3,759 research outputs found
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
Candidate Tidal Dwarf Galaxies in the Compact Group CG J1720-67.8
This is the second part of a detailed study of the ultracompact group CG
J1720-67.8: in the first part we have focused the attention on the three main
galaxies of the group and we have identified a number of candidate tidal dwarf
galaxies (TDGs). Here we concentrate on these candidate TDGs. Absolute
photometry of these objects in BVRJHKs bands confirms their relatively blue
colors, as we already expected from the inspection of optical and near-infrared
color maps and from the presence of emission-lines in the optical spectra. The
physical conditions in such candidate TDGs are investigated through the
application of photoionization models, while the optical colors are compared
with grids of spectrophotometric evolutionary synthesis models from the
literature. Although from our data self-gravitation cannot be proved for these
objects, their general properties are consistent with those of other TDG
candidates. Additionally we present the photometry of a few ``knots'' detected
in the immediate surroundings of CG J1720-67.8 and consider the possibility
that these objects might belong to a dwarf population associated with the
compact group.Comment: Accepted for publication in the Astrophysical Journa
A New Ultra-dense Group of Obscured Emission-Line Galaxies
We present the discovery of an isolated compact group of galaxies that is
extremely dense (median projected galaxy separation: 6.9 kpc), has a very low
velocity dispersion ( = 67 km s), and where all
observed members show emission lines and are morphologically disturbed. These
properties, together with the lack of spirals and the presence of a prominent
tidal tail make this group one of the most evolved compact groups.Comment: 15 pages,LaTeX, 2figures. A Postscript figure with spectra is
available at ftp://astro.uibk.ac.at/pub/weinberger/ . Accepted for
publication in ApJ Letter
Tuning independently Fermi energy and spin splitting in Rashba systems: Ternary surface alloys on Ag(111)
By detailed first-principles calculations we show that the Fermi energy and
the Rashba splitting in disordered ternary surface alloys (BiPbSb)/Ag(111) can
be independently tuned by choosing the concentrations of Bi and Pb. The
findings are explained by three fundamental mechanisms, namely the relaxation
of the adatoms, the strength of the atomic spin-orbit coupling, and band
filling. By mapping the Rashba characteristics,i.e.the splitting and the Rashba
energy, and the Fermi energy of the surface states in the complete range of
concentrations. Our results suggest to investigate experimentally effects which
rely on the Rashba spin-orbit coupling in dependence on spin-orbit splitting
and band filling.Comment: 11 pages, 3 figure
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
Role of C in MgC_xNi_3 investigated from first principles
The influence of vacancies in the sub-lattice of , on its
structural, electronic and magnetic properties are studied by means of the
density-functional based Korringa-Kohn-Rostoker Green's function method
formulated in the atomic sphere approximation. Disorder is taken into account
by means of coherent-potential approximation. Characterizations representing
the change in the lattice properties include the variation in the equilibrium
lattice constants, bulk modulus and pressure derivative of the bulk modulus,
and that of electronic structure include the changes in the, total, partial and
-resolved density of states. The incipient magnetic properties are
studied by means of fixed-spin moment method of alloy theory, together in
conjunction with the phenomenological Ginzburg-Landau equation for magnetic
phase transition. The first-principles calculations reveal that due to the
breaking of the - bonds, some of the 3d states, which were lowered
in energy due to strong hybridization, are transfered back to higher energies
thereby increasing the itinerant character in the material. The Bloch spectral
densities evaluated at the high symmetry points however reveal that the charge
redistribution is not uniform over the cubic Brillouin zone, as new states are
seen to be created at the point, while a shift in the states on the
energy scale are seen at other high symmetry points
Electrical Resistivity of a Thin Metallic Film
The electrical resistivity of a pure sample of a thin metallic film is found
to depend on the boundary conditions. This conclusion is supported by a
free-electron model calculation and confirmed by an ab initio relativistic
Korringa-Kohn-Rostoker computation. The low-temperature resistivity is found to
be zero for a free-standing film (reflecting boundary conditions) but nonzero
when the film is sandwiched between two semi-infinite samples of the same
material (outgoing boundary conditions). In the latter case, this resistivity
scales inversely with the number of monolayers and is due to the background
diffusive scattering by a finite lattice.Comment: 20 pages. To be published in Physical Review B, December 15, 199
Age-related alterations in default mode network: Impact on working memory performance
The default mode network (DMN) is a set of functionally connected brain regions which shows deactivation (task-induced deactivation, TID) during a cognitive task. Evidence shows an age-related decline in task-load-related modulation of the activity within the DMN during cognitive tasks. However, the effect of age on the functional coupling within the DMN and their relation to cognitive performance has hitherto been unexplored. Using functional magnetic resonance imaging, we investigated functional connectivity within the DMN in older and younger subjects during a working memory task with increasing task load. Older adults showed decreased connectivity and ability to suppress low frequency oscillations of the DMN. Additionally, the strength of the functional coupling of posterior cingulate (pCC) with medial prefrontal cortex (PFC) correlated positively with performance and was lower in older adults. pCC was also negatively coupled with task-related regions, namely the dorsolateral PFC and cingulate regions. Our results show that in addition to changes in canonical task-related brain regions, normal aging is also associated with alterations in the activity and connectivity of brain regions within the DMN. These changes may be a reflection of a deficit in cognitive control associated with advancing age that results in deficient resource allocation to the task at hand
Maximal and minimal spreading speeds for reaction diffusion equations in nonperiodic slowly varying media
This paper investigates the asymptotic behavior of the solutions of the
Fisher-KPP equation in a heterogeneous medium, associated with a compactly supported initial datum. A typical
nonlinearity we consider is , where
is a 1-periodic function and is a increasing function
that satisfies and . Although quite specific, the choice of such a reaction
term is motivated by its highly heterogeneous nature. We exhibit two different
behaviors for for large times, depending on the speed of the convergence of
at infinity. If grows sufficiently slowly, then we prove that the
spreading speed of oscillates between two distinct values. If grows
rapidly, then we compute explicitly a unique and well determined speed of
propagation , arising from the limiting problem of an infinite
period. We give a heuristic interpretation for these two behaviors
Influence of the carbon and the titanium sublayers on the eletrophysical preporties of the thin film system ALN/SI
The carbon (C) and titanium (Ti) ions were implanted onto the surface of the silicon (Si), and the thin films of aluminum nitride (AlN) were prepared on Si(100) substrate by magnetron sputtering after the implantations. The compositions of these thin film systems were studied by optical microscopy and Raman spectroscopy, and the surface conductivity of those samples were measured. It was indicated that the implanted C and Ti ions did have made a difference to the electro physical properties of the systems AlN/Si
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