2,776 research outputs found
The implications of resonant x-ray scattering data on the physics of the insulating phase of V_2O_3
We have performed a quantitative analysis of recent resonant x-ray scattering
experiments carried out in the antiferromagnetic phase of V_2O_3 by means of
numerical ab-initio simulations. In order to treat magnetic effects, we have
developed a method based on multiple scattering theory (MST) and a relativistic
extension of the Schr\"{o}dinger Equation, thereby working with the usual non
relativistic set of quantum numbers for angular and spin momenta.
Electric dipole-dipole (E1-E1), dipole-quadrupole (E1-E2) and
quadrupole-quadrupole (E2-E2) transition were considered altogether. We obtain
satisfactory agreement with experiments, both in energy and azimuthal scans.
All the main features of the V K edge Bragg-forbidden reflections with
odd can be interpreted in terms of the antiferromagnetic ordering only,
{\it ie}, they are of magnetic origin. In particular the ab-initio simulation
of the energy scan around the (1,1,1)-monoclinic reflection excludes the
possibility of any symmetry reduction due to a time-reversal breaking induced
by orbital ordering.Comment: 11 pages, 6 figure
Spin-1 effective Hamiltonian with three degenerate orbitals: An application to the case of V_2O_3
Motivated by recent neutron and x-ray observations in V_2O_3, we derive the
effective Hamiltonian in the strong coupling limit of an Hubbard model with
three degenerate t_{2g} states containing two electrons coupled to spin S = 1,
and use it to re-examine the low-temperature ground-state properties of this
compound. An axial trigonal distortion of the cubic states is also taken into
account. Since there are no assumptions about the symmetry properties of the
hopping integrals involved, the resulting spin-orbital Hamiltonian can be
generally applied to any crystallographic configuration of the transition metal
ion giving rise to degenerate t_{2g} orbitals. Specializing to the case of
V_2O_3 we consider the antiferromagnetic insulating phase. We find two
variational regimes, depending on the relative size of the correlation energy
of the vertical pairs and the in-plane interaction energy. The former favors
the formation of stable molecules throughout the crystal, while the latter
tends to break this correlated state. We determine in both cases the minimizing
orbital solutions for various spin configurations, and draw the corresponding
phase diagrams. We find that none of the symmetry-breaking stable phases with
the real spin structure presents an orbital ordering compatible with the
magnetic space group indicated by very recent observations of non-reciprocal
x-ray gyrotropy in V_2O_3. We do however find a compatible solution with very
small excitation energy in two distinct regions of the phase space, which might
turn into the true ground state of V_2O_3 due to the favorable coupling with
the lattice. We illustrate merits and drawbacks of the various solutions and
discuss them in relation to the present experimental evidence.Comment: 36 pages, 19 figure
Variational study of the antiferromagnetic insulating phase of V2O3 based on Nth order Muffin-Tin-Orbitals
Motivated by recent results of th order muffin-tin orbital (NMTO)
implementation of the density functional theory (DFT), we re-examine
low-temperature ground-state properties of the anti-ferromagnetic insulating
phase of vanadium sesquioxide VO. The hopping matrix elements obtained
by the NMTO-downfolding procedure differ significantly from those previously
obtained in electronic structure calculations and imply that the in-plane
hopping integrals are as important as the out-of-plane ones. We use the NMTO
hopping matrix elements as input and perform a variational study of the ground
state. We show that the formation of stable molecules throughout the crystal is
not favorable in this case and that the experimentally observed magnetic
structure can still be obtained in the atomic variational regime. However the
resulting ground state (two electrons occupying the degenerate
doublet) is in contrast with many well established experimental observations.
We discuss the implications of this finding in the light of the non-local
electronic correlations certainly present in this compound.Comment: 7 pages, 2 figure
Structural dichroism in the antiferromagnetic insulating phase of V_2O_3
We performed near-edge x-ray absorption spectroscopy (XANES) at V K edge in
the antiferromagnetic insulating (AFI) phase of a 2.8% Cr-doped V_2O_3 single
crystal. Linear dichroism of several percent is measured in the hexagonal plane
and found to be in good agreement with ab-initio calculations based on multiple
scattering theory. This experiment definitively proves the structural origin of
the signal and therefore solves a controversy raised by previous
interpretations of the same dichroism as non-reciprocal. It also calls for a
further investigation of the role of the magnetoelectric annealing procedure in
cooling to the AFI phase.Comment: 4 pages 3 figures. To be published in Phys. Rev. B (2005
Local tetragonal distortion in La_{0.7}Sr_{0.3}MnO_3 strained thin films probed by x-ray absorption spectroscopy
We report on an angular resolved X-ray Absorption Spectroscopy study of
thin films epitaxially grown by pulsed laser
deposition on slightly mismatched substrates which induce tensile or
compressive strains. XANES spectra give evidence of tetragonal distortion
within the octahedra, with opposite directions for tensile and
compressive strains. Quantitative analysis has been done and a model of
tetragonal distortion reflecting the strain has been established. EXAFS data
collected in plane for tensile substrate confirm the change in the
average bond distance and the increase of length matching with the
enlargement of the cell parameter. From these results we conclude that there is
no significant change in the angle. Our observations conflict with
the scenarios which this angle is the main driving parameter in the sensitivity
of manganite films properties to external strains and suggest that the
distortion within the octahedra plays a key role in the modification of the
transport and magnetic properties.Comment: 8 pages, 6 figure
The Planck mission: From first results to cosmology
Planck is a ESA satellite, currently in operation, whose main objective is to accurately image the anisotropies of the Cosmic Microwave Background Radiation in intensity and polarization. Benefiting from an unprecedented combination
of sensitivity, angular resolution, and frequency leverage, Planck will provide high quality data to be mined in cosmology and astrophysics. The first Planck results have been released in January 2011 and include both Galactic and extragalactic source catalogues, a list of galaxy clusters selected by the Sunyaev-Zel’dovich effect, and a cold cores catalogue. The first cosmological data products are awaited for early 2013. Planck has a wide list of scientific targets. Here we focus on one specific aspect which is also of interest to the high energy physics community: constraining the Parity and CPT symmetries through CMB datasets. We describe the basic formalism, the relevant estimators and the overall analysis strategy. We provide
marginal evidence for large scale Parity anomaly in the WMAP data that may be soon confirmed or discarded by the Planck satellite. Planck is currently measuring CMB anisotropies and their polarization with a level of precision that will remain unparalleled for many years to come. We also show how the CMB can be used to constrain fundamental symmetry violations in the photon sector through the so-called cosmological birefringence phenomenon
Correlated Component Analysis for diffuse component separation with error estimation on simulated Planck polarization data
We present a data analysis pipeline for CMB polarization experiments, running
from multi-frequency maps to the power spectra. We focus mainly on component
separation and, for the first time, we work out the covariance matrix
accounting for errors associated to the separation itself. This allows us to
propagate such errors and evaluate their contributions to the uncertainties on
the final products.The pipeline is optimized for intermediate and small scales,
but could be easily extended to lower multipoles. We exploit realistic
simulations of the sky, tailored for the Planck mission. The component
separation is achieved by exploiting the Correlated Component Analysis in the
harmonic domain, that we demonstrate to be superior to the real-space
application (Bonaldi et al. 2006). We present two techniques to estimate the
uncertainties on the spectral parameters of the separated components. The
component separation errors are then propagated by means of Monte Carlo
simulations to obtain the corresponding contributions to uncertainties on the
component maps and on the CMB power spectra. For the Planck polarization case
they are found to be subdominant compared to noise.Comment: 17 pages, accepted in MNRA
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