904 research outputs found
Anomalous population of He states in reactions with Li
Structure with the lowest energy observed in the He spectrum populated
in the proton knockout reaction with Li beam has a peak at
MeV. This peak is usually interpreted as a resonant ground state of
He. Our theoretical calculations indicate that this peak is likely to be
a pileup of , , and excitations with very similar shapes. %We
predict a very specific nature of the excitation in He. Moreover,
the ``soft'' excitation appears to be the lowest one in energy. Such an
anomalous continuum response is traced to the halo structure of Li
providing extreme low energy shift to all the expected continuum excitations.
Competitions of the initial state structure (ISS) and the final state
interaction (FSI) effects on the spectrum and three-body correlations in
He are discussed. Analogous effect of the extreme low-energy shift could
also be expected in other cases of emitters populated in reactions with
halo nuclei. Simplified example of the He spectrum in knockout
from Be, is given. We also discuss limits on the properties of He
stemming from the observed He spectrum.Comment: 10 pages, 13 figure
Problems with interpretation of He ground state
The continuum of He nucleus is studied theoretically in a three-body
He++ model basing on the recent information concerning He
spectrum [Golovkov, \textit{et al.}, Phys. Rev. C \textbf{76}, 021605(R)
(2007)]. The He ground state (g.s.) candidate with structure
for new g.s. energy of He is predicted to be at about
MeV. The peak in the cross section associated with this state may be
shifted to a lower energy (e.g. MeV) when He is populated in
reactions with Li due to peculiar reaction mechanism. Formation of the
low-energy ( keV) ``alternative'' ground state with structure
is highly probable in He in the case of considerable
attraction (e.g. fm) in the s-wave He channel, which properties are
still quite uncertain. This result either questions the existing experimental
low-energy spectrum of He or place a limit on the scattering length in
He channel, which contradicts existing data.Comment: 14 pages, 13 figures, 1 tabl
Optimized design of universal two-qubit gates
We construct optimized implementations of the CNOT and other universal
two-qubit gates that, unlike many of the previously proposed protocols, are
carried out in a single step. The new protocols require tunable inter-qubit
couplings but, in return, show a significant improvements in the quality of
gate operations. Our optimization procedure can be further extended to the
combinations of elementary two-qubit as well as irreducible many-qubit gates.Comment: 6 pages, 2 figure
Two-proton radioactivity and three-body decay. V. Improved momentum distributions
Nowadays quantum-mechanical theory allows one to reliably calculate the
processes of 2p radioactivity (true three-body decays) and the corresponding
energy and angular correlations up to distances of the order of 1000 fm.
However, the precision of modern experiments has now become sufficient to
indicate some deficiency of the predicted theoretical distributions. In this
paper we discuss the extrapolation along the classical trajectories as a method
to improve the convergence of the theoretical energy and angular correlations
at very large distances (of the order of atomic distances), where only the
long-range Coulomb forces are still operating. The precision of this approach
is demonstrated using the "exactly" solvable semianalytical models with
simplified three-body Hamiltonians. It is also demonstrated that for heavy 2p
emitters, the 2p decay momentum distributions can be sensitive to the effect of
the screening by atomic electrons. We compare theoretical results with
available experimental data.Comment: 13 pages, 18 figure
Transition from direct to sequential two-proton decay in - shell nuclei
Transitions among different mechanisms of two-proton decay are studied in
general. The introduced improved direct-decay model generalizes the
semi-analytical models used before and provides flawless phenomenological
description of three-body correlations in decays. This is demonstrated by
examples of the low-lying Ne state decays. Different forms of transition
dynamic are shown to be highly probable beyond the proton dripline for the
- shell nuclei. It is demonstrated that transition dynamic of
emitters can provide means for extraction of a width of the ground-state
resonance of a core+ subsystem of the core+ system. Practical
applicability of the method is demonstrated by properties of the F
ground state derived from the ^{15}\mbox{Ne}\rightarrow
^{\,13\!\!}\mbox{O}+2p decay data and of the Cl ground state derived
from the ^{30}\mbox{Ar}\rightarrow ^{\,28\!\!}\mbox{S}+2p decay data.Comment: 8 pages, 10 figure
Two-proton events in the 17F(p,2p)16O reaction
In a recent experimental study (Gomez del Campo et al, PRL 86, 43 (2001)) of
the reaction 17F(p,2p)16O, two-proton events were measured from excitations
near a 1-, E*=6.15 MeV state in 18Ne. We calculate by means of R-matrix theory
the resonant two-proton production cross section and branching ratios. We
conclude that it is unlikely that two-proton production via population of the
1- state is sufficient to explain the observed two-proton events. Alternative
sources of such events are discussed.Comment: 4 pages, 4 figures. Resubmission to Physical Review C (first received
6 March 2001
From Coulomb excitation cross sections to non-resonant astrophysical rates in three-body systems: Ne case
Coulomb and nuclear dissociation of Ne on light and heavy targets are
studied theoretically. The dipole E1 strength function is determined in a broad
energy range including energies of astrophysical interest. Dependence of the
strength function on different parameters of the Ne ground state
structure and continuum dynamics is analyzed in a three-body model. The
discovered dependence plays an important role for studies of the strength
functions for the three-body E1 dissociation and radiative capture. The
constraints on the configuration mixing in Ne and on
-wave interaction in the O+ channel are imposed based on
experimental data for Ne Coulomb dissociation on heavy target.Comment: 12 pages, 13 figure
Nanofabricated media with negative permeability at visible frequencies
We report a nanofabricated medium made of electromagnetically coupled pairs
of gold dots with geometry carefully designed at a 10-nm level. The medium
exhibits strong magnetic response at visible-light frequencies, including bands
with negative \mu. The magnetism arises due to the excitation of quadrupole
plasmon resonances. Our approach shows for the first time the feasibility of
magnetism at optical frequencies and paves a way towards magnetic and
left-handed components for visible optics.Comment: 16 pages, 4 figures. submitted to Nature on 1 April 200
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