240 research outputs found
Faddeev-Merkuriev equations for resonances in three-body Coulombic systems
We reconsider the homogeneous Faddeev-Merkuriev integral equations for
three-body Coulombic systems with attractive Coulomb interactions and point out
that the resonant solutions are contaminated with spurious resonances. The
spurious solutions are related to the splitting of the attractive Coulomb
potential into short- and long-range parts, which is inherent in the approach,
but arbitrary to some extent. By varying the parameters of the splitting the
spurious solutions can easily be ruled out. We solve the integral equations by
using the Coulomb-Sturmian separable expansion approach. This solution method
provides an exact description of the threshold phenomena. We have found several
new S-wave resonances in the e- e+ e- system in the vicinity of thresholds.Comment: LaTeX with elsart.sty 13 pages, 5 figure
Engineered nonlinear lattices
We show that with the quasi-phase-matching technique it is possible to fabricate stripes of nonlinearity that trap and guide light like waveguides. We investigate an array of such stripes and find that when the stripes are sufficiently narrow, the beam dynamics is governed by a quadratic nonlinear discrete equation. The proposed structure therefore provides an experimental setting for exploring discrete effects in a controlled manner. In particular, we show propagation of breathers that are eventually trapped by discreteness. When the stripes are wide the beams evolve in a structure we term a quasilattice, which interpolates between a lattice system and a continuous system.Peer ReviewedPostprint (published version
Spatially valid proprioceptive cues improve the detection of a visual stimulus
Vision and proprioception are the main sensory modalities that convey hand location and direction of movement. Fusion of these sensory signals into a single robust percept is now well documented. However, it is not known whether these modalities also interact in the spatial allocation of attention, which has been demonstrated for other modality pairings. The aim of this study was to test whether proprioceptive signals can spatially cue a visual target to improve its detection. Participants were instructed to use a planar manipulandum in a forward reaching action and determine during this movement whether a near-threshold visual target appeared at either of two lateral positions. The target presentation was followed by a masking stimulus, which made its possible location unambiguous, but not its presence. Proprioceptive cues were given by applying a brief lateral force to the participantâs arm, either in the same direction (validly cued) or in the opposite direction (invalidly cued) to the on-screen location of the mask. The dâČ detection rate of the target increased when the direction of proprioceptive stimulus was compatible with the location of the visual target compared to when it was incompatible. These results suggest that proprioception influences the allocation of attention in visual spac
Algebraic Model for scattering of three-s-cluster systems. II. Resonances in the three-cluster continuum of 6He and 6Be
The resonance states embedded in the three-cluster continuum of 6He and 6Be
are obtained in the Algebraic Version of the Resonating Group Method. The model
accounts for a correct treatment of the Pauli principle. It also provides the
correct three-cluster continuum boundary conditions by using a Hyperspherical
Harmonics basis. The model reproduces the observed resonances well and achieves
good agreement with other models. A better understanding for the process of
formation and decay of the resonance states in six-nucleon systems is obtained.Comment: 8 pages, 10 postscript figures, submitted to Phys. Rev.
Localization of shadow poles by complex scaling
Through numerical examples we show that the complex scaling method is suited
to explore the pole structure in multichannel scattering problems. All poles
lying on the multisheeted Riemann energy surface, including shadow poles, can
be revealed and the Riemann sheets on which they reside can be identified.Comment: 6 pages, Latex with Revtex, 3 figures (not included) available on
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Three-body resonances in He-6, Li-6, and Be-6, and the soft dipole mode problem of neutron halo nuclei
Using the complex scaling method, the low-lying three-body resonances of
He, Li, and Be are investigated in a parameter-free microscopic
three-cluster model. In He a 2, in Li a 2 and a 1, and in
Be the 0 ground state and a 2 excited state is found. The other
experimentally known 2 state of Li cannot be localized by our present
method. We have found no indication for the existence of the predicted 1
soft dipole state in He. We argue that the sequential decay mode of He
through the resonant states of its two-body subsystem can lead to peaks in the
excitation function. This process can explain the experimental results in the
case of Li, too. We propose an experimental analysis, which can decide
between the soft dipole mode and the sequential decay mode.Comment: REVTEX, Submitted to Phys. Rev. C, 12 pages, 2 postscript figures are
available upon request. CALTECH, MAP-16
Searching for three-nucleon resonances
We search for three-neutron resonances which were predicted from pion double
charge exchange experiments on He-3. All partial waves up to J=5/2 are
nonresonant except the J=3/2^+ one, where we find a state at E=14 MeV energy
with 13 MeV width. The parameters of the mirror state in the three-proton
system are E=15 MeV and Gamma=14 MeV. The possible existence of an excited
state in the triton, which was predicted from a H(He-6,alpha) experiment, is
also discussed.Comment: LaTex with RevTe
Nuclear Alpha-Particle Condensates
The -particle condensate in nuclei is a novel state described by a
product state of 's, all with their c.o.m. in the lowest 0S orbit. We
demonstrate that a typical -particle condensate is the Hoyle state
( MeV, state in C), which plays a crucial role for
the synthesis of C in the universe. The influence of antisymmentrization
in the Hoyle state on the bosonic character of the particle is
discussed in detail. It is shown to be weak. The bosonic aspects in the Hoyle
state, therefore, are predominant. It is conjectured that -particle
condensate states also exist in heavier nuclei, like O,
Ne, etc. For instance the state of O at MeV
is identified from a theoretical analysis as being a strong candidate of a
condensate. The calculated small width (34 keV) of ,
consistent with data, lends credit to the existence of heavier Hoyle-analogue
states. In non-self-conjugated nuclei such as B and C, we discuss
candidates for the product states of clusters, composed of 's,
triton's, and neutrons etc. The relationship of -particle condensation
in finite nuclei to quartetting in symmetric nuclear matter is investigated
with the help of an in-medium modified four-nucleon equation. A nonlinear order
parameter equation for quartet condensation is derived and solved for
particle condensation in infinite nuclear matter. The strong qualitative
difference with the pairing case is pointed out.Comment: 71 pages, 41 figures, review article, to be published in "Cluster in
Nuclei (Lecture Notes in Physics) - Vol.2 -", ed. by C. Beck,
(Springer-Verlag, Berlin, 2011
Structure of the mirror nuclei Be and B in a microscopic cluster model
The structure of the mirror nuclei Be and B is studied in a
microscopic and three-cluster model
using a fully antisymmetrized 9-nucleon wave function. The two-nucleon
interaction includes central and spin-orbit components and the Coulomb
potential. The ground state of Be is obtained accurately with the
stochastic variational method, while several particle-unbound states of both
Be and B are investigated with the complex scaling method.The
calculation for Be supports the recent identification for the existence of
two broad states around 6.5 MeV, and predicts the and
states at about 4.5 MeV and 8 MeV, respectively. The
similarity of the calculated spectra of Be and B enables one to
identify unknown spins and parities of the B states. Available data on
electromagnetic moments and elastic electron scatterings are reproduced very
well. The enhancement of the 1 transition of the first excited state in
Be is well accounted for. The calculated density of Be is found to
reproduce the reaction cross section on a Carbon target. The analysis of the
beta decay of Li to Be clearly shows that the wave function of Be
must contain a small component that cannot be described by the simple model. This small component can be well accounted for by extending a
configuration space to include the distortion of the -particle to
and partitions.Comment: 24 page
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