4,164 research outputs found
Photoassociation spectra and the validity of the dipole approximation for weakly bound dimers
Photoassociation (PA) of ultracold metastable helium to the 2s2p manifold is
theoretically investigated using a non-perturbative close-coupled treatment in
which the laser coupling is evaluated without assuming the dipole
approximation. The results are compared with our previous study [Cocks and
Whittingham, Phys. Rev. A 80, 023417 (2009)] that makes use of the dipole
approximation. The approximation is found to strongly affect the PA spectra
because the photoassociated levels are weakly bound, and a similar impact is
predicted to occur in other systems of a weakly bound nature. The inclusion or
not of the approximation does not affect the resonance positions or widths,
however significant differences are observed in the background of the spectra
and the maximum laser intensity at which resonances are discernable. Couplings
not satisfying the dipole selection rule |J-1| <= J' <= |J+1| do not lead to
observable resonances.Comment: 5 pages, 2 figures; Minor textual revision
Exotic solutions in string theory
Solutions of classical string theory, correspondent to the world sheets,
mapped in Minkowsky space with a fold, are considered. Typical processes for
them are creation of strings from vacuum, their recombination and annihilation.
These solutions violate positiveness of square of mass and Regge condition. In
quantum string theory these solutions correspond to physical states |DDF>+|sp>
with non-zero spurious component.Comment: accepted in Il Nuovo Cimento A for publication in 199
Relativistic Coulomb problem for particles with arbitrary half-integer spin
Using relativistic tensor-bispinorial equations proposed in hep-th/0412213 we
solve the Kepler problem for a charged particle with arbitrary half-integer
spin interacting with the Coulomb potential.Comment: Misprints are correcte
Curve crossing in linear potential grids: the quasidegeneracy approximation
The quasidegeneracy approximation [V. A. Yurovsky, A. Ben-Reuven, P. S.
Julienne, and Y. B. Band, J. Phys. B {\bf 32}, 1845 (1999)] is used here to
evaluate transition amplitudes for the problem of curve crossing in linear
potential grids involving two sets of parallel potentials. The approximation
describes phenomena, such as counterintuitive transitions and saturation
(incomplete population transfer), not predictable by the assumption of
independent crossings. Also, a new kind of oscillations due to quantum
interference (different from the well-known St\"uckelberg oscillations) is
disclosed, and its nature discussed. The approximation can find applications in
many fields of physics, where multistate curve crossing problems occur.Comment: LaTeX, 8 pages, 8 PostScript figures, uses REVTeX and psfig,
submitted to Physical Review
Optimal processing of noisy images in a photodetector with limited dynamic range
A study aimed at optimizing noisy image processing under conditions of strong additive noise has been performed. An algorithm of optimal signal processing was developed and a possibility of improving image quality due to the subtraction of excess additive noise (which limits the photodetector dynamic range) was substantiated. The possibility of technical implementation of noise subtraction due to forced recombination of charge carriers in the photodetector is experimentally confirmed. The proposed approach to design processing systems makes it possible to improve the quality of recorded images under noisy conditions without any changes in the photodetector desig
Landau-Zener Problem for Trilinear Hamiltonians
We consider a nonlinear version of the Landau-Zener problem, focusing on
photoassociation of a Bose-Einstein condensate as a specific example. Contrary
to the exponential rate dependence obtained for the linear problem, a series
expansion technique indicates that, when the resonance is crossed slowly, the
probability for failure of adiabaticity is directly proportional to the rate at
which the resonance is crossed.Comment: 4.5 pages, 1 figure, transferred to PRA; v2 adds discussion,
clarification, and explicit numbers for Na and 87R
Magnetic and spectral properties of multi-sublattice oxides SrY2O4:Er3+ and SrEr2O4
SrEr2O4 is a geometrically frustrated magnet which demonstrates rather
unusual properties at low temperatures including a coexistence of long- and
short-range magnetic order, characterized by two different propagation vectors.
In the present work, the effects of crystal fields (CF) in this compound
containing four magnetically inequivalent erbium sublattices are investigated
experimentally and theoretically. We combine the measurements of the CF levels
of the Er3+ ions made on a powder sample of SrEr2O4 using neutron spectroscopy
with site-selective optical and electron paramagnetic resonance measurements
performed on single crystal samples of the lightly Er-doped nonmagnetic
analogue, SrY2O4. Two sets of CF parameters corresponding to the Er3+ ions at
the crystallographically inequivalent lattice sites are derived which fit all
the available experimental data well, including the magnetization and dc
susceptibility data for both lightly doped and concentrated samples.Comment: 14 pages, 9 figure
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