845 research outputs found
Chiral properties of hematite ({\alpha}-Fe2O3) inferred from resonant Bragg diffraction using circularly polarized x-rays
Chiral properties of the two phases - collinear motif (below Morin transition
temperature, TM=250 K) and canted motif (above TM) - of magnetically ordered
hematite ({\alpha}-Fe2O3) have been identified in single crystal resonant x-ray
Bragg diffraction, using circular polarized incident x-rays tuned near the iron
K-edge. Magneto-electric multipoles, including an anapole, fully characterize
the high-temperature canted phase, whereas the low-temperature collinear phase
supports both parity-odd and parity-even multipoles that are time-odd. Orbital
angular momentum accompanies the collinear motif, while it is conspicuously
absent with the canted motif. Intensities have been successfully confronted
with analytic expressions derived from an atomic model fully compliant with
chemical and magnetic structures. Values of Fe atomic multipoles previously
derived from independent experimental data, are shown to be completely
trustworthy
Coherent Acoustic Perturbation of Second-Harmonic-Generation in NiO
We investigate the structural and magnetic origins of the unusual ultrafast
second-harmonicgeneration (SHG) response of femtosecond-laser-excited nickel
oxide (NiO) previously attributed to oscillatory reorientation dynamics of the
magnetic structure induced by d-d excitations. Using time-resolved x-ray
diffraction from the (3/2 3/2 3/2) magnetic planes, we show that changes in the
magnitude of the magnetic structure factor following ultrafast optical
excitation are limited to = 1.5% in the first 30 ps. An
extended investigation of the ultrafast SHG response reveals a strong
dependence on wavelength as well as characteristic echoes, both of which give
evidence for an acoustic origin of the dynamics. We therefore propose an
alternative mechanism for the SHG response based on perturbations of the
nonlinear susceptibility via optically induced strain in a spatially confined
medium. In this model, the two observed oscillation periods can be understood
as the times required for an acoustic strain wave to traverse one coherence
length of the SHG process in either the collinear or anti-collinear geometries.Comment: 26 pages, 7 figure
Magnetic and electronic Co states in layered cobaltate GdBaCo2O5.5-x
We have performed non-resonant x-ray diffraction, resonant soft and hard
x-ray magnetic diffraction, soft x-ray absorption and x-ray magnetic circular
dichroism measurements to clarify the electronic and magnetic states of the
Co3+ ions in GdBaCo2O5.5. Our data are consistent with a 3+ Py Co HS state at
the pyramidal sites and a 3+ Oc Co LS state at the octahedral sites. The
structural distortion, with a doubling of the a axis (2ap x 2ap x 2ap cell),
shows alternating elongations and contractions of the pyramids and indicates
that the metal-insulator transition is associated with orbital order in the t2g
orbitals of the 3+ Py Co HS state. This distortion corresponds to an
alternating ordering of xz and yz orbitals along the a and c axes for the 3+ Py
Co . The orbital ordering and pyramidal distortion lead to deformation of the
octahedra, but the 3+ Oc Co LS state does not allow an orbital order to occur
for the 3+ Oc Co ions. The soft x-ray magnetic diffraction results indicate
that the magnetic moments are aligned in the ab plane but are not parallel to
the crystallographic a or b axes. The orbital order and the doubling of the
magnetic unit cell along the c axis support a non-collinear magnetic structure.
The x-ray magnetic circular dichroism data indicate that there is a large
orbital magnetic contribution to the total ordered Co moment
Dynamical charge inhomogeneity and crystal-field fluctuations for 4f ions in high-Tc cuprates
The main relaxation mechanism of crystal-field excitations in rare-earth ions
in cuprates is believed to be provided by the fluctuations of crystalline
electric field induced by a dynamic charge inhomogeneity generic for the doped
cuprates. We address the generalized granular model as one of the model
scenario for such an ingomogeneity where the cuprate charge subsystem remind
that of Wigner crystal with the melting transition and phonon-like positional
excitation modes. Formal description of R-ion relaxation coincides with that of
recently suggested magnetoelastic mechanism.Comment: 4 page
Denying humanness to victims: How gang members justify violent behavior
The high prevalence of violent offending amongst gang-involved youth has been established in the literature. Yet the underlying psychological mechanisms that enable youth to engage in such acts of violence remain unclear. 189 young people were recruited from areas in London, UK, known for their gang activity. We found that gang members, in comparison to non-gang youth, described the groups they belong to as having recognized leaders, specific rules and codes, initiation rituals, and special clothing. Gang members were also more likely than non-gang youth to engage in violent behavior and endorse moral disengagement strategies (i.e., moral justification, euphemistic language, advantageous comparison, displacement of responsibility, attribution of blame, and dehumanization). Finally, we found that dehumanizing victims partially mediated the relationship between gang membership and violent behavior. These findings highlight the effects of groups at the individual level and an underlying psychological mechanism that explains, in part, how gang members engage in violence
Direct observation of charge order and orbital glass state in multiferroic LuFe2O4
Geometrical frustration of the Fe ions in LuFe2O4 leads to intricate charge
and magnetic order and a strong magnetoelectric coupling. Using resonant x-ray
diffraction at the Fe K edge, the anomalous scattering factors of both Fe sites
are deduced from the (h/3 k/3 l/2) reflections. The chemical shift between the
two types of Fe ions equals 4.0(1) eV corresponding to full charge separation
into Fe2+ and Fe3+. Polarization and azimuthal angle dependence of the
superlattice reflections demonstrates the absence of anisotropic scattering
revealing random orientations of the Fe2+ orbitals characteristic of an orbital
glass state.Comment: v2: refined analysis, added supplement, PRL, accepted for publicatio
Dark matter scenarios in the minimal SUSY B-L model
We perform a study of the dark matter candidates of a constrained version of
the minimal R-parity-conserving supersymmetric model with a gauged
. It turns out that there are four additional candidates for dark
matter in comparison to the MSSM: two kinds of neutralino, which either
correspond to the gaugino of the or to a fermionic bilepton, as
well as "right-handed" CP-even and -odd sneutrinos. The correct dark matter
relic density of the neutralinos can be obtained due to different mechanisms
including new co-annihilation regions and resonances. The large additional
Yukawa couplings required to break the radiatively often lead to
large annihilation cross sections for the sneutrinos. The correct treatment of
gauge kinetic mixing is crucial to the success of some scenarios. All
candidates are consistent with the exclusion limits of Xenon100.Comment: 45 pages, 22 figures; v2: extended discussion of direct detection
cross section, matches published versio
Direct observation of electron density reconstruction at the metal-insulator transition in NaOsO3
5d transition metal oxides offer new opportunities to test our understanding
of the interplay of correlation effects and spin-orbit interactions in
materials in the absence of a single dominant interaction. The subtle balance
between solid-state interactions can result in new mechanisms that minimize the
interaction energy, and in material properties of potential use for
applications. We focus here on the 5d transition metal oxide NaOsO3, a strong
candidate for the realization of a magnetically driven transition from a
metallic to an insulating state exploiting the so-called Slater mechanism.
Experimental results are derived from non-resonant and resonant x-ray single
crystal diffraction at the Os L-edges. A change in the crystallographic
symmetry does not accompany the metal-insulator transition in the Slater
mechanism and, indeed, we find no evidence of such a change in NaOsO3. An
equally important experimental observation is the emergence of the (300) Bragg
peak in the resonant condition with the onset of magnetic order. The intensity
of this space-group forbidden Bragg peak continuously increases with decreasing
temperature in line with the square of intensity observed for an allowed
magnetic Bragg peak. Our main experimental results, the absence of crystal
symmetry breaking and the emergence of a space-group forbidden Bragg peak with
developing magnetic order, support the use of the Slater mechanism to interpret
the metal-insulator transition in NaOsO3. We successfully describe our
experimental results with simulations of the electronic structure and, also,
with an atomic model based on the established symmetry of the crystal and
magnetic structure.Comment: 6 figure
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