5,796 research outputs found
Characterization of polyacrylonitrile ultrafiltration membranes
Various methods have been used to characterize ultrafiltration membranes, such as gas flux measurements, (field emission) scanning electron microscopy, permporometry and liquid-liquid displacement. Significant differences in the pore size distributions determined from permporometry and liquid-liquid displacement were found
Symmetric Versus Nonsymmetric Structure of the Phosphorus Vacancy on InP(110)
The atomic and electronic structure of positively charged P vacancies on
InP(110) surfaces is determined by combining scanning tunneling microscopy,
photoelectron spectroscopy, and density-functional theory calculations. The
vacancy exhibits a nonsymmetric rebonded atomic configuration with a charge
transfer level 0.75+-0.1 eV above the valence band maximum. The scanning
tunneling microscopy (STM) images show only a time average of two degenerate
geometries, due to a thermal flip motion between the mirror configurations.
This leads to an apparently symmetric STM image, although the ground state
atomic structure is nonsymmetric.Comment: 5 pages including 3 figures. related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Electronic, magnetic and transport properties of Fe intercalated 2H-TaS studied by means of the KKR-CPA method
The electronic, magnetic and transport properties of Fe intercalated
2H-TaS have been investigated by means of the Korringa-Kohn-Rostoker (KKR)
method. The non-stoichiometry and disorder in the system has been accounted for
using the Coherent Potential Approximation (CPA) alloy theory. A pronounced
influence of disorder on the spin magnetic moment has been found for the
ferro-magnetically ordered material. The same applies for the spin-orbit
induced orbital magnetic moment and magneto-crystalline anisotropy energy. The
temperature-dependence of the resistivity of disordered 2H-FeTaS
investigated on the basis of the Kubo-St\v{r}eda formalism in combination with
the alloy analogy model has been found in very satisfying agreement with
experimental data. This also holds for the temperature dependent anomalous Hall
resistivity . The role of thermally induced lattice
vibrations and spin fluctuations for the transport properties is discussed in
detail
Spin-orbit induced longitudinal spin-polarized currents in non-magnetic solids
For certain non-magnetic solids with low symmetry the occurrence of
spin-polarized longitudinal currents is predicted. These arise due to an
interplay of spin-orbit interaction and the particular crystal symmetry. This
result is derived using a group-theoretical scheme that allows investigating
the symmetry properties of any linear response tensor relevant to the field of
spintronics. For the spin conductivity tensor it is shown that only the
magnetic Laue group has to be considered in this context. Within the introduced
general scheme also the spin Hall- and additional related transverse effects
emerge without making reference to the two-current model. Numerical studies
confirm these findings and demonstrate for (AuPt)Sc that
the longitudinal spin conductivity may be in the same order of magnitude as the
conventional transverse one. The presented formalism only relies on the
magnetic space group and therefore is universally applicable to any type of
magnetic order.Comment: 5 pages, 1 table, 2 figures (3 & 2 subfigures
Effect of chemical disorder on NiMnSb investigated by Appearance Potential Spectroscopy: a theoretical study
The half-Heusler alloy NiMnSb is one of the local-moment ferromagnets with
unique properties for future applications. Band structure calculations predict
exclusively majority bands at the Fermi level, thus indicating {100%} spin
polarization there. As one thinks about applications and the design of
functional materials, the influence of chemical disorder in these materials
must be considered. The magnetization, spin polarization, and electronic
structure are expected to be sensitive to structural and stoichiometric
changes. In this contribution, we report on an investigation of the
spin-dependent electronic structure of NiMnSb. We studied the influence of
chemical disorder on the unoccupied electronic density of states by use of the
ab-initio Coherent Potential Approximation method. The theoretical analysis is
discussed along with corresponding spin-resolved Appearance Potential
Spectroscopy measurements. Our theoretical approach describes the spectra as
the fully-relativistic self-convolution of the matrix-element weighted,
orbitally resolved density of states.Comment: JPD submitte
Bethe-Salpeter Equation Calculations of Core Excitation Spectra
We present a hybrid approach for GW/Bethe-Salpeter Equation (BSE)
calculations of core excitation spectra, including x-ray absorption (XAS),
electron energy loss spectra (EELS), and non-resonant inelastic x-ray
scattering (NRIXS). The method is based on {\it ab initio} wavefunctions from
the plane-wave pseudopotential code ABINIT; atomic core-level states and
projector augmented wave (PAW) transition matrix elements; the NIST core-level
BSE solver; and a many-pole GW self-energy model to account for final-state
broadening and self-energy shifts. Multiplet effects are also accounted for.
The approach is implemented using an interface dubbed OCEAN (Obtaining Core
Excitations using ABINIT and NBSE). To demonstrate the utility of the code we
present results for the K-edges in LiF as probed by XAS and NRIXS, the K-edges
of KCl as probed by XAS, the Ti L_2,3-edge in SrTiO_3 as probed by XAS, and the
Mg L_2,3-edge in MgO as probed by XAS. We compare the results to experiments
and results obtained using other theoretical approaches
Spontaneous Branching of Anode-Directed Streamers between Planar Electrodes
Non-ionized media subject to strong fields can become locally ionized by
penetration of finger-shaped streamers. We study negative streamers between
planar electrodes in a simple deterministic continuum approximation. We observe
that for sufficiently large fields, the streamer tip can split. This happens
close to Firsov's limit of `ideal conductivity'. Qualitatively the tip
splitting is due to a Laplacian instability quite like in viscous fingering.
For future quantitative analytical progress, our stability analysis of planar
fronts identifies the screening length as a regularization mechanism.Comment: 4 pages, 6 figures, submitted to PRL on Nov. 16, 2001, revised
version of March 10, 200
Chalcogen Height Dependence of Magnetism and Fermiology in FeTe_xSe_{1-x}
FeTexSe1-x (x=0, 0.25, 0.50, 0.75 and 1) system has been studied using
density functional theory. Our results show that for FeSe, LDA seems better
approximation in terms of magnitude of magnetic energy whereas GGA
overestimates it largely. On the other hand for FeTe, GGA is better
approximation that gives experimentally observed magnetic state. It has been
shown that the height of chalcogen atoms above Fe layers has significant effect
on band structure, electronic density of states (DOS) at Fermi level N(EF) and
Fermi surfaces. For FeSe the value of N(EF) is small so as to satisfy Stoner
criteria for ferromagnetism, (I\timesN(EF)\geq1) whereas for FeTe, since the
value of N(EF) is large, the same is close to be satisfied. Force minimization
done for FeTexSe1-x using supercell approach shows that in disordered system Se
and Te do not share same site and have two distinct z coordinates. This has
small effect on magnetic energy but no significant difference in band structure
and DOS near EF when calculated using either relaxed or average value of z for
chalcogen atoms. Thus substitution of Se at Te site decreases average value of
chalcogen height above Fe layers which in turn affect the magnetism and
Fermiology in the system. By using coherent-potential approximation for
disordered system we found that height of chalcogen atoms above Fe layer rather
than chalcogen species or disorder in the anion planes, affect magnetism and
shape of Fermi surfaces (FS), thus significantly altering nesting conditions,
which govern antiferromagnetic spin fluctuations in the system.Comment: 24 pages Text+Figs: comments/suggestions welcome
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