4,220 research outputs found
Radiative rates and electron impact excitation rates for transitions in He II
We report calculations of energy levels, radiative rates, collision
strengths, and effective collision strengths for transitions among the lowest
25 levels of the n <= 5 configurations of He~II. The general-purpose
relativistic atomic structure package (GRASP) and Dirac atomic R-matrix code
(DARC) are adopted for the calculations. Radiative rates, oscillator strengths,
and line strengths are reported for all electric dipole (E1), magnetic dipole
(M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among
the 25 levels. Furthermore, collision strengths and effective collision
strengths are listed for all 300 transitions among the above 25 levels over a
wide energy (temperature) range up to 9 Ryd (10**5.4 K). Comparisons are made
with earlier available results and the accuracy of the data is assessed.Comment: 30 pages of text including 12 figures and 5 Tables will appear in
ATOMS 5 (2017
Spin Waves in Quantum Antiferromagnets
Using a self-consistent mean-field theory for the Heisenberg
antiferromagnet Kr\"uger and Schuck recently derived an analytic expression for
the dispersion. It is exact in one dimension () and agrees well with
numerical results in . With an expansion in powers of the inverse
coordination number () we investigate if this expression can be
{\em exact} for all . The projection method of Mori-Zwanzig is used for the
{\em dynamical} spin susceptibility. We find that the expression of Kr\"uger
and Schuck deviates in order from our rigorous result. Our method is
generalised to arbitrary spin and to models with easy-axis anisotropy \D.
It can be systematically improved to higher orders in . We clarify its
relation to the expansion.Comment: 8 pages, uuencoded compressed PS-file, accepted as Euro. Phys. Lette
Resonant x-ray scattering spectra from multipole orderings: Np M_{4,5} edges in NpO2
We study resonant x-ray scattering (RXS) at Np M_{4,5} edges in the
triple-\textbf{k} multipole ordering phase in NpO_{2}, on the basis of a
localized electron model. We derive an expression for RXS amplitudes to
characterize the spectra under the assumption that a rotational invariance is
preserved in the intermediate state of scattering process. This assumption is
justified by the fact that energies of the crystal electric field and the
intersite interaction is smaller than the energy of multiplet structures. This
expression is found useful to calculate energy profiles with taking account of
the intra-Coulomb and spin-orbit interactions. Assuming the \Gamma_{8}-quartet
ground state, we construct the triple-\textbf{k} ground state, and analyze the
RXS spectra. The energy profiles are calculated in good agreement with the
experiment, providing a sound basis to previous phenomenological analyses.Comment: 10 pages, 7 figure
Spin Excitations and Sum Rules in the Heisenberg Antiferromagnet
Various bounds for the energy of collective excitations in the Heisenberg
antiferromagnet are presented and discussed using the formalism of sum rules.
We show that the Feynman approximation significantly overestimates (by about
30\% in the square lattice) the spin velocity due to the non
negligible contribution of multi magnons to the energy weighted sum rule. We
also discuss a different, Goldstone type bound depending explicitly on the
order parameter (staggered magnetization). This bound is shown to be
proportional to the dispersion of classical spin wave theory with a
q-independent normalization factor. Rigorous bounds for the excitation energies
in the anisotropic Heisenberg model are also presented.Comment: 26 pages, Plain TeX including 1 PostScript figure, UTF-307-10/9
Climatological studies of the quasi 16-day oscillations in the mesosphere and lower thermosphere at Yamagawa (31.2° N, 130.6° E), Japan
International audienceThe 16-day wave climatology at Yamagawa (31.2° N, 130.6° E) is investigated by analyzing the mesosphere and lower thermosphere (MLT) wind data collected with the recently installed MF radar. We have utilized the continuous data gathered during the five-year period 1996?2000. The wave climatology clearly shows some seasonal variations. The period of late autumn-spring is marked with larger wave activity, with the strongest waves being observed in the winter months. The maximum amplitude observed at Yamagawa is about 20 m/s, which is comparatively larger than the amplitudes observed at mid-latitude stations. The height dependence of the 16-day wave suggests that the maximum amplitude is observed at altitudes below 80 km. The summer months are characterized with much weaker wave activity. The vertical wavelength appears to be larger in the winter months and shorter in the summer months. The present analysis again confirms that the 16-day wave is highly sensitive to the background mean winds. Eastward motion of the background winds is a more favourable condition for the 16-day wave penetration to the MLT heights. The wave features show some signs of interannual variability. Overall, the observed features of the 16-day wave at Yamagawa, which is located at the edge of the subtropical latitudes, show some correspondence with the results reported for mid-latitude stations
Interaction Effect in the Kondo Energy of the Periodic Anderson-Hubbard Model
We extend the periodic Anderson model by switching on a Hubbard for the
conduction electrons. The nearly integral valent (Kondo) limit of the
Anderson--Hubbard model is studied with the Gutzwiller variational method. The
new formula for the Kondo energy contains the -dependent chemical
potential of the Hubbard subsystem in the exponent, and the correlation-induced
band narrowing in the prefactor. Both effects tend to suppress the Kondo scale,
which can be understood to result from the blocking of hybridization (this
behaviour is the opposite of that found for Kondo--Hubbard models). At
half-filling, we find a Brinkman--Rice-type transition which leads from a
small-gap Kondo insulator to a Mott insulator.Comment: 4 pages (ReVTeX), submitted for publicatio
Spectral properties of the t-J model in the presence of hole-phonon interaction
We examine the effects of electron-phonon interaction on the dynamics of the
charge carriers doped in two-dimensional (2D) Heisenberg antiferromagnet. The
- model Hamiltonian with a Fr\"ohlich term which couples the holes to a
dispersionless (optical) phonon mode is considered for low doping
concentration. The evolution of the spectral density function, the density of
states, and the momentum distribution function of the holes with an increase of
the hole-phonon coupling constant is studied numerically. As the coupling
to a phonon mode increases the quasiparticle spectral weight decreases and a
``phonon satellite'' feature close to the quasi-particle peak becomes more
pronounced. Furthermore, strong electron-phonon coupling smears the
multi-magnon resonances (``string states'') in the incoherent part of the
spectral function. The jump in the momentum distribution function at the Fermi
surface is reduced without changing the hole pocket volume, thereby providing a
numerical verification of Luttinger theorem for this strongly interacting
system. The vertex corrections due to electron- phonon interaction are
negligible in spite of the fact that the ratio of the phonon frequency to the
effective bandwidth is not small.Comment: REVTeX, 20 pages, 9 figures, to be published in Phys. Rev. B (Nov. 1,
1996
X-Ray Magnetic Circular Dichroism at the K edge of Mn3GaC
We theoretically investigate the origin of the x-ray magnetic circular
dichroism (XMCD) spectra at the K edges of Mn and Ga in the ferromagnetic phase
of Mn3GaC on the basis of an ab initio calculation. Taking account of the
spin-orbit interaction in the LDA scheme, we obtain the XMCD spectra in
excellent agreement with the recent experiment. We have analyzed the origin of
each structure, and thus elucidated the mechanism of inducing the orbital
polarization in the p symmetric states. We also discuss a simple sum rule
connecting the XMCD spectra with the orbital moment in the p symmetric states.Comment: 5 pages, 5 figures, accepted for publication in Physical Review
Tailoring confining jacket for concrete column using ultra high performance-fiber reinforced cementitious composites (UHP-FRCC) with high volume fly ash (HVFA)
Ultra-High Performance Fibre-Reinforced Cementitious Composites (UHP-FRCC) show excellent mechanical performances in terms of strength, ductility, and durability. Therefore, these cementitious materials have been successfully used for repairing, strengthening, and seismic retrofitting of old structures. However, UHP-FRCCs are not always environmental friendly products, especially in terms of the initial cost, due to the large quantity of cement that is contained in the mixture. Different rates of fly ash substitute herein part of the cement, and the new UHP-FRCCs are used to retrofit concrete columns to overcome this problem. To simulate the mechanical response of these columns, cylindrical specimens, which are made of normal concrete and reinforced with different UHP-FRCC jackets, are tested in uniaxial compression. Relationships between the size of the jacket, the percentage of cement replaced by fly ash, and the strength of the columns are measured and analyzed by means of the eco-mechanical approach. As a result, a replacement of approximately 50% of cement with fly ash, and a suitable thickness of the UHP-FRCC jacket, might ensure the lowest environmental impact without compromising the mechanical performances
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