14,348 research outputs found
Nuclear fragmentation by tunneling
Fragmentation of nuclear system by tunneling is discussed in a molecular
dynamics simulation coupled with imaginary time method. In this way we obtain
informations on the fragmenting systems at low densities and temperatures.
These conditions cannot be reached normally (i.e. above the barrier) in
nucleus-nucleus or nucleon-nucleus collisions. The price to pay is the small
probability of fragmentation by tunneling but we obtain observables which can
be a clear signature of such phenomena.Comment: Phys.Rev.C (submitted
Electron-Hole Asymmetry in Single-Walled Carbon Nanotubes Probed by Direct Observation of Transverse Quasi-Dark Excitons
We studied the asymmetry between valence and conduction bands in
single-walled carbon nanotubes (SWNTs) through the direct observation of
spin-singlet transverse dark excitons using polarized photoluminescence
excitation spectroscopy. The intrinsic electron-hole (e-h) asymmetry lifts the
degeneracy of the transverse exciton wavefunctions at two equivalent K and K'
valleys in momentum space, which gives finite oscillator strength to transverse
dark exciton states. Chirality-dependent spectral weight transfer to transverse
dark states was clearly observed, indicating that the degree of the e-h
asymmetry depends on the specific nanotube structure. Based on comparison
between theoretical and experimental results, we evaluated the band asymmetry
parameters in graphene and various carbon nanotube structures.Comment: 11 pages, 4 figure
Superconductivity without Local Inversion Symmetry; Multi-layer Systems
While multi-layer systems can possess global inversion centers, they can have
regions with locally broken inversion symmetry. This can modify the
superconducting properties of such a system. Here we analyze two dimensional
multi-layer systems yielding spatially modulated antisymmetric spin-orbit
coupling (ASOC) and discuss superconductivity with mixed parity order
parameters. In particular, the influence of ASOC on the spin susceptibility is
investigated at zero temperature. For weak inter-layer coupling we find an
enhanced spin susceptibility induced by ASOC, which hints the potential
importance of this aspect for superconducting phase in specially structured
superlattices.Comment: 4 pages, 2 figures, proceedings of the 26th International Conference
on Low Temperature Physics (LT26
Nucleon Flow and Fragment Flow in Heavy Ion Reactions
The collective flow of nucleons and that of fragments in the 12C + 12C
reaction below 150 MeV/nucleon are calculated with the antisymmetrized version
of molecular dynamics combined with the statistical decay calculation. Density
dependent Gogny force is used as the effective interaction. The calculated
balance energy is about 100 MeV/nucleon, which is close to the observed value.
Below the balance energy, the absolute value of the fragment flow is larger
than that of nucleon flow, which is also in accordance with data. The
dependence of the flow on the stochastic collision cross section and its origin
are discussed. All the results are naturally understood by introducing the
concept of two components of flow: the flow of dynamically emitted nucleons and
the flow of the nuclear matter which contributes to both the flow of fragments
and the flow of nucleons due to the statistical decay.Comment: 20 pages, PostScript figures, LaTeX with REVTeX and EPSF, KUNS 121
K^+ momentum spectrum from (K^-,K^+) reactions in intranuclear cascade model
In a framework of intranuclear cascade (INC) type calculation, we study a
momentum spectrum in reactions \KK at a beam momentum of 1.65 GeV/c. INC model
calculations are compared with the relativistic impulse approximation (RIA)
calculations to perform the detailed study of the reaction mechanism. We find
that the INC model can reproduce the experimental data on various targets.
Especially, in the low-momentum region, the forward-angle cross sections of the
reaction on from light to heavy targets are consistently explained
with the two-step strangeness exchange and production processes with various
intermediate mesons, and , and productions and their decay
into . In the two-step processes, inclusion of meson and hyperon
resonances is found to be essential.Comment: LaTeX file and 12ps figure
Isoscalar Giant Quadrupole Resonance State in the Relativistic Approach with the Momentum-Dependent Self-Energies
We study the excited energy of the isoscalar giant quadrupole resonance with
the scaling method in the relativistic many-body framework. In this calculation
we introduce the momentum-dependent parts of the Dirac self-energies arising
from the one-pion exchange on the assumption of the pseudo-vector coupling with
nucleon field. It is shown that this momentum-dependence enhances the Landau
mass significantly and thus suppresses the quadrupole resonance energy even
giving the small Dirac effective mass which causes a problem in the
momentum-independent mean-field theory.Comment: 12pages, 2 Postscript figure
Axion Production from Landau Quantization in the Strong Magnetic Field of Magnetars
We utilize an exact quantum calculation to explore axion emission from
electrons and protons in the presence of the strong magnetic field of
magnetars. The axion is emitted via transitions between the Landau levels
generated by the strong magnetic field. The luminosity of axions emitted by
protons is shown to be much larger than that of electrons and becomes stronger
with increasing matter density. Cooling by axion emission is shown to be much
larger than neutrino cooling by the Urca processes. Consequently, axion
emission in the crust may significantly contribute to the cooling of magnetars.
In the high-density core, however, it may cause heating of the magnetar.Comment: 14 pages, 3 figure
Noise-induced behaviors in neural mean field dynamics
The collective behavior of cortical neurons is strongly affected by the
presence of noise at the level of individual cells. In order to study these
phenomena in large-scale assemblies of neurons, we consider networks of
firing-rate neurons with linear intrinsic dynamics and nonlinear coupling,
belonging to a few types of cell populations and receiving noisy currents.
Asymptotic equations as the number of neurons tends to infinity (mean field
equations) are rigorously derived based on a probabilistic approach. These
equations are implicit on the probability distribution of the solutions which
generally makes their direct analysis difficult. However, in our case, the
solutions are Gaussian, and their moments satisfy a closed system of nonlinear
ordinary differential equations (ODEs), which are much easier to study than the
original stochastic network equations, and the statistics of the empirical
process uniformly converge towards the solutions of these ODEs. Based on this
description, we analytically and numerically study the influence of noise on
the collective behaviors, and compare these asymptotic regimes to simulations
of the network. We observe that the mean field equations provide an accurate
description of the solutions of the network equations for network sizes as
small as a few hundreds of neurons. In particular, we observe that the level of
noise in the system qualitatively modifies its collective behavior, producing
for instance synchronized oscillations of the whole network, desynchronization
of oscillating regimes, and stabilization or destabilization of stationary
solutions. These results shed a new light on the role of noise in shaping
collective dynamics of neurons, and gives us clues for understanding similar
phenomena observed in biological networks
Quantum Molecular Dynamics Approach to the Nuclear Matter Below the Saturation Density
Quantum molecular dynamics is applied to study the ground state properties of
nuclear matter at subsaturation densities. Clustering effects are observed as
to soften the equation of state at these densities. The structure of nuclear
matter at subsaturation density shows some exotic shapes with variation of the
density.Comment: 21 pages of Latex (revtex), 9 Postscript figure
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