607 research outputs found
Critical point for the strong field magnetoresistance of a normal conductor/perfect insulator/perfect conductor composite with a random columnar microstructure
A recently developed self-consistent effective medium approximation, for
composites with a columnar microstructure, is applied to such a
three-constituent mixture of isotropic normal conductor, perfect insulator, and
perfect conductor, where a strong magnetic field {\bf B} is present in the
plane perpendicular to the columnar axis. When the insulating and perfectly
conducting constituents do not percolate in that plane, the
microstructure-induced in-plane magnetoresistance is found to saturate for
large {\bf B}, if the volume fraction of the perfect conductor is greater
than that of the perfect insulator . By contrast, if , that
magnetoresistance keeps increasing as without ever saturating. This
abrupt change in the macroscopic response, which occurs when , is a
critical point, with the associated critical exponents and scaling behavior
that are characteristic of such points. The physical reasons for the singular
behavior of the macroscopic response are discussed. A new type of percolation
process is apparently involved in this phenomenon.Comment: 4 pages, 1 figur
Electron Transfer in Donor-Acceptor Systems: Many-Particle Effects and Influence of Electronic Correlations
We investigate electron transfer processes in donor-acceptor systems with a
coupling of the electronic degrees of freedom to a common bosonic bath. The
model allows to study many-particle effects and the influence of the local
Coulomb interaction U between electrons on donor and acceptor sites. Using the
non-perturbative numerical renormalization group approach we find distinct
differences between the electron transfer characteristics in the single- and
two-particle subspaces. We calculate the critical electron-boson coupling
alpha_c as a function of and show results for density-density correlation
functions in the whole parameter space. The possibility of many-particle
(bipolaronic) and Coulomb-assisted transfer is discussed.Comment: 4 pages, 4 figure
Effects of the magnetic moment interaction between nucleons on observables in the 3N continuum
The influence of the magnetic moment interaction of nucleons on
nucleon-deuteron elastic scattering and breakup cross sections and on elastic
scattering polarization observables has been studied. Among the numerous
elastic scattering observables only the vector analyzing powers were found to
show a significant effect, and of opposite sign for the proton-deuteron and
neutron-deuteron systems. This finding results in an even larger discrepancy
than the one previously established between neutron-deuteron data and
theoretical calculations. For the breakup reaction the largest effect was found
for the final-state-interaction cross sections. The consequences of this
observation on previous determinations of the ^1S_0 scattering lengths from
breakup data are discussed.Comment: 24 pages, 6 ps figures, 1 png figur
Polarization observables in the semiexclusive photoinduced three-body breakup of 3He
The photon and 3He analyzing powers as well as spin correlation coefficients
in the semiexclusive three-body photodisintegration of 3He are investigated for
incoming photon laboratory energies E=12, 40 and 120 MeV. The nuclear states
are obtained by solving three-body Faddeev equations with the AV18
nucleon-nucleon potential alone or supplemented with the UrbanaIX three-nucleon
force. Explicit pi- and rho-meson exchange currents are taken into account, but
we also compare to other models of the electromagnetic current. In some
kinematical conditions we have found strong effects of the three-nucleon force
for the 3He analyzing power and spin correlation coefficients, as well strong
sensitivities to the choice of the currents. This set of predictions should be
a useful guidance for the planning of measurements. In addition, we compare our
results for two-body 3He breakup induced by polarized photons with a few
existing data.Comment: 23 pages, 16 figure
Earth-like Habitats in Planetary Systems
Understanding the concept of habitability is related to an evolutionary
knowledge of the particular planet-in-question. Additional indications
so-called "systemic aspects" of the planetary system as a whole governs a
particular planet's claim on habitability. Here we focus on such systemic
aspects and discuss their relevance to the formation of an 'Earth-like'
habitable planet. We summarize our results obtained by lunar sample work and
numerical models within the framework of the Research Alliance "Planetary
Evolution and Life". We consider various scenarios which simulate the dynamical
evolution of the Solar System and discuss the likelihood of forming an
Earth-like world orbiting another star. Our model approach is constrained by
observations of the modern Solar System and the knowledge of its history.
Results suggest that the long-term presence of terrestrial planets is
jeopardized due to gravitational interactions if giant planets are present. But
habitability of inner rocky planets may be supported in those planetary systems
hosting giant planets.
Gravitational interactions within a complex multiple-body structure including
giant planets may supply terrestrial planets with materials which formed in the
colder region of the proto-planetary disk. During these processes, water, the
prime requisite for habitability, is delivered to the inner system. This may
occur either during the main accretion phase of terrestrial planets or via
impacts during a post-accretion bombardment. Results for both processes are
summarized and discussed with reference to the lunar crater record.
Starting from a scenario involving migration of the giant planets this
contribution discusses the delivery of water to Earth, the modification of
atmospheres by impacts in a planetary system context and the likelihood of the
existence of extrasolar Earth-like habitable worlds.Comment: 36 Pages, 6 figures, 2014, Special Issue in Planetary and Space
Science on the Helmholtz Research Alliance on Planetary Evolution and Lif
Neutron-proton analyzing power at 12 MeV and inconsistencies in parametrizations of nucleon-nucleon data
We present the most accurate and complete data set for the analyzing power
Ay(theta) in neutron-proton scattering. The experimental data were corrected
for the effects of multiple scattering, both in the center detector and in the
neutron detectors. The final data at En = 12.0 MeV deviate considerably from
the predictions of nucleon-nucleon phase-shift analyses and potential models.
The impact of the new data on the value of the charged pion-nucleon coupling
constant is discussed in a model study.Comment: Six pages, four figures, one table, to be published in Physics
Letters
The decay of quadrupole-octupole states in Ca and Ce
Background: Two-phonon excitations originating from the coupling of two
collective one-phonon states are of great interest in nuclear structure
physics. One possibility to generate low-lying excitations is the coupling
of quadrupole and octupole phonons.
Purpose: In this work, the -decay behavior of candidates for the
state in the doubly-magic nucleus Ca and in
the heavier and semi-magic nucleus Ce is investigated.
Methods: experiments have been carried out at the
High Intensity -ray Source (HIS) facility in combination with
the high-efficiency -ray spectroscopy setup consisting of
HPGe and LaBr detectors. The setup enables the acquisition of
- coincidence data and, hence, the detection of direct decay
paths.
Results: In addition to the known ground-state decays, for Ca the
decay into the state was observed, while for Ce the direct
decays into the and the state were detected. The experimentally
deduced transition strengths and excitation energies are compared to
theoretical calculations in the framework of EDF theory plus QPM approach and
systematically analyzed for isotones. In addition, negative parities for
two states in Ca were deduced simultaneously.
Conclusions: The experimental findings together with the theoretical
calculations support the two-phonon character of the excitation in the
light-to-medium-mass nucleus Ca as well as in the stable even-even
nuclei.Comment: 11 pages, 6 figures, as accepted in Phys. Rev.
Isospin properties of electric dipole excitations in 48Ca
Two different experimental approaches were combined to study the electric
dipole strength in the doubly-magic nucleus 48Ca below the neutron threshold.
Real-photon scattering experiments using bremsstrahlung up to 9.9 MeV and
nearly mono-energetic linearly polarized photons with energies between 6.6 and
9.51 MeV provided strength distribution and parities, and an
(\alpha,\alpha'\gamma) experiment at E_{\alpha}=136 MeV gave cross sections for
an isoscalar probe. The unexpected difference observed in the dipole response
is compared to calculations using the first-order random-phase approximation
and points to an energy-dependent isospin character. A strong isoscalar state
at 7.6 MeV was identified for the first time supporting a recent theoretical
prediction.Comment: 6 pages, 5 figures, as accepted in Phys. Lett.
Analyzing power in nucleon-deuteron scattering and three-nucleon forces
Three-nucleon forces have been considered to be one possibility to resolve
the well known discrepancy between experimental values and theoretical
calculations of the nucleon analyzing power in low energy nucleon-deuteron
scattering. In this paper, we investigate possible effects of two-pion exchange
three-nucleon forces on the analyzing power and the differential cross section.
We found that the reason for different effects on the analyzing power by
different three-nucleon forces found in previous calculations is related to the
existence of the contact term. Effects of some variations of two-pion exchange
three-nucleon forces are investigated. Also, an expression for the measure of
the nucleon analyzing power with quartet P-wave phase shifts is presented.Comment: 11 pages including 2 eps figures, use epsfig.sty, to appear in Phys.
Rev.
A distributed environment for developing, teaching, and learning algorithmic concepts
Thesis (S.B. and M.Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1998.Includes bibliographical references (p. 78-80).by Nicholas J. Tornow.S.B.and M.Eng
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