79,427 research outputs found
Three isoparametric solid elements for NASTRAN
Linear, quadratic, and cubic isoparametric hexahedral solid elements have been added to the element library of NASTRAN. These elements are available for static, dynamic, buckling, and heat-transfer analyses. Because the isoparametric element matrices are generated by direct numerical integration over the volume of the element, variations in material properties, temperatures, and stresses within the elements are represented in the computations. In order to compare the accuracy of the new elements, three similar models of a slender cantilever were developed, one for each element. All elements performed well. As expected, however, the linear element model yielded excellent results only when shear behavior predominated. In contrast, the results obtained from the quadratic and cubic element models were excellent in both shear and bending
Excitation energies, polarizabilities, multipole transition rates, and lifetimes of ions along the francium isoelectronic sequence
Relativistic many-body perturbation theory is applied to study properties of
ions of the francium isoelectronic sequence. Specifically, energies of the 7s,
7p, 6d, and 5f states of Fr-like ions with nuclear charges Z = 87 - 100 are
calculated through third order; reduced matrix elements, oscillator strengths,
transition rates, and lifetimes are determined for 7s - 7p, 7p - 6d, and 6d -
5f electric-dipole transitions; and 7s - 6d, 7s - 5f, and 5f_5/2 - 5f_7/2
multipole matrix elements are evaluated to obtain the lifetimes of low-lying
excited states. Moreover, for the ions Z = 87 - 92 calculations are also
carried out using the relativistic all-order single-double method, in which
single and double excitations of Dirac-Fock wave functions are included to all
orders in perturbation theory. With the aid of the SD wave functions, we obtain
accurate values of energies, transition rates, oscillator strengths, and the
lifetimes of these six ions. Ground state scalar polarizabilities in Fr I, Ra
II, Ac III, and Th IV are calculated using relativistic third-order and
all-order methods. Ground state scalar polarizabilities for other Fr-like ions
are calculated using a relativistic second-order method. These calculations
provide a theoretical benchmark for comparison with experiment and theory.Comment: 13 figures, 11 table
Excitation energies, polarizabilities, multipole transition rates, and lifetimes in Th IV
Excitation energies of the ns_{1/2} (n=7-10), np_j (n=7-9), nd_j (n=6-8),
nf_{j} (n=5-7), and ng_{j} (n=5-6) states in Th IV are evaluated. First-,
second-, third-, and all-order Coulomb energies and first- and second-order
Coulomb-Breit energies are calculated. Reduced matrix elements, oscillator
strengths, transition rates, and lifetimes are determined for the 96 possible
nl_j-n'l'_j' electric-dipole transitions. Multipole matrix elements
(7s_{1/2}-6d_j, 7s_{1/2}-5f_j, and 5f_{5/2}-5f_{7/2}) are evaluated to obtain
the lifetimes of the and 7s_{1/2}$ states. Matrix elements are
calculated using both relativistic many-body perturbation theory, complete
through third order, and a relativistic all-order method restricted to single
and double (SD) excitations. Scalar and tensor polarizabilities for the
5f_{5/2} ground state in Th3+ are calculated using relativistic third-order and
all-order methods. These calculations provide a theoretical benchmark for
comparison with experiment and theory.Comment: 9 pages, 9 figure
Addition of three-dimensional isoparametric elements to NASA structural analysis program (NASTRAN)
Implementation is made of the three-dimensional family of linear, quadratic and cubic isoparametric solid elements into the NASA Structural Analysis program, NASTRAN. This work included program development, installation, testing, and documentation. The addition of these elements to NASTRAN provides a significant increase in modeling capability particularly for structures requiring specification of temperatures, material properties, displacements, and stresses which vary throughout each individual element. Complete program documentation is presented in the form of new sections and updates for direct insertion to the three NASTRAN manuals. The results of demonstration test problems are summarized. Excellent results are obtained with the isoparametric elements for static, normal mode, and buckling analyses
Relativistic many-body calculation of energies, lifetimes, polarizabilities, blackbody radiative shift and hyperfine constants in Lu2+
Energy levels of 30 low-lying states of Lu2+ and allowed electric-dipole
matrix elements between these states are evaluated using a relativistic
all-order method in which all single, double and partial triple excitations of
Dirac-Fock wave functions are included to all orders of perturbation theory.
Matrix elements are critically evaluated for their accuracy and recommended
values of the matrix elements are given together with uncertainty estimates.
Line strengths, transition rates and lifetimes of the metastable 5d(3/2) and
5d(5/2) states are calculated. Recommended values are given for static
polarizabilities of the 6s, 5d and 6p states and tensor polarizabilities of the
5d and 6p(3/2) states. Uncertainties of the polarizability values are estimated
in all cases. The blackbody radiation shift of the 6s(1/2)-5d(5/2) transition
frequency of the Lu2+ ion is calculated with the aid of the recommended scalar
polarizabilities of the 6s(1/2) and 5d(5/2) states. Finally, A and B hyperfine
constants are determined for states of 175Lu2+ with n <= 9. This work provides
recommended values of transition matrix elements, polarizabilities and
hyperfine constants of Lu2+, critically evaluated for accuracy, for benchmark
tests of high-precision theoretical methodology and planning of future
experiments.Comment: 9 pages, 10 table
Exact dynamical response of an N-electron quantum dot subject to a time-dependent potential
We calculate analytically the exact dynamical response of a droplet of N
interacting electrons in a quantum dot with an arbitrarily time-dependent
parabolic confinement potential \omega(t) and a perpendicular magnetic field.
We find that, for certain frequency ranges, a sinusoidal perturbation acts like
an attractive effective interaction between electrons. In the absence of a
time-averaged confinement potential, the N electrons can bind together to form
a stable, free-standing droplet.Comment: 10 pages, RevTex, 3 Postscript figures. This version to appear as a
Rapid Communication in PR
Forbidden M1 and E2 transitions in monovalent atoms and ions
We carried out a systematic high-precision relativistic study of the
forbidden magnetic-dipole and electric-quadrupole transitions in Ca+, Rb, Sr+,
Cs, Ba+, Fr, Ra+, Ac2+, and Th3+. This work is motivated by the importance of
these transitions for tests of fundamental physics and precision measurements.
The relative importance of the relativistic, correlation, Breit correction, and
contributions of negative-energy states is investigated. Recommended values of
reduced matrix elements are presented together with their uncertainties. The
matrix elements and resulting lifetimes are compared with other theoretical
values and with experiment where available.Comment: 9 pages, 6 table
Imaging and quantum efficiency measurement of chromium emitters in diamond
We present direct imaging of the emission pattern of individual
chromium-based single photon emitters in diamond and measure their quantum
efficiency. By imaging the excited state transition dipole intensity
distribution in the back focal plane of high numerical aperture objective, we
determined that the emission dipole is oriented nearly orthogonal to the
diamond-air interface. Employing ion implantation techniques, the emitters were
engineered with various proximities from the diamond-air interface. By
comparing the decay rates from the single chromium emitters at different depths
in the diamond crystal, an average quantum efficiency of 28% was measured.Comment: 11 pages and 4 figure
Radio emission from the massive stars in the Galactic Super Star Cluster Westerlund 1
Current mass-loss rate estimates imply that main sequence winds are not
sufficient to strip away the H-rich envelope to yield Wolf-Rayet (WR) stars.
The rich transitional population of Westerlund 1 (Wd 1) provides an ideal
laboratory to observe mass-loss processes throughout the transitional phase of
stellar evolution. An analysis of deep radio continuum observations of Wd 1 is
presented. We detect 18 cluster members. The radio properties of the sample are
diverse, with thermal, non-thermal and composite thermal/non-thermal sources
present. Mass-loss rates are ~10^{-5} solar mass/year across all spectral
types, insufficient to form WRs during a massive star lifetime, and the stars
must undergo a period of enhanced mass loss. The sgB[e] star W9 may provide an
example, with a mass-loss rate an order of magnitude higher than the other
cluster members, and an extended nebula of density ~3 times the current wind.
This structure is reminiscent of luminous blue variables, and one with evidence
of two eras of high, possibly eruptive, mass loss. Three OB supergiants are
detected, implying unusually dense winds. They also may have composite spectra,
suggesting binarity. Spatially resolved nebulae are associated with three of
the four RSGs and three of the six YHGs in the cluster, which are due to
quiescent mass loss rather than outbursts. For some of the cool star winds, the
ionizing source may be a companion star though the cluster radiation density is
sufficiently high to provide the necessary ionizing radiation. Five WR stars
are detected with composite spectra, interpreted as arising in colliding-wind
binaries.Comment: 15 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
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