic structure of VO_2
We present cluster-DMFT (CTQMC) calculations based on a downfolded
tight-binding model in order to study the electronic structure of vanadium
dioxide (VO_2) both in the low-temperature (M_1) and high-temperature (rutile)
phases. Motivated by the recent efforts directed towards tuning the physical
properties of VO_2 by depositing films on different supporting surfaces of
different orientations we performed calculations for different geometries for
both phases. In order to investigate the effects of the different growing
geometries we applied both contraction and expansion for the lattice parameter
along the rutile c-axis in the 3-dimensional translationally invariant systems
miming the real situation. Our main focus is to identify the mechanisms
governing the formation of the gap characterizing the M_1 phase and its
dependence on strain. We found that the increase of the band-width with
compression along the axis corresponding to the rutile c-axis is more important
than the Peierls bonding-antibonding splitting
Spin-polarized surface states close to adatoms on Cu(111)
We present a theoretical study of surface states close to 3d transition metal
adatoms (Cr, Mn, Fe, Co, Ni and Cu) on a Cu(111) surface in terms of an
embedding technique using the fully relativistic Korringa-Kohn-Rostoker method.
For each of the adatoms we found resonances in the s-like states to be
attributed to a localization of the surface states in the presence of an
impurity. We studied the change of the s-like densities of states in the
vicinity of the surface state band-edge due to scattering effects mediated via
the adatom's d-orbitals. The obtained results show that a magnetic impurity
causes spin-polarization of the surface states. In particular, the long-range
oscillations of the spin-polarized s-like density of states around an Fe adatom
are demonstrated.Comment: 5 pages, 5 figures, submitted to PR
Theory of anisotropic Rashba splitting of surface states
We investigate the surface Rashba effect for a surface of reduced in-plane
symmetry. Formulating a k.p perturbation theory, we show that the Rashba
splitting is anisotropic, in agreement with symmetry-based considerations. We
show that the anisotropic Rashba splitting is due to the admixture of bulk
states of different symmetry to the surface state, and it cannot be explained
within the standard theoretical picture supposing just a normal-to-surface
variation of the crystal potential. Performing relativistic ab initio
calculations we find a remarkably large Rashba anisotropy for an
unreconstructed Au(110) surface that is in the experimentally accessible range.Comment: 4 pages, 5 figure
Giant magnetic anisotropy of the bulk antiferromagnets IrMn and IrMn3
Theoretical predictions of the magnetic anisotropy of antiferromagnetic
materials are demanding due to a lack of experimental techniques which are
capable of a direct measurement of this quantity. At the same time it is highly
significant due to the use of antiferromagnetic components in magneto-resistive
sensor devices where the stability of the antiferromagnet is of upmost
relevance. We perform an ab-initio study of the ordered phases of IrMn and
IrMn3, the most widely used industrial antiferromagnets. Calculating the form
and the strength of the magnetic anisotropy allows the construction of an
effective spin model, which is tested against experimental measurements
regarding the magnetic ground state and the Neel temperature. Our most
important result is the extremely strong second order anisotropy for IrMn3
appearing in its frustrated triangular magnetic ground state, a surprising fact
since the ordered L12 phase has a cubic symmetry. We explain this large
anisotropy by the fact that cubic symmetry is locally broken for each of the
three Mn sub-lattices.Comment: 4 pages, 4 figure
Ab initio study of canted magnetism of finite atomic chains at surfaces
By using ab initio methods on different levels we study the magnetic ground
state of (finite) atomic wires deposited on metallic surfaces. A
phenomenological model based on symmetry arguments suggests that the
magnetization of a ferromagnetic wire is aligned either normal to the wire and,
generally, tilted with respect to the surface normal or parallel to the wire.
From a first principles point of view, this simple model can be best related
to the so--called magnetic force theorem calculations being often used to
explore magnetic anisotropy energies of bulk and surface systems. The second
theoretical approach we use to search for the canted magnetic ground state is
first principles adiabatic spin dynamics extended to the case of fully
relativistic electron scattering. First, for the case of two adjacent Fe atoms
an a Cu(111) surface we demonstrate that the reduction of the surface symmetry
can indeed lead to canted magnetism. The anisotropy constants and consequently
the ground state magnetization direction are very sensitive to the position of
the dimer with respect to the surface. We also performed calculations for a
seven--atom Co chain placed along a step edge of a Pt(111) surface. As far as
the ground state spin orientation is concerned we obtain excellent agreement
with experiment. Moreover, the magnetic ground state turns out to be slightly
noncollinear.Comment: 8 pages, 5 figures; presented on the International Conference on
Nanospintronics Design and Realizations, Kyoto, Japan, May 2004; to appear in
J. Phys.: Cond. Matte
Magnetism in systems with various dimensionality: A comparison between Fe and Co
A systematic ab initio study is performed for the spin and orbital moments
and for the validity of the sum rules for x-ray magnetic circular dichroism for
Fe systems with various dimensionality (bulk, Pt-supported monolayers and
monatomic wires, free-standing monolayers and monatomic wires). Qualitatively,
the results are similar to those for the respective Co systems, with the main
difference that for the monatomic Fe wires the term in the spin sum rule
is much larger than for the Co wires. The spin and orbital moments induced in
the Pt substrate are also discussed.Comment: 4 page
Magnetic properties of Quantum Corrals from first principles calculations
We present calculations for electronic and magnetic properties of surface
states confined by a circular quantum corral built of magnetic adatoms (Fe) on
a Cu(111) surface. We show the oscillations of charge and magnetization
densities within the corral and the possibility of the appearance of
spin--polarized states. In order to classify the peaks in the calculated
density of states with orbital quantum numbers we analyzed the problem in terms
of a simple quantum mechanical circular well model. This model is also used to
estimate the behaviour of the magnetization and energy with respect to the
radius of the circular corral. The calculations are performed fully
relativistically using the embedding technique within the
Korringa-Kohn-Rostoker method.Comment: 14 pages, 9 figures, submitted to J. Phys. Cond. Matt. special issue
on 'Theory and Simulation of Nanostructures
Thermally activated magnetization reversal in monoatomic magnetic chains on surfaces studied by classical atomistic spin-dynamics simulations
We analyze the spontaneous magnetization reversal of supported monoatomic
chains of finite length due to thermal fluctuations via atomistic spin-dynamics
simulations. Our approach is based on the integration of the Landau-Lifshitz
equation of motion of a classical spin Hamiltonian at the presence of
stochastic forces. The associated magnetization lifetime is found to obey an
Arrhenius law with an activation barrier equal to the domain wall energy in the
chain. For chains longer than one domain-wall width, the reversal is initiated
by nucleation of a reversed magnetization domain primarily at the chain edge
followed by a subsequent propagation of the domain wall to the other edge in a
random-walk fashion. This results in a linear dependence of the lifetime on the
chain length, if the magnetization correlation length is not exceeded. We
studied chains of uniaxial and tri-axial anisotropy and found that a tri-axial
anisotropy leads to a reduction of the magnetization lifetime due to a higher
reversal attempt rate, even though the activation barrier is not changed.Comment: 2nd version contains some improvements and new Appendi
Abstracts of papers presented at an international workshop on management of soilborne pathogens sponsored by the United States-Israel binational agricultural research and development fund (bard): March 1-5,1998 Ramat Rachel, Jerusalem, Israel
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