285 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
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
Local electronic structure and magnetic properties of LaMn0.5Co0.5O3 studied by x-ray absorption and magnetic circular dichroism spectroscopy
We have studied the local electronic structure of LaMn0.5Co0.5O3 using
soft-x-ray absorption spectroscopy at the Co-L_3,2 and Mn-L_3,2 edges. We found
a high-spin Co^{2+}--Mn^{4+} valence state for samples with the optimal Curie
temperature. We discovered that samples with lower Curie temperatures contain
low-spin nonmagnetic Co^{3+} ions. Using soft-x-ray magnetic circular dichroism
we established that the Co^{2+} and Mn^{4+} ions are ferromagnetically aligned.
We revealed also that the Co^{2+} ions have a large orbital moment:
m_orb/m_spin ~ 0.47. Together with model calculations, this suggests the
presence of a large magnetocrystalline anisotropy in the material and predicts
a non-trivial temperature dependence for the magnetic susceptibility.Comment: 8 pages, 7 figure
Congenital disorder of glycosylation type Id (CDG Id): phenotypic, biochemical and molecular characterization of a new patient
Summary: Congenital disorders of glycosylation (CDG) are a family of multisystem inherited disorders caused by defects in the biosynthesis of N- or O-glycans. Among the many different subtypes of CDG, the defect of a mannosyltransferase encoded by the human ALG3 gene (chromosome 3q27) is known to cause CDG Id. Six patients with CDG Id have been described in the literature so far. We further delineate the clinical, biochemical, neuroradiological and molecular features of CDG Id by reporting an additional patient bearing a novel missense mutation in the ALG3 gene. All patients with CDG Id display a slowly progressive encephalopathy with microcephaly, severe psychomotor retardation and epileptic seizures. They also share some typical dysmorphic features but they do not present the multisystem involvement observed in other CDG syndromes or any biological marker abnormalities. Unusually marked osteopenia is a feature in some patients and may remain undiagnosed until revealed by pathological fractures. Serum transferrin screening for CDG should be extended to all patients with encephalopathy of unknown origin, even in the absence of multisystem involvemen
Wilson-Polchinski exact renormalization group equation for O(N) systems: Leading and next-to-leading orders in the derivative expansion
With a view to study the convergence properties of the derivative expansion
of the exact renormalization group (RG) equation, I explicitly study the
leading and next-to-leading orders of this expansion applied to the
Wilson-Polchinski equation in the case of the -vector model with the
symmetry . As a test, the critical exponents and as well as the subcritical exponent (and higher ones) are estimated
in three dimensions for values of ranging from 1 to 20. I compare the
results with the corresponding estimates obtained in preceding studies or
treatments of other exact RG equations at second order. The
possibility of varying allows to size up the derivative expansion method.
The values obtained from the resummation of high orders of perturbative field
theory are used as standards to illustrate the eventual convergence in each
case. A peculiar attention is drawn on the preservation (or not) of the
reparametrisation invariance.Comment: Dedicated to Lothar Sch\"afer on the occasion of his 60th birthday.
Final versio
Immune phenotypes and checkpoint molecule expression of clonally expanded lymph node-infiltrating T cells in classical Hodgkin lymphoma
Lymph node-infiltrating T cells have been of particular interest in classical Hodgkin lymphoma (cHL). High rates of complete therapeutic responses to antibody-mediated immune checkpoint blockade, even in relapsed/refractory patients, suggest the existence of a T cell-dominated, antigen-experienced, functionally inhibited and lymphoma-directed immune microenvironment. We asked whether clonally expanded T cells (1) were detectable in cHL lymph nodes, (2) showed characteristic immune phenotypes, and (3) were inhibited by immune checkpoint molecule expression. We applied high-dimensional FACS index sorting and single cell T cell receptor αβ sequencing to lymph node-infiltrating T cells from 10 treatment-naïve patients. T cells were predominantly CD4(+) and showed memory differentiation. Expression of classical immune checkpoint molecules (CTLA-4, PD-1, TIM-3) was generally low (< 12.0% of T cells) and not different between CD4(+) and CD8(+) T cells. Degrees of clonal T cell expansion varied between patients (range: 1-18 expanded clones per patient) and was almost exclusively restricted to CD8(+) T cells. Clonally expanded T cells showed non-naïve phenotypes and low checkpoint molecule expression similar to non-expanded T cells. Our data suggest that the therapeutic effects of immune checkpoint blockade require mechanisms in addition to dis-inhibition of pre-existing lymphoma-directed T cell responses. Future studies on immune checkpoint blockade-associated effects will identify molecular T cell targets, address dynamic aspects of cell compositions over time, and extend their focus beyond lymph node-infiltrating T cells
Threshold electronic structure at the oxygen K edge of 3d transition metal oxides: a configuration interaction approach
It has been generally accepted that the threshold structure observed in the
oxygen K edge X-ray absorption spectrum in 3d transition metal oxides
represents the electronic structure of the 3d transition metal. There is,
however, no consensus about the correct description. We present an
interpretation, which includes both ground state hybridization and electron
correlation. It is based on a configuration interaction cluster calculation
using a MO6 cluster. The oxygen K edge spectrum is calculated by annihilating a
ligand hole in the ground state and is compared to calculations representing
inverse photoemission experiments in which a 3d transition metal electron is
added. Clear differences are observed related to the amount of ligand hole
created in the ground state. Two "rules" connected to this are discussed.
Comparison with experimental data of some early transition metal compounds is
made and shows that this simple cluster approach explains the experimental
features quite well.Comment: 10 pages, submitted to Phys. Rev. B, tried to make a better PS file
X-ray emission from magnetized neutron star atmospheres at low mass accretion rates. I. Phase-averaged spectrum
Recent observations of X-ray pulsars at low luminosities allow, for the first
time, to compare theoretical models for the emission from highly magnetized
neutron star atmospheres at low mass accretion rates ( g s) with the broadband X-ray data. The purpose of this paper
is to investigate the spectral formation in the neutron star atmosphere at low
and to conduct a parameter study of physical properties of the
emitting region. We obtain the structure of the static atmosphere, assuming
that Coulomb collisions are the dominant deceleration process. The upper part
of the atmosphere is strongly heated by the braking plasma, reaching
temperatures of 30-40 keV, while its denser isothermal interior is much cooler
(~2 keV). We numerically solve the polarized radiative transfer in the
atmosphere with magnetic Compton scattering, free-free processes, and
non-thermal cyclotron emission due to possible collisional excitations of
electrons. The strongly polarized emitted spectrum has a double-hump shape that
is observed in low-luminosity X-ray pulsars. A low-energy "thermal" component
is dominated by extraordinary photons that can leave the atmosphere from deeper
layers due to their long mean free path at soft energies. We find that a
high-energy component is formed due to resonant Comptonization in the heated
non-isothermal part of the atmosphere even in the absence of collisional
excitations. The latter, however, affect the ratio of the two components. A
strong cyclotron line originates from the optically thin, uppermost zone. A fit
of the model to NuSTAR and Swift/XRT observations of GX 304-1 provides an
accurate description of the data with reasonable parameters. The model can thus
reproduce the characteristic double-hump spectrum observed in low-luminosity
X-ray pulsars and provides insights into spectral formation.Comment: 18 pages, 10 figures, A&A accepte
- …