560 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.
Molecular orbital theory: an introductory lecture note and reprint volume
These notes are based on lectures on molecular orbital theory that we have presented at the University of Copenhagen and Columbia University. They were designed primarily for advanced-undergraduate and first-year graduate students as an introduction to molecular orbital theory.
It is apparent that the molecular orbital theory is a very useful method of classifying the ground and excited states of small molecules. The transition metal complexes occupy a special place here, and the last chapter is devoted entirely to this subject. We believe that modern inorganic chemists should be acquainted with the methods of the theory, and that they will find approximate one-electron calculations as helpful as the organic chemists have found simple Hückel calculations. For this reason, we have included a calculation of the permanganate ion in Chapter 8. On the other hand, we have not considered conjugated pi systems because they are excellently discussed in a number of books.
Our intuitive approach in the use of symmetry methods is admittedly nonrigorous and therefore will be unsatisfactory to purists, but we believe this is the best way to introduce symmetry ideas to the majority of students. Once the student has learned how to use symmetry methods, it will be easier for him to appreciate more formal and rigorous treatments.
Several reprints of papers on molecular orbital theory are included in the back of the book. The papers treat a substantial number of the important molecular geometries. The reader should be able to follow the discussions after reading through the lecture notes.
We thank our colleagues in New York and Copenhagen for help with the manuscript. We gratefully acknowledge the help of Dr. Arlen Viste and Mr. Harold Basch in preparing Appendix 8-B. Finally, it is a pleasure to acknowledge the expert assistance of Mrs. Diane Celeste in preparing the final manuscript.
C. J. BALLHAUSEN, Kobenhavn
HARRY B. GRAY, New York
October 196
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
Adult age differences in prospective memory in the laboratory : Are they related to higher stress levels in the elderly?
Peer reviewedPublisher PD
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
A possible phase dependent absorption feature in the transient X-ray pulsar SAX J2103.5+4545
We present an X-ray spectral and timing analysis of two observations
of the transient Be X-ray binary SAX J2103.5+4545 during its April 2016
outburst, which was characterized by the highest flux since 's launch.
These observations provide detailed hard X-ray spectra of this source during
its bright precursor flare and subsequent fainter regular outburst for the
first time. In this work, we model the phase-averaged spectra for these
observations with a negative and positive power law with an exponential cut-off
(NPEX) model and compare the pulse profiles at different flux states. We found
that the broad-band pulse profile changes from a three peaked pulse in the
first observation to a two peaked pulse in the second observation, and that
each of the pulse peaks has some energy dependence. We also perform pulse-phase
spectroscopy and fit phase-resolved spectra with NPEX to evaluate how spectral
parameters change with pulse phase. We find that while the continuum parameters
are mostly constant with pulse phase, a weak absorption feature at ~12 keV that
might, with further study, be classified as a cyclotron line, does show strong
pulse phase dependence.Comment: 10 pages, 7 figures, accepted by ApJ, acknowledgements update
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
Cumulative life course adversity, mental health, and cognition in the UK biobank
Abstract
The association between adversity and cognition varies according to the specific adversity, when the adversity was experienced, and the cognitive domains investigated. Disentangling the effect of adversity and the underlying mechanistic pathway is therefore difficult. The association between adversity (i.e., maltreatment) accumulated over the life course and cognitive flexibility, as well as two potential mediators (i.e., intra-individual variability in reaction time and depression) of this association, were investigated. Data stem from the baseline population of the UK Biobank study ( N = 73,489, Mdn age = 56, SD age = 7.628, 55.740% of women). Cumulative life course adversity (specifically maltreatment) was measured with items based on the Childhood Trauma Questionnaire (CTS-5) and items adapted from the British Crime Survey. Depression was assessed with the Patient Health Questionnaire-9 (PHQ-9). Intra-individual variability in reaction time was measured with a reaction time test “snap game” and the Trail Making Test A and B were used as a measure of cognitive flexibility. A path analysis was performed on these data. Higher cumulative adverse experiences were associated with lower performance in cognitive flexibility (β = .016, p < .001, 95% CI [0.009, 0.024]), and this effect was partly mediated by the level of depression (22.727% of the total effect of cumulative life course adversity on cognitive flexibility was mediated by depression (β = .005, p < .001, 95% CI [0.004, 0.007])). No association between cumulative life course adverse experiences and intra-individual variability in reaction time was found, nor was any indirect association between cumulative life course adversity and performance in cognitive flexibility via intra-individual variability in reaction time. The association between cumulative life course adversity, depression, and performance in cognitive flexibility has been highlighted. In contrast, no indirect effect between cumulative life course adversity and performance in cognitive flexibility via intra-individual variability in reaction time was found, suggesting that it is not a potential mechanism underlying the association between cumulative life course adversity and executive function. </p
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