18,452 research outputs found
A comparison between detailed and configuration-averaged collisional-radiative codes applied to non-local thermal equilibrium plasma
A collisional-radiative model describing nonlocal-thermodynamic-equilibrium
plasmas is developed. It is based on the HULLAC (Hebrew University Lawrence
Livermore Atomic Code) suite for the transitions rates, in the zero-temperature
radiation field hypothesis. Two variants of the model are presented: the first
one is configuration averaged, while the second one is a detailed level
version. Comparisons are made between them in the case of a carbon plasma; they
show that the configuration-averaged code gives correct results for an
electronic temperature Te=10 eV (or higher) but fails at lower temperatures
such as Te=1 eV. The validity of the configuration-averaged approximation is
discussed: the intuitive criterion requiring that the average
configuration-energy dispersion must be less than the electron thermal energy
turns out to be a necessary but far from sufficient condition. Another
condition based on the resolution of a modified rate-equation system is
proposed. Its efficiency is emphasized in the case of low-temperature plasmas.
Finally, it is shown that near-threshold autoionization cascade processes may
induce a severe failure of the configuration-average formalism.Comment: 9
Recombination in polymer-fullerene bulk heterojunction solar cells
Recombination of photogenerated charge carriers in polymer bulk
heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and
the fill factor (FF). Identifying the mechanism of recombination is, therefore,
fundamentally important for increasing the power conversion efficiency. Light
intensity and temperature dependent current-voltage measurements on polymer BHJ
cells made from a variety of different semiconducting polymers and fullerenes
show that the recombination kinetics are voltage dependent and evolve from
first order recombination at short circuit to bimolecular recombination at open
circuit as a result of increasing the voltage-dependent charge carrier density
in the cell. The "missing 0.3V" inferred from comparison of the band gaps of
the bulk heterojunction materials and the measured open circuit voltage at room
temperature results from the temperature dependence of the quasi-Fermi-levels
in the polymer and fullerene domains - a conclusion based upon the fundamental
statistics of Fermions.Comment: Accepted for publication in Physical Review B.
http://prb.aps.org/accepted/B/6b07cO3aHe71bd1b149e1425e58bf2868cda2384d?ajax=1&height=500&width=50
Ferromagnetic fluid as a model of social impact
The paper proposes a new model of spin dynamics which can be treated as a
model of sociological coupling between individuals. Our approach takes into
account two different human features: gregariousness and individuality. We will
show how they affect a psychological distance between individuals and how the
distance changes the opinion formation in a social group. Apart from its
sociological aplications the model displays the variety of other interesting
phenomena like self-organizing ferromagnetic state or a second order phase
transition and can be studied from different points of view, e.g. as a model of
ferromagnetic fluid, complex evolving network or multiplicative random process.Comment: 8 pages, 5 figure
Relativistic many-body calculations of the Stark-induced amplitude of the 6P1/2 -7P1/2 transition in thallium
Stark-induced amplitudes for the 6P1/2 - 7P1/2 transition in Tl I are
calculated using the relativistic SD approximation in which single and double
excitations of Dirac-Hartree-Fock levels are summed to all orders in
perturbation theory. Our SD values alpha S = 368 a0 3 and beta S= 298 a 0 3 are
in good agreement with the measurements alpha S=377(8) a 0 3$ and beta S =
313(8) a 0 3 by D. DeMille, D. Budker, and E. D. Commins [Phys. Rev. A 50, 4657
(1994)]. Calculations of the Stark shifts in the 6P1/2 - 7P1/2 and 6P1/2 -
7S1/2 transitions are also carried out. The Stark shifts predicted by our
calculations agree with the most accurate measured values within the
experimental uncertainties for both transitions
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
Composite Fermions and quantum Hall systems: Role of the Coulomb pseudopotential
The mean field composite Fermion (CF) picture successfully predicts angular
momenta of multiplets forming the lowest energy band in fractional quantum Hall
(FQH) systems. This success cannot be attributed to a cancellation between
Coulomb and Chern-Simons interactions beyond the mean field, because these
interactions have totally different energy scales. Rather, it results from the
behavior of the Coulomb pseudopotential V(L) (pair energy as a function of pair
angular momentum) in the lowest Landau level (LL). The class of short range
repulsive pseudopotentials is defined that lead to short range Laughlin like
correlations in many body systems and to which the CF model can be applied.
These Laughlin correlations are described quantitatively using the formalism of
fractional parentage. The discussion is illustrated with an analysis of the
energy spectra obtained in numerical diagonalization of up to eleven electrons
in the lowest and excited LL's. The qualitative difference in the behavior of
V(L) is shown to sometimes invalidate the mean field CF picture when applied to
higher LL's. For example, the nu=7/3 state is not a Laughlin nu=1/3 state in
the first excited LL. The analysis of the involved pseudopotentials also
explains the success or failure of the CF picture when applied to other systems
of charged Fermions with Coulomb repulsion, such as the Laughlin quasiparticles
in the FQH hierarchy or charged excitons in an electron-hole plasma.Comment: 27 pages, 23 figures, revised version (significant changes in text
and figures), submitted to Phil. Mag.
The Radio Recovery of SN 1970G: The Continuing Radio Evolution of SN 1970G
Using the Very Large Array, we have detected radio emission from the site of
SN 1970G in the Sc galaxy M101. These observations are 31 years after the
supernova event, making SN 1970G the longest monitored radio supernova. With
flux densities of 0.12 +/- 0.020 mJy at 6 cm and 0.16 +/- 0.015 mJy at 20 cm,
the spectral index of -0.24 +/- 0.20 appears to have flattened somewhat when
compared with the previously reported value of -0.56 +/- 0.11, taken in 1990.
The radio emission at 20 cm has decayed since the 1990 observations with a
power-law index of beta_20cm = -0.28 +/- 0.13. We discuss the radio properties
of this source and compare them to those of other Type II radio supernovae.Comment: 11 pages, 1 table and 2 figures; To appear in Astrophysical Journal
Letter
Theoretical study of resonant x-ray emission spectroscopy of Mn films on Ag
We report a theoretical study on resonant x-ray emission spectra (RXES) in
the whole energy region of the Mn white lines for three prototypical
Mn/Ag(001) systems: (i) a Mn impurity in Ag, (ii) an adsorbed Mn monolayer on
Ag, and (iii) a thick Mn film. The calculated RXES spectra depend strongly on
the excitation energy. At excitation, the spectra of all three systems
are dominated by the elastic peak. For excitation energies around , and
between and , however, most of the spectral weight comes from
inelastic x-ray scattering. The line shape of these inelastic ``satellite''
structures changes considerably between the three considered Mn/Ag systems, a
fact that may be attributed to changes in the bonding nature of the Mn-
orbitals. The system-dependence of the RXES spectrum is thus found to be much
stronger than that of the corresponding absorption spectrum. Our results
suggest that RXES in the Mn region may be used as a sensitive probe
of the local environment of Mn atoms.Comment: 9 pages, 11 figure
Evaluation of left ventricular torsion by cardiovascular magnetic resonance
Recently there has been considerable interest in LV torsion and its relationship with symptomatic and pre-symptomatic disease processes. Torsion gives useful additional information about myocardial tissue performance in both systolic and diastolic function. CMR assessment of LV torsion is simply and efficiently performed. However, there is currently a wide variation in the reporting of torsional motion and the procedures used for its calculation. For example, torsion has been presented as twist (degrees), twist per length (degrees/mm), shear angle (degrees), and shear strain (dimensionless). This paper reviews current clinical applications and shows how torsion can give insights into LV mechanics and the influence of LV geometry and myocyte fiber architecture on cardiac function. Finally, it provides recommendations for CMR measurement protocols, attempts to stimulate standardization of torsion calculation, and suggests areas of useful future research
Systematic computation of crystal field multiplets for X-ray core spectroscopies
We present a new approach to computing multiplets for core spectroscopies,
whereby the crystal field is constructed explicitly from the positions and
charges of surrounding atoms. The simplicity of the input allows the
consideration of crystal fields of any symmetry, and in particular facilitates
the study of spectroscopic effects arising from low symmetry environments. The
interplay between polarization directions and crystal field can also be
conveniently investigated. The determination of the multiplets proceeds from a
Dirac density functional atomic calculation, followed by the exact
diagonalization of the Coulomb, spin-orbit and crystal field interactions for
the electrons in the open shells. The eigenstates are then used to simulate
X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering spectra.
In examples ranging from high symmetry down to low symmetry environment,
comparisons with experiments are done with unadjusted model parameters as well
as with semi-empirically optimized ones. Furthermore, predictions for the RIXS
of low-temperature MnO and for Dy in a molecular complex are proposed.Comment: Accepted for publication in Phys. Rev.
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