28,708 research outputs found
Mn L edge resonant x-ray scattering in manganites: Influence of the magnetic state
We present an analysis of the dependence of the resonant orbital order and
magnetic scattering spectra on the spin configuration. We consider an arbitrary
spin direction with respect to the local crystal field axis, thus lowering
significantly the local symmetry. To evaluate the atomic scattering in this
case, we generalized the Hannon-Trammel formula and implemented it inside the
framework of atomic multiplet calculations in a crystal field. For an
illustration, we calculate the magnetic and orbital scattering in the CE phase
of \lsmo in the cases when the spins are aligned with the crystal lattice
vector (or equivalently ) and when they are rotated in the
-plane by 45 with respect to this axis. Magnetic spectra differ
for the two cases. For the orbital scattering, we show that for the former
configuration there is a non negligible ()
scattering component, which vanishes in the 45 case, while the () components are similar in the two cases. From the
consideration of two 90 spin canted structures, we conclude there is a
significant dependence of the orbital scattering spectra on the spin
arrangement. Recent experiments detected a sudden decrease of the orbital
scattering intensity upon increasing the temperature above the N\' eel
temperature in \lsmo. We discuss this behavior considering the effect of
different types of misorientations of the spins on the orbital scattering
spectrum.Comment: 8 figures. In the revised version, we added a note, a reference, and
a few minor changes in Figure 1 and the text. Accepted in Physical Review
Jahn-Teller effect versus Hund's rule coupling in C60N-
We propose variational states for the ground state and the low-energy
collective rotator excitations in negatively charged C60N- ions (N=1...5). The
approach includes the linear electron-phonon coupling and the Coulomb
interaction on the same level. The electron-phonon coupling is treated within
the effective mode approximation (EMA) which yields the linear t_{1u} x H_g
Jahn-Teller problem whereas the Coulomb interaction gives rise to Hund's rule
coupling for N=2,3,4. The Hamiltonian has accidental SO(3) symmetry which
allows an elegant formulation in terms of angular momenta. Trial states are
constructed from coherent states and using projection operators onto angular
momentum subspaces which results in good variational states for the complete
parameter range. The evaluation of the corresponding energies is to a large
extent analytical. We use the approach for a detailed analysis of the
competition between Jahn-Teller effect and Hund's rule coupling, which
determines the spin state for N=2,3,4. We calculate the low-spin/high-spin gap
for N=2,3,4 as a function of the Hund's rule coupling constant J. We find that
the experimentally measured gaps suggest a coupling constant in the range
J=60-80meV. Using a finite value for J, we recalculate the ground state
energies of the C60N- ions and find that the Jahn-Teller energy gain is partly
counterbalanced by the Hund's rule coupling. In particular, the ground state
energies for N=2,3,4 are almost equal
Time-varying Learning and Content Analytics via Sparse Factor Analysis
We propose SPARFA-Trace, a new machine learning-based framework for
time-varying learning and content analytics for education applications. We
develop a novel message passing-based, blind, approximate Kalman filter for
sparse factor analysis (SPARFA), that jointly (i) traces learner concept
knowledge over time, (ii) analyzes learner concept knowledge state transitions
(induced by interacting with learning resources, such as textbook sections,
lecture videos, etc, or the forgetting effect), and (iii) estimates the content
organization and intrinsic difficulty of the assessment questions. These
quantities are estimated solely from binary-valued (correct/incorrect) graded
learner response data and a summary of the specific actions each learner
performs (e.g., answering a question or studying a learning resource) at each
time instance. Experimental results on two online course datasets demonstrate
that SPARFA-Trace is capable of tracing each learner's concept knowledge
evolution over time, as well as analyzing the quality and content organization
of learning resources, the question-concept associations, and the question
intrinsic difficulties. Moreover, we show that SPARFA-Trace achieves comparable
or better performance in predicting unobserved learner responses than existing
collaborative filtering and knowledge tracing approaches for personalized
education
Research study of some RAM antennas Final report, 18 Nov. 1964 - 18 Jun. 1965
Input impedance and radiation pattern determinations for cylindrical gap, waveguide excited and circular waveguide slot antenna array
Microscopic expressions for the thermodynamic temperature
We show that arbitrary phase space vector fields can be used to generate
phase functions whose ensemble averages give the thermodynamic temperature. We
describe conditions for the validity of these functions in periodic boundary
systems and the Molecular Dynamics (MD) ensemble, and test them with a
short-ranged potential MD simulation.Comment: 21 pages, 2 figures, Revtex. Submitted to Phys. Rev.
The Stabilized Poincare-Heisenberg algebra: a Clifford algebra viewpoint
The stabilized Poincare-Heisenberg algebra (SPHA) is the Lie algebra of
quantum relativistic kinematics generated by fifteen generators. It is obtained
from imposing stability conditions after attempting to combine the Lie algebras
of quantum mechanics and relativity which by themselves are stable, however not
when combined. In this paper we show how the sixteen dimensional Clifford
algebra CL(1,3) can be used to generate the SPHA. The Clifford algebra path to
the SPHA avoids the traditional stability considerations, relying instead on
the fact that CL(1,3) is a semi-simple algebra and therefore stable. It is
therefore conceptually easier and more straightforward to work with a Clifford
algebra. The Clifford algebra path suggests the next evolutionary step toward a
theory of physics at the interface of GR and QM might be to depart from working
in space-time and instead to work in space-time-momentum.Comment: 14 page
Dissipation in nanocrystalline-diamond nanomechanical resonators
We have measured the dissipation and frequency of nanocrystalline-diamond nanomechanical resonators with resonant frequencies between 13.7 MHz and 157.3 MHz, over a temperature range of 1.4–274 K. Using both magnetomotive network analysis and a time-domain ring-down technique, we have found the dissipation in this material to have a temperature dependence roughly following T^(0.2), with Q^(–1) ≈ 10^(–4) at low temperatures. The frequency dependence of a large dissipation feature at ~35–55 K is consistent with thermal activation over a 0.02 eV barrier with an attempt frequency of 10 GHz
Four new planets around giant stars and the mass-metallicity correlation of planet-hosting stars
CONTEXT. Exoplanet searches have demonstrated that giant planets are
preferentially found around metal-rich stars and that their fraction increases
with the stellar mass. AIMS. During the past six years, we have conducted a
radial velocity follow-up program of 166 giant stars, to detect substellar
companions, and characterizing their orbital properties. Using this
information, we aim to study the role of the stellar evolution in the orbital
parameters of the companions, and to unveil possible correlations between the
stellar properties and the occurrence rate of giant planets. METHODS. Using
FEROS and CHIRON spectra, we have computed precision radial velocities and we
have derived atmospheric and physical parameters for all of our targets.
Additionally, velocities computed from UCLES spectra are presented here. By
studying the periodic radial velocity signals, we have detected the presence of
several substellar companions. RESULTS. We present four new planetary systems
around the giant stars HIP8541, HIP74890, HIP84056 and HIP95124. Additionally,
we find that giant planets are more frequent around metal-rich stars, reaching
a peak in the detection of = 16.7% around stars with
[Fe/H] 0.35 dex. Similarly, we observe a positive correlation of the
planet occurrence rate with the stellar mass, between M 1.0 -2.1
M, with a maximum of = 13.0%, at M = 2.1
M. CONCLUSIONS. We conclude that giant planets are preferentially
formed around metal-rich stars. Also, we conclude that they are more
efficiently formed around more massive stars, in the mass range of M
1.0 - 2.1 M. These observational results confirm previous
findings for solar-type and post-MS hosting stars, and provide further support
to the core-accretion formation model.Comment: Accepted for publication in A&
Measurement of the energy resolution and calibration of hybrid pixel detectors with GaAs:Cr sensor and Timepix readout chip
This paper describes an iterative method of per-pixel energy calibration of
hybrid pixel detectors with GaAs:Cr sensor and Timepix readout chip. A
convolution of precisely measured spectra of characteristic X-rays of different
metals with the resolution and the efficiency of the pixel detector is used for
the calibration. The energy resolution of the detector is also measured during
the calibration. The use of per-pixel calibration allows to achieve a good
energy resolution of the Timepix detector with GaAs:Cr sensor: 8% and 13% at 60
keV and 20 keV, respectively
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