2,096 research outputs found
Isotope Shifts in Beryllium-, Boron-, Carbon-, and Nitrogen-like Ions from Relativistic Configuration Interaction Calculations
Energy levels, normal and specific mass shift parameters as well as
electronic densities at the nucleus are reported for numerous states along the
beryllium, boron, carbon, and nitrogen isoelectronic sequences. Combined with
nuclear data, these electronic parameters can be used to determine values of
level and transition isotope shifts. The calculation of the electronic
parameters is done using first-order perturbation theory with relativistic
configuration interaction wave functions that account for valence, core-valence
and core-core correlation effects as zero-order functions. Results are compared
with experimental and other theoretical values, when available.Comment: 56 pages, 1 figure, Atomic Data and Nuclear Data Tables (2014
Finite-size effects in amorphous Fe90Zr10/Al75Zr25 multilayers
The thickness dependence of the magnetic properties of amorphous Fe90Zr10
layers has been explored using Fe90Zr10/Al75Zr25 multilayers. The Al75Zr25
layer thickness is kept at 40 \AA, while the thickness of the Fe90Zr10 layers
is varied between 5 and 20 \AA. The thickness of the Al75Zr25 layers is
sufficiently large to suppress any significant interlayer coupling. Both the
Curie temperature and the spontaneous magnetization decrease non-linearly with
decreasing thickness of the Fe90Zr10 layers. No ferromagnetic order is observed
in the multilayer with 5 {\AA} Fe90Zr10 layers. The variation of the Curie
temperature with the Fe90Zr10 layer thickness is fitted with a
finite-size scaling formula [1-\Tc(t)/\Tc(\infty)]=[(t-t')/t_0]^{-\lambda},
yielding , and a critical thickness \AA, below which the
Curie temperature is zero.Comment: 8 pages, 8 figure
Absence of Conventional Spin-Glass Transition in the Ising Dipolar System LiHo_xY_{1-x}F_4
The magnetic properties of single crystals of LiHo_xY_{1-x}F_4 with x=16.5%
and x=4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While
this system is considered as the archetypal quantum spin glass, the detailed
analysis of our magnetization data indicates the absence of a phase transition,
not only in a transverse applied magnetic field, but also without field. A
zero-Kelvin phase transition is also unlikely, as the magnetization seems to
follow a non-critical exponential dependence on the temperature. Our analysis
thus unmasks the true, short-ranged nature of the magnetic properties of the
LiHo_xY_{1-x}F_4 system, validating recent theoretical investigations
suggesting the lack of phase transition in this system.Comment: 5 pages, 4 figure
Extended calculations of energy levels, radiative properties, , hyperfine interaction constants, and Land\'e -factors for nitrogen-like \mbox{Ge XXVI}
Employing two state-of-the-art methods, multiconfiguration
Dirac--Hartree--Fock and second-order many-body perturbation theory, highly
accurate calculations are performed for the lowest 272 fine-structure levels
arising from the , , , ~(), (), and ()
configurations in nitrogen-like Ge XXVI. Complete and consistent atomic data,
including excitation energies, lifetimes, wavelengths, hyperfine structures,
Land\'e -factors, and E1, E2, M1, M2 line strengths, oscillator
strengths, and transition rates among these 272 levels are provided.
Comparisons are made between the present two data sets, as well as with other
available experimental and theoretical values. The present data are accurate
enough for identification and deblending of emission lines involving the
levels, and are also useful for modeling and diagnosing fusion plasmas
Exploring Biorthonormal Transformations of Pair-Correlation Functions in Atomic Structure Variational Calculations
Multiconfiguration expansions frequently target valence correlation and
correlation between valence electrons and the outermost core electrons.
Correlation within the core is often neglected. A large orbital basis is needed
to saturate both the valence and core-valence correlation effects. This in turn
leads to huge numbers of CSFs, many of which are unimportant. To avoid the
problems inherent to the use of a single common orthonormal orbital basis for
all correlation effects in the MCHF method, we propose to optimize independent
MCHF pair-correlation functions (PCFs), bringing their own orthonormal
one-electron basis. Each PCF is generated by allowing single- and double-
excitations from a multireference (MR) function. This computational scheme has
the advantage of using targeted and optimally localized orbital sets for each
PCF. These pair-correlation functions are coupled together and with each
component of the MR space through a low dimension generalized eigenvalue
problem. Nonorthogonal orbital sets being involved, the interaction and overlap
matrices are built using biorthonormal transformation of the coupled basis sets
followed by a counter-transformation of the PCF expansions.
Applied to the ground state of beryllium, the new method gives total energies
that are lower than the ones from traditional CAS-MCHF calculations using large
orbital active sets. It is fair to say that we now have the possibility to
account for, in a balanced way, correlation deep down in the atomic core in
variational calculations
Extended Calculations of Spectroscopic Data: Energy Levels, Lifetimes and Transition rates for O-like ions from Cr XVII to Zn XXIII
Employing two state-of-the-art methods, multiconfiguration
Dirac--Hartree--Fock and second-order many-body perturbation theory, the
excitation energies and lifetimes for the lowest 200 states of the ,
, , , , , , , and configurations, and multipole (electric
dipole (E1), magnetic dipole (M1), and electric quadrupole (E2)) transition
rates, line strengths, and oscillator strengths among these states are
calculated for each O-like ion from Cr XVII to Zn XXIII. Our two data sets are
compared with the NIST and CHIANTI compiled values, and previous calculations.
The data are accurate enough for identification and deblending of new emission
lines from the sun and other astrophysical sources. The amount of data of high
accuracy is significantly increased for the states of several O-like
ions of astrophysics interest, where experimental data are very scarce
Memory and superposition in a spin glass
Non-equilibrium dynamics in a Ag(Mn) spin glass are investigated by
measurements of the temperature dependence of the remanent magnetisation. Using
specific cooling protocols before recording the thermo- or isothermal remanent
magnetisations on re-heating, it is found that the measured curves effectively
disclose non-equilibrium spin glass characteristics such as ageing and memory
phenomena as well as an extended validity of the superposition principle for
the relaxation. The usefulness of this "simple" dc-method is discussed, as well
as its applicability to other disordered magnetic systems.Comment: REVTeX style; 8 pages, 4 figure
Extended atomic data for oxygen abundance analyses
As the most abundant element in the universe after hydrogen and helium,
oxygen plays a key role in planetary, stellar, and galactic astrophysics. Its
abundance is especially influential on stellar structure and evolution, and as
the dominant opacity contributor at the base of the Sun's convection zone it is
central to the discussion around the solar modelling problem. However,
abundance analyses require complete and reliable sets of atomic data. We
present extensive atomic data for O I, by using the multiconfiguration
Dirac-Hartree-Fock and relativistic configuration interaction methods.
Lifetimes and transition probabilities for radiative electric dipole
transitions are given and compared with results from previous calculations and
available measurements. The accuracy of the computed transition rates is
evaluated by the differences between the transition rates in Babushkin and
Coulomb gauges, as well as by a cancellation factor analysis. Out of the 989
computed transitions in this work, 205 are assigned to the accuracy classes
AA-B, that is, with uncertainties less than 10%, following the criteria defined
by the National Institute of Standards and Technology Atomic Spectra Database.
We discuss the influence of the new log(gf) values on the solar oxygen
abundance and ultimately advocate .Comment: 13 pages, 5 figures; Accepted for publication in Astronomy &
Astrophysic
Aging and scaling laws in -hydroquinone-clathrate
The dielectric permittivity of the orientational glass
methanol(x=0.73)--hydroquinone-clathrate has been studied as function of
temperature and waiting time using different temperature-time-protocols. We
study aging, rejuvenation and memory effects in the glassy phase and discuss
similarities and differences to aging in spin-glasses. We argue that the
diluted methanol-clathrate, although conceptually close to its magnetic
pendants, takes an intermediate character between a true spin-glass and a pure
random field system
Non-equilibrium dynamics in an interacting nanoparticle system
Non-equilibrium dynamics in an interacting Fe-C nanoparticle sample,
exhibiting a low temperature spin glass like phase, has been studied by low
frequency ac-susceptibility and magnetic relaxation experiments. The
non-equilibrium behavior shows characteristic spin glass features, but some
qualitative differences exist. The nature of these differences is discussed.Comment: 7 pages, 11 figure
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