9,399 research outputs found
Transport parameters in neutron stars from in-medium NN cross sections
We present a numerical study of shear viscosity and thermal conductivity of
symmetric nuclear matter, pure neutron matter and -stable nuclear
matter, in the framework of the Brueckner theory. The calculation of in-medium
cross sections and nucleon effective masses is performed with a consistent two
and three body interaction. The investigation covers a wide baryon density
range as requested in the applications to neutron stars. The results for the
transport coefficients in -stable nuclear matter are used to make
preliminary predictions on the damping time scales of non radial modes in
neutron stars
Microscopic three-body force for asymmetric nuclear matter
Brueckner calculations including a microscopic three-body force have been
extended to isospin asymmetric nuclear matter. The effects of the three-body
force on the equation of state and on the single-particle properties of nuclear
matter are discussed with a view to possible applications in nuclear physics
and astrophysics. It is shown that, even in the presence of the three-body
force, the empirical parabolic law of the energy per nucleon vs isospin
asymmetry is fulfilled in the whole asymmetry range
up to high densities. The three-body force provides a strong
enhancement of symmetry energy increasing with the density in good agreement
with relativistic approaches. The Lane's assumption that proton and neutron
mean fields linearly vary vs the isospin parameter is violated at high density
in the presence of the three-body force. Instead the momentum dependence of the
mean fields is rather insensitive to three body force which brings about a
linear isospin deviation of the neutron and proton effective masses. The
isospin effects on multifragmentation events and collective flows in heavy-ion
collisions are briefly discussed along with the conditions for direct URCA
processes to occur in the neutron-star cooling.Comment: 11 pages, 7 figure
Su(3) Algebraic Structure of the Cuprate Superconductors Model based on the Analogy with Atomic Nuclei
A cuprate superconductor model based on the analogy with atomic nuclei was
shown by Iachello to have an structure. The mean-field approximation
Hamiltonian can be written as a linear function of the generators of
algebra. Using algebraic method, we derive the eigenvalues of the reduced
Hamiltonian beyond the subalgebras and of
algebra. In particular, by considering the coherence between s- and d-wave
pairs as perturbation, the effects of coherent term upon the energy spectrum
are investigated
Microscopic three-body forces and kaon condensation in cold neutrino-trapped matter
We investigate the composition and the equation of state of the kaon
condensed phase in neutrino-free and neutrino-trapped star matter within the
framework of the Brueckner-Hartree-Fock approach with three-body forces. We
find that neutrino trapping shifts the onset density of kaon condensation to a
larger baryon density, and reduces considerably the kaon abundance. As a
consequence, when kaons are allowed, the equation of state of neutrino-trapped
star matter becomes stiffer than the one of neutrino free matter. The effects
of different three-body forces are compared and discussed. Neutrino trapping
turns out to weaken the role played by the symmetry energy in determining the
composition of stellar matter, and thus reduces the difference between the
results obtained by using different three-body forces.Comment: 9 pages, 7 figures, accepted in Phys. Rev.
Landau parameters of nuclear matter in the spin and spin-isospin channels
The equation of state of spin and isospin polarized nuclear matter is
determined in the framework of the Brueckner theory including three-body
forces. The Landau parameters in the spin and spin-isospin sectors are derived
as a function of the baryonic density. The results are compared with the
Gamow-Teller collective modes. The relevance of and for neutron
stars is shortly discussed, including the magnetic susceptibility and the
neutron star cooling.Comment: 2 pages, 2 figures, RevTex4 forma
Transparent magnetic state in single crystal Nd(1.85)Ce(0.15)CuO(4-y) superconductors
Several experimental studies have been reported as evidence of Josephson coupling between the superconducting layers in the highly anisotropic oxide such as the Bi2Sr2CaCu2O8 and Tl2Ba2CuO6 systems. These include the large penetration depth of 100 mu m measured, ac and dc Josephson effects. Recently two critical temperatures corresponding to Josephson coupling in between the layers and the Berezinskii-Kosterlitz-Thouless transition in the ab-plane have been directly observed in the transport measurements. If the field is applied parallel to the superconducting layers, the magnetic excitation is not the conventional Abrikosov vortices, but the Josephson vortices which extend lambda(sub ab) in the c-axis direction and lambda(sub J) = gamma s in the plane (s is the interlayer distance, gamma is the anisotropy constant). Because of the weak screening effect associated with the Josephson vortices, there have been predictions of magnetic transparent states at magnetic field above a characteristic field H(sub J), a behavior distinctively different from that of the type-II superconductors. In this paper, we report an experimental result which illustrates a transition from the Meissner state to the magnetic transparent state in single crystal of Nd(1.85)Ce(0.15)CuO(4-y). Magnetization has been measured as a function of temperature and field in the magnetic field parallel or close to ab-plane geometry. For a fixed magnetic field, the magnetization shows a two-step transition in M(T); for a fixed temperature, the magnetization shows an abrupt change to almost zero value above a characteristic field H(sub J), an indication of magnetic transparent state. The data of magnetization as a function of field clearly deviates from the behavior predicted by the Abrikosov theory for type-II superconductors. Instead, the data fit well into the picture of Josephson decoupling between the CuO2 layers
Dynamic fluctuations coincide with periods of high and low modularity in resting-state functional brain networks
We investigate the relationship of resting-state fMRI functional connectivity
estimated over long periods of time with time-varying functional connectivity
estimated over shorter time intervals. We show that using Pearson's correlation
to estimate functional connectivity implies that the range of fluctuations of
functional connections over short time scales is subject to statistical
constraints imposed by their connectivity strength over longer scales. We
present a method for estimating time-varying functional connectivity that is
designed to mitigate this issue and allows us to identify episodes where
functional connections are unexpectedly strong or weak. We apply this method to
data recorded from participants, and show that the number of
unexpectedly strong/weak connections fluctuates over time, and that these
variations coincide with intermittent periods of high and low modularity in
time-varying functional connectivity. We also find that during periods of
relative quiescence regions associated with default mode network tend to join
communities with attentional, control, and primary sensory systems. In
contrast, during periods where many connections are unexpectedly strong/weak,
default mode regions dissociate and form distinct modules. Finally, we go on to
show that, while all functional connections can at times manifest stronger
(more positively correlated) or weaker (more negatively correlated) than
expected, a small number of connections, mostly within the visual and
somatomotor networks, do so a disproportional number of times. Our statistical
approach allows the detection of functional connections that fluctuate more or
less than expected based on their long-time averages and may be of use in
future studies characterizing the spatio-temporal patterns of time-varying
functional connectivityComment: 47 Pages, 8 Figures, 4 Supplementary Figure
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