53,766 research outputs found
Nucleon-nucleon potentials in phase-space representation
A phase-space representation of nuclear interactions, which depends on the
distance and relative momentum of the nucleons, is
presented. A method is developed that permits to extract the interaction
from antisymmetrized matrix elements given in a spherical
basis with angular momentum quantum numbers, either in momentum or coordinate
space representation. This representation visualizes in an intuitive way the
non-local behavior introduced by cutoffs in momentum space or renormalization
procedures that are used to adapt the interaction to low momentum many-body
Hilbert spaces, as done in the unitary correlation operator method or with the
similarity renormalization group. It allows to develop intuition about the
various interactions and illustrates how the softened interactions reduce the
short-range repulsion in favor of non-locality or momentum dependence while
keeping the scattering phase shifts invariant. It also reveals that these
effective interactions can have undesired complicated momentum dependencies at
momenta around and above the Fermi momentum. Properties, similarities and
differences of the phase-space representations of the Argonne and the N3LO
chiral potential, and their UCOM and SRG derivatives are discussed
From nucleon-nucleon interaction matrix elements in momentum space to an operator representation
Starting from the matrix elements of the nucleon-nucleon interaction in
momentum space we present a method to derive an operator representation with a
minimal set of operators that is required to provide an optimal description of
the partial waves with low angular momentum. As a first application we use this
method to obtain an operator representation for the Argonne potential
transformed by means of the unitary correlation operator method and discuss the
necessity of including momentum dependent operators. The resulting operator
representation leads to the same results as the original momentum space matrix
elements when applied to the two-nucleon system and various light nuclei. For
applications in fermionic and antisymmetrized molecular dynamics, where an
operator representation of a soft but realistic effective interaction is
indispensable, a simplified version using a reduced set of operators is given
Evaluation of roughness lengths at the NSSL- WKY meteorological tower
Wind and temperature sensors installed on NSSL-WKY /Oklahoma City/ meteorological tower to evaluate roughness length
Evidence for crossed Andreev reflection in superconductor-ferromagnet hybrid structures
We have measured the non-local resistance of aluminum-iron spin-valve
structures fabricated by e-beam lithography and shadow evaporation. The sample
geometry consists of an aluminum bar with two or more ferromagnetic wires
forming point contacts to the aluminum at varying distances from each other. In
the normal state of aluminum, we observe a spin-valve signal which allows us to
control the relative orientation of the magnetizations of the ferromagnetic
contacts. In the superconducting state, at low temperatures and excitation
voltages well below the gap, we observe a spin-dependent non-local resistance
which decays on a smaller length scale than the normal-state spin-valve signal.
The sign, magnitude and decay length of this signal is consistent with
predictions made for crossed Andreev reflection (CAR).Comment: RevTeX, 4 page
Electron shielding studies - Experimental program Technical summary report, 1 Aug. 1968 - 31 Dec. 1969
Electron shielding and bremsstrahlung energy spectra for tin, gold, and silve
Nonaxisymmetric Neutral Modes in Rotating Relativistic Stars
We study nonaxisymmetric perturbations of rotating relativistic stars.
modeled as perfect-fluid equilibria. Instability to a mode with angular
dependence sets in when the frequency of the mode vanishes. The
locations of these zero-frequency modes along sequences of rotating stars are
computed in the framework of general relativity. We consider models of
uniformly rotating stars with polytropic equations of state, finding that the
relativistic models are unstable to nonaxisymmetric modes at significantly
smaller values of rotation than in the Newtonian limit. Most strikingly, the
m=2 bar mode can become unstable even for soft polytropes of index , while in Newtonian theory it becomes unstable only for stiff polytropes
of index . If rapidly rotating neutron stars are formed by the
accretion-induced collapse of white dwarfs, instability associated with these
nonaxisymmetric, gravitational-wave driven modes may set an upper limit on
neutron-star rotation. Consideration is restricted to perturbations that
correspond to polar perturbations of a spherical star. A study of axial
perturbations is in progress.Comment: 57 pages, 9 figure
On the Cooling of the Neutron Star in Cassiopeia A
We demonstrate that the high-quality cooling data observed for the young
neutron star in the supernova remnant Cassiopeia A over the past 10 years--as
well as all other reliably known temperature data of neutron stars--can be
comfortably explained within the "nuclear medium cooling" scenario. The cooling
rates of this scenario account for medium-modified one-pion exchange in dense
matter and polarization effects in the pair-breaking formations of superfluid
neutrons and protons. Crucial for the successful description of the observed
data is a substantial reduction of the thermal conductivity, resulting from a
suppression of both the electron and nucleon contributions to it by medium
effects. We also find that possibly in as little as about ten years of
continued observation, the data may tell whether or not fast cooling processes
are active in this neutron star.Comment: 4 pages, 3 figure
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