202 research outputs found
Unified character of correlation effects in unconventional Pu-based superconductors and \delta-Pu
Electronic structure calculations combining the local-density approximation
with an exact diagonalization of the Anderson impurity model show an
intermediate 5f^5-5f^6-valence ground state and delocalization of the 5f^5
multiplet of the Pu atom 5f-shell in PuCoIn_5, PuCoGa_5, and \delta-Pu. The
5f-local magnetic moment is compensated by a moment formed in the surrounding
cloud of conduction electrons. For PuCoGa_5 and \delta-Pu the compensation is
complete and the Anderson impurity ground state is a singlet. For PuCoIn_5 the
compensation is partial and the Pu ground state is magnetic. We suggest that
the unconventional d-wave superconductivity is likely mediated by the 5f-states
antiferromagnetic fluctuations in PuCoIn_5, and by valence fluctuations in
PuCoGa_5.Comment: 5 pages, 3 figure
Lessons from the harmonic oscillator -- a reconciliation of the Frequency-Resolved Frozen Phonon Multislice Method with other theoretical approaches
We compare the Frequency-Resolved Frozen Phonon Multislice (FRFPMS) method,
introduced in Phys. Rev. Lett. 124, 025501 (2020), with other theoretical
approaches used to account for the inelastic scattering of high energy
electrons, namely the first-order Born approximation and the quantum excitation
of phonons model. We show, that these theories lead to similar expressions for
the single inelastically scattered intensity as a function of momentum transfer
for an anisotropic quantum harmonic oscillator in a weak phase object
approximation of the scattered waves, except for a too small smearing of the
scattering potential by the effective Debye-Waller factor (DWF) in the FRFPMS
method. We propose that this issue can be fixed by including an explicit DWF
smearing into the potential and demonstrate numerically, that in any realistic
situation, a FRFPMS approach revised in this way, correctly accounts for the
single inelastically scattered intensity and the correct elastic scattering
intensity. Furthermore our simulations illustrate that the only requirement for
such a revised FRFPMS method is the smallness of mean squared displacements for
all atomic species in all frequency bins. The analytical considerations for the
FRFPMS method also explain the -scaling of FRFPMS spectra observed
in Phys. Rev. B 104, 104301 (2021) by the use of classical statistics in the
molecular dynamics simulation. Moreover, we find that the FRFPMS method
inherently adds the contributions of phonon loss and gain within each frequency
bin. Both of these issues related to the frequency-scaling can be fixed by a
system-independent post-processing step
Electronic structure theory of the hidden order material URuSi
We report a comprehensive electronic structure investigation of the
paramagnetic (PM), the large moment antiferromagnetic (LMAF), and the hidden
order (HO) phases of URuSi. We have performed relativistic
full-potential calculations on the basis of the density functional theory
(DFT), employing different exchange-correlation functionals to treat electron
correlations within the open -shell of uranium. Specifically, we
investigate---through a comparison between calculated and low-temperature
experimental properties---whether the electrons are localized or
delocalized in URuSi. We also performed dynamical mean field theory
calculations (LDA+DMFT) to investigate the temperature evolution of the
quasi-particle states at 100~K and above, unveiling a progressive opening of a
quasi-particle gap at the chemical potential when temperature is reduced. A
detailed comparison of calculated properties with known experimental data
demonstrates that the LSDA and GGA approaches, in which the uranium
electrons are treated as itinerant, provide an excellent explanation of the
available low-temperature experimental data of the PM and LMAF phases. We show
furthermore that due to a materials-specific Fermi surface instability a large,
but partial, Fermi surface gapping of up to 750 K occurs upon antiferromagnetic
symmetry breaking. The occurrence of the HO phase is explained through
dynamical symmetry breaking induced by a mode of long-lived antiferromagnetic
spin-fluctuations. This dynamical symmetry breaking model explains why the
Fermi surface gapping in the HO phase is similar but smaller than that in the
LMAF phase and it also explains why the HO and LMAF phases have the same Fermi
surfaces yet different order parameters. Suitable derived order parameters for
the HO are proposed to be the Fermi surface gap or the dynamic spin-spin
correlation function.Comment: 23 pages, 20 figure
Spatial and temporal characteristics of gait as outcome measures in multiple sclerosis (EDSS 0 to 6.5)
Background: Gait impairment represents one of the most common and disabling symptom of multiple sclerosis. Quantification of the gait is an important aspect of clinical trials. In order to identify which temporal or spatial parameters of gait could be used as outcome measures in interventional studies of patients with different levels of disability, we evaluated characteristics of these parameters in MS patients across the whole spectrum of mobility from EDSS 0 to 6.5.
Methods: This is a cross-sectional study of spatial and temporal parameters of gait at self selected speed and at fast speed of walking in 284 patients with multiple sclerosis (108 men, mean age 38 years ± SD 10.8 years, range 18–64) divided into seven levels of disability (EDSS 0 to 1.5, EDSS 2.0 to 2.5, EDSS 3.0 to 3.5, EDSS 4.0 to 4.5, EDSS 5.0 to 5.5, EDSS 6.0, EDSS 6.5).
Results: The velocity of gait decreases with increasing EDSS levels. Hovewer, the spatio-temporal parameters of gait that are involved in this process differ across the EDSS levels. The step length is decreased at higher EDSS levels up to the EDSS 6.0, but was not different between EDSS 6.0 and 6.5. The step time is significantly longer at EDSS 6.0 and 6.5, while the step length remains the same at those levels. The increase in percentage of double support time becomes statistically significant at EDSS 3.0-3.5 and continues to increase until EDSS 6.5. Variability of step time, step length or step width did not show significant difference between studied EDSS levels.
Conclusions: There is no single spatio-temporal parameter of gait (other than velocity of gait) that would show significant differences among all levels of EDSS. The step length reflects shortening of steps at lower EDSS levels (2.0 to 6.0), and percentage of double support time better reflects changes at higher EDSS levels 3.0 – 6.5. Gait variability is not associated with disability in MS and therefore would not be a suitable outcome measure. T
Magnetic properties of (FeCo)B alloys and the effect of doping by 5 elements
We have explored, computationally and experimentally, the magnetic properties
of \fecob{} alloys. Calculations provide a good agreement with experiment in
terms of the saturation magnetization and the magnetocrystalline anisotropy
energy with some difficulty in describing CoB, for which it is found that
both full potential effects and electron correlations treated within dynamical
mean field theory are of importance for a correct description. The material
exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations
between and . A simple model for the temperature dependence of
magnetic anisotropy suggests that the complicated non-monotonous temperature
behaviour is mainly due to variations in the band structure as the exchange
splitting is reduced by temperature. Using density functional theory based
calculations we have explored the effect of substitutional doping the
transition metal sublattice by the whole range of 5 transition metals and
found that doping by Re or W elements should significantly enhance the
magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed
in enhancing the magnetic anisotropy due to formation of other phases. On the
other hand, doping by Ir and Re was successful and resulted in magnetic
anisotropies that are in agreement with theoretical predictions. In particular,
doping by 2.5~at.\% of Re on the Fe/Co site shows a magnetocrystalline
anisotropy energy which is increased by 50\% compared to its parent
(FeCo)B compound, making this system interesting, for
example, in the context of permanent magnet replacement materials or in other
areas where a large magnetic anisotropy is of importance.Comment: 15 pages 17 figure
Cancellation of probe effects in measurements of spin polarized momentum density by electron positron annihilation
Measurements of the two dimensional angular correlation of the
electron-positron annihilation radiation have been done in the past to detect
the momentum spin density and the Fermi surface. We point out that the momentum
spin density and the Fermi Surface of ferromagnetic metals can be revealed
within great detail owing to the large cancellation of the electron-positron
matrix elements which in paramagnetic multiatomic systems plague the
interpretation of the experiments. We prove our conjecture by calculating the
momentum spin density and the Fermi surface of the half metal CrO2, who has
received large attention due to its possible applications as spintronics
material
First principles theory of chiral dichroism in electron microscopy applied to 3d ferromagnets
Recently it was demonstrated (Schattschneider et al., Nature 441 (2006),
486), that an analogue of the X-ray magnetic circular dichroism (XMCD)
experiment can be performed with the transmission electron microscope (TEM).
The new phenomenon has been named energy-loss magnetic chiral dichroism (EMCD).
In this work we present a detailed ab initio study of the chiral dichroism in
the Fe, Co and Ni transition elements. We discuss the methods used for the
simulations together with the validity and accuracy of the treatment, which
can, in principle, apply to any given crystalline specimen. The dependence of
the dichroic signal on the sample thickness, accuracy of the detector position
and the size of convergence and collection angles is calculated.Comment: 9 pages, 6 figures, submitted to Physical Review
Retaining Expression on De-identified Faces
© Springer International Publishing AG 2017The extensive use of video surveillance along with advances in face recognition has ignited concerns about the privacy of the people identifiable in the recorded documents. A face de-identification algorithm, named k-Same, has been proposed by prior research and guarantees to thwart face recognition software. However, like many previous attempts in face de-identification, kSame fails to preserve the utility such as gender and expression of the original data. To overcome this, a new algorithm is proposed here to preserve data utility as well as protect privacy. In terms of utility preservation, this new algorithm is capable of preserving not only the category of the facial expression (e.g., happy or sad) but also the intensity of the expression. This new algorithm for face de-identification possesses a great potential especially with real-world images and videos as each facial expression in real life is a continuous motion consisting of images of the same expression with various degrees of intensity.Peer reviewe
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