4,144 research outputs found
Evaluation of the optical conductivity tensor in terms of contour integrations
For the case of finite life-time broadening the standard Kubo-formula for the
optical conductivity tensor is rederived in terms of Green's functions by using
contour integrations, whereby finite temperatures are accounted for by using
the Fermi-Dirac distribution function. For zero life-time broadening, the
present formalism is related to expressions well-known in the literature.
Numerical aspects of how to calculate the corresponding contour integrals are
also outlined.Comment: 8 pages, Latex + 2 figure (Encapsulated Postscript
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
Electrical transport properties of bulk NiFe alloys and related spin-valve systems
Within the Kubo-Greenwood formalism we use the fully relativistic,
spin-polarized, screened Korringa-Kohn-Rostoker method together with the
coherent-potential approximation for layered systems to calculate the
resistivity for the permalloy series NiFe. We are able to
reproduce the variation of the resistivity across the entire series; notably
the discontinuous behavior in the vicinity of the structural phase transition
from bcc to fcc. The absolute values for the resistivity are within a factor of
two of the experimental data. Also the giant magnetoresistance of a series of
permalloy-based spin-valve structures is estimated; we are able to reproduce
the trends and values observed on prototypical spin-valve structures.Comment: 6 pages, ReVTeX + 4 figures (Encapsulated Postscript), submitted to
PR
Non-collinear magnetic structures: a possible cause for current induced switching
Current induced switching in Co/Cu/Co trilayers is described in terms of
ab-initio determined magnetic twisting energies and corresponding sheet
resistances. In viewing the twisting energy as an energy flux the
characteristic time thereof is evaluated by means of the
Landau-Lifshitz-Gilbert equation using ab-initio parameters. The obtained
switching times are in very good agreement with available experimental data. In
terms of the calculated currents, scalar quantities since a classical Ohm's law
is applied, critical currents needed to switch magnetic configurations from
parallel to antiparallel and vice versa can unambiguously be defined. It is
found that the magnetoresistance viewed as a function of the current is
essentially determined by the twisting energy as a function of the relative
angle between the orientations of the magnetization in the magnetic slabs,
which in turn can also explain in particular cases the fact that after having
switched off the current the system remains in the switched magnetic
configuration. For all ab-initio type calculations the fully relativistic
Screened Korringa-Kohn-Rostoker method and the corresponding Kubo-Greenwood
equation in the context of density functional theory are applied.Comment: 20 pages, 4 tables and 15 figures, submitted to PR
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
Numerically improved computational scheme for the optical conductivity tensor in layered systems
The contour integration technique applied to calculate the optical
conductivity tensor at finite temperatures in the case of layered systems
within the framework of the spin-polarized relativistic screened
Korringa-Kohn-Rostoker band structure method is improved from the computational
point of view by applying the Gauss-Konrod quadrature for the integrals along
the different parts of the contour and by designing a cumulative special points
scheme for two-dimensional Brillouin zone integrals corresponding to cubic
systems.Comment: 17 pages, LaTeX + 4 figures (Encapsulated PostScript), submitted to
J. Phys.: Condensed Matter (19 Sept. 2000
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
Hubble Space Telescope Images of Magellanic Cloud Planetary Nebulae: Data and Correlations across Morphological Classes
The morphology of planetary nebulae (PNe) provides an essential tool for
understanding their origin and evolution, as it reflects both the dynamics of
the gas ejected during the TP-AGB phase, and the central star energetics. Here
we study the morphology of 27 Magellanic Cloud planetary nebulae (MCPNe) and
present an analysis of their physical characteristics across morphological
classes. Similar studies have been successfully carried out for galactic PNe,
but were compromised by the uncertainty of individual PN distances. We present
our own HST/FOC images of 15 Magellanic Cloud PNe (MCPNe) acquired through a
narrow-band lambda 5007 [O III] filter. We use the Richardson-Lucy
deconvolution technique on these pre-COSTAR images to achieve post-COSTAR
quality. Three PNe imaged before and after COSTAR confirm the high reliability
of our deconvolution procedure. We derive morphological classes, dimensions,
and surface photometry for all these PNe. We have combined this sample with
HST/PC1 images of 15 MCPNe, three of which are in common with the FOC set,
acquired by Dopita et al. (1996), to obtain the largest MCPN sample ever
examined from the morphological viewpoint. By using the whole database,
supplemented with published data from the literature, we have analyzed the
properties of the MCPNe and compared them to a typical, complete galactic
sample. Morphology of the MCPNe is then correlated with PN density, chemistry,
and evolution.Comment: text file lstanghe_mcpn.tex (LaTex); Figures 2 through 10, Figure 5
is in 3 parts (a,b,c); Figure 1 available by regular mail only; ApJ, in
press, November 10, 199
Postharvest loss in the supply chain for vegetables – The case of chili and tomato in Viet Nam
AVRDC Publication 06-685Christian Genova II, Katinka Weinberger, Hoang Bang An, Dang Dinh Dam, Nguyen Thi Tan Loc, Le Nhu Thinh, Nguyen Thi Thanh Thu
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