434 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.
Evidence for a temperature-induced spin-state transition of Co3+ in La2-xSrxCoO4
We study the magnetic susceptibility of mixed-valent La2-xSrxCoO4 single
crystals in the doping range of 0.5<= x <= 0.8 for temperatures up to 1000 K.
The magnetism below room temperature is described by paramagnetic Co2+ in the
high-spin state and by Co3+ in the non-magnetic low-spin state. Above room
temperature, an increase in susceptibility compared to the behavior expected
from Co2+ is seen, which we attribute to a spin-state transition of Co3+. The
susceptibility is analyzed by comparison to full-multiplet calculations for the
thermal population of the high- and intermediate-spin states of Co3+
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
Magnetic properties of 3d-impurities substituted in GaAs
We have calculated the magnetic properties of substituted 3d-impurities
(Cr-Ni) in a GaAs host by means of first principles electronic structure
calculations. We provide a novel model explaining the ferromagnetic long rang
order of III-V dilute magnetic semiconductors. The origin of the ferromagnetism
is shown to be due to delocalized spin-uncompensated As dangling bond
electrons. Besides the quantitative prediction of the magnetic moments, our
model provides an understanding of the halfmetallicity, and the raise of the
critical temperature with the impurity concentration
Critical Phenomena in Continuous Dimension
We present a calculation of critical phenomena directly in continuous
dimension d employing an exact renormalization group equation for the effective
average action. For an Ising-type scalar field theory we calculate the critical
exponents nu(d) and eta(d) both from a lowest--order and a complete
first--order derivative expansion of the effective average action. In
particular, this can be used to study critical behavior as a function of
dimensionality at fixed temperature.Comment: 5 pages, 1 figure, PLB version, references adde
Polarization dependence of x-ray absorption spectra in Na_xCoO_2
In order to shed light on the electronic structure of Na_xCoO_2, and
motivated by recent Co L-edge X-ray absorption spectra (XAS) experiments with
polarized light, we calculate the electronic spectrum of a CoO_6 cluster
including all interactions between 3d orbitals. We obtain the ground state for
two electronic occupations in the cluster that correspond nominally to all O in
the O^{-2} oxidation state, and Co^{+3} or Co^{+4}. Then, all excited states
obtained by promotion of a Co 2p electron to a 3d electron, and the
corresponding matrix elements are calculated. A fit of the observed
experimental spectra is good and points out a large Co-O covalency and cubic
crystal field effects, that result in low spin Co 3d configurations. Our
results indicate that the effective hopping between different Co atoms plays a
major role in determining the symmetry of the ground state in the lattice.
Remaining quantitative discrepancies with the XAS experiments are expected to
come from composition effects of itineracy in the ground and excited states.Comment: 10 pages, 4 figure
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
On the Optical Spectra of Some Transition Metal Pentacyanonitrosyl Complexes
The visible absorption spectra of the ions [Fe(CN)_(5)NO]^(2-), [Mn(CN)_(5)NO]^(3—), and [Cr(CN)_(5)NO]^(3—) are interpreted using a crystal field level scheme of C_(4v) symmetry. Assuming a strong axial compression to be present a self‐consistent assignment is obtained for both the 3d^5 and 3d^6 electronic configurations
Little boxes: A simple implementation of the Greenberger, Horne, and Zeilinger result for spatial degrees of freedom
A Greenberger, Horne, and Zeilinger-type construction is realized in the position properties of three particles whose wavefunctions are distributed over three two-chambered boxes. The same system is modeled more realistically using three spatially separated, singly ionized hydrogen molecules. © 2011 American Association of Physics Teachers
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