477 research outputs found
Magnetic excitations in vanadium spinels
We study magnetic excitations in vanadium spinel oxides AVO (A=Zn,
Mg, Cd) using two models: first one is a superexchange model for vanadium S=1
spins, second one includes in addition spin-orbit coupling, and crystal
anisotropy. We show that the experimentally observed magnetic ordering can be
obtained in both models, however the orbital ordering is different with and
without spin-orbit coupling and crystal anisotropy. We demonstrate that this
difference strongly affects the spin-wave excitation spectrum above the
magnetically ordered state, and argue that the neutron measurement of such
dispersion is a way to distinguish between the two possible orbital orderings
in AVO.Comment: accepted in Phys. Rev.
Non-resonant inelastic x-ray scattering involving excitonic excitations
In a recent publication Larson \textit{et al.} reported remarkably clear
- excitations for NiO and CoO measured with x-ray energies well below the
transition metal edge. In this letter we demonstrate that we can obtain an
accurate quantitative description based on a local many body approach. We find
that the magnitude of can be tuned for maximum sensitivity for
dipole, quadrupole, etc. excitations. We also find that the direction of
with respect to the crystal axes can be used as an equivalent to
polarization similar to electron energy loss spectroscopy, allowing for a
determination of the local symmetry of the initial and final state based on
selection rules. This method is more generally applicable and combined with the
high resolution available, could be a powerful tool for the study of local
distortions and symmetries in transition metal compounds including also buried
interfaces
Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system III. Dipping in the low/hard state
We present an analysis of three Chandra High Energy Transmission Gratings
observations of the black hole binary Cyg X-1/HDE 226868 at different orbital
phases. The stellar wind that is powering the accretion in this system is
characterized by temperature and density inhomogeneities including structures,
or "clumps", of colder, more dense material embedded in the photoionized gas.
As these clumps pass our line of sight, absorption dips appear in the light
curve. We characterize the properties of the clumps through spectral changes
during various dip stages. Comparing the silicon and sulfur absorption line
regions (1.6-2.7 keV 7.7-4.6 {\AA}) in four levels of varying column
depth reveals the presence of lower ionization stages, i.e., colder or denser
material, in the deeper dip phases. The Doppler velocities of the lines are
roughly consistent within each observation, varying with the respective orbital
phase. This is consistent with the picture of a structure that consists of
differently ionized material, in which shells of material facing the black hole
shield the inner and back shells from the ionizing radiation. The variation of
the Doppler velocities compared to a toy model of the stellar wind, however,
does not allow us to pin down an exact location of the clump region in the
system. This result, as well as the asymmetric shape of the observed lines,
point at a picture of a complex wind structure.Comment: 19 pages, 15 figures, accepted for publication in A&
Local electronic structure of Fe impurities in MgO thin films: Temperature-dependent soft x-ray absorption spectroscopy study
We report on the local electronic structure of Fe impurities in MgO thin
films. Using soft x-ray absorption spectroscopy (XAS) we verified that the Fe
impurities are all in the 2+ valence state. The fine details in the line shape
of the Fe edges provide direct evidence for the presence of a
dynamical Jahn-Teller distortion. We are able to determine the magnitude of the
effective crystal field energies. We also observed a strong
temperature dependence in the spectra which we can attribute to the thermal
population of low-lying excited states that are present due to the spin-orbit
coupling in the Fe 3d. Using this Fe impurity system as an example, we
show that an accurate measurement of the orbital moment in FeO will
provide a direct estimate for the effective local low-symmetry crystal fields
on the Fe sites, important for the theoretical modeling of the formation
of orbital ordering
Multiplet ligand-field theory using Wannier orbitals
We demonstrate how ab initio cluster calculations including the full Coulomb
vertex can be done in the basis of the localized, generalized Wannier orbitals
which describe the low-energy density functional (LDA) band structure of the
infinite crystal, e.g. the transition metal 3d and oxygen 2p orbitals. The
spatial extend of our 3d Wannier orbitals (orthonormalized Nth order muffin-tin
orbitals) is close to that found for atomic Hartree-Fock orbitals. We define
Ligand orbitals as those linear combinations of the O 2p Wannier orbitals which
couple to the 3d orbitals for the chosen cluster. The use of ligand orbitals
allows for a minimal Hilbert space in multiplet ligand-field theory
calculations, thus reducing the computational costs substantially. The result
is a fast and simple ab initio theory, which can provide useful information
about local properties of correlated insulators. We compare results for NiO,
MnO and SrTiO3 with x-ray absorption, inelastic x-ray scattering, and
photoemission experiments. The multiplet ligand field theory parameters found
by our ab initio method agree within ~10% to known experimental values
Crystal-field splitting for low symmetry systems in ab initio calculations
In the framework of the LDA+U approximation we propose the direct way of
calculation of crystal-field excitation energy and apply it to La and Y
titanates. The method developed can be useful for comparison with the results
of spectroscopic measurements because it takes into account fast relaxations of
electronic system. For titanates these relaxation processes reduce the value of
crystal-field splitting by as compared with the difference of LDA one
electron energies. However, the crystal-field excitation energy in these
systems is still large enough to make an orbital liquid formation rather
unlikely and experimentally observed isotropic magnetism remains unexplained.Comment: 13 pages, 5 figures, 3 table
Cyclotron resonant scattering feature simulations. I. Thermally averaged cyclotron scattering cross sections, mean free photon-path tables, and electron momentum sampling
Electron cyclotron resonant scattering features (CRSFs) are observed as
absorption-like lines in the spectra of X-ray pulsars. A significant fraction
of the computing time for Monte Carlo simulations of these quantum mechanical
features is spent on the calculation of the mean free path for each individual
photon before scattering, since it involves a complex numerical integration
over the scattering cross section and the (thermal) velocity distribution of
the scattering electrons.
We aim to numerically calculate interpolation tables which can be used in
CRSF simulations to sample the mean free path of the scattering photon and the
momentum of the scattering electron. The tables also contain all the
information required for sampling the scattering electron's final spin.
The tables were calculated using an adaptive Simpson integration scheme. The
energy and angle grids were refined until a prescribed accuracy is reached. The
tables are used by our simulation code to produce artificial CRSF spectra. The
electron momenta sampled during these simulations were analyzed and justified
using theoretically determined boundaries.
We present a complete set of tables suited for mean free path calculations of
Monte Carlo simulations of the cyclotron scattering process for conditions
expected in typical X-ray pulsar accretion columns (0.01<B/B_{crit}<=0.12,
where B_{crit}=4.413x10^{13} G and 3keV<=kT<15keV). The sampling of the tables
is chosen such that the results have an estimated relative error of at most
1/15 for all points in the grid. The tables are available online at
http://www.sternwarte.uni-erlangen.de/research/cyclo.Comment: A&A, in pres
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