3,237 research outputs found
Isovector Vibrations in Nuclear Matter at Finite Temperature
We consider the propagation and damping of isovector excitations in heated
nuclear matter within the Landau Fermi-liquid theory. Results obtained for
nuclear matter are applied to calculate the Giant Dipole Resonance (GDR) at
finite temperature in heavy spherical nuclei within Steinwedel and Jensen
model.
The centroid energy of the GDR slightly decreases with increasing temperature
and the width increases as for temperatures MeV in agreement with
recent experimental data for GDR in Pb and Sn.
The validity of the method for other Fermi fluids is finally suggested.Comment: gzipped LaTeX file with text: 19 pages, 26 blocks; 3 gzipped *.ps
files with figures: 50 block
Nuclear collective dynamics within Vlasov approach
We discuss, in an investigation based on Vlasov equation, the properties of
the isovector modes in nuclear matter and atomic nuclei in relation with the
symmetry energy. We obtain numerically the dipole response and determine the
strength function for various systems, including a chain of Sn isotopes. We
consider for the symmetry energy three parametrizations with density providing
similar values at saturation but which manifest very different slopes around
this point. In this way we can explore how the slope affects the collective
response of finite nuclear systems. We focus first on the dipole polarizability
and show that while the model is able to describe the expected mass dependence,
A^{5/3}, it also demonstrates that this quantity is sensitive to the slope
parameter of the symmetry energy. Then, by considering the Sn isotopic chain,
we investigate the emergence of a collective mode, the Pygmy Dipole Resonance
(PDR), when the number of neutrons in excess increases. We show that the total
energy-weighted sum rule exhausted by this mode has a linear dependence with
the square of isospin I=(N-Z)/A, again sensitive to the slope of the symmetry
energy with density. Therefore the polarization effects in the isovector
density have to play an important role in the dynamics of PDR. These results
provide additional hints in the investigations aiming to extract the properties
of symmetry energy below saturation.Comment: 7 pages, 6 figure
Dynamics of Phase Transitions in Asymmetric Nuclear Matter
We present several possibilities offered by the reaction dynamics of
dissipative heavy ion collisions to study in detail the symmetry term of the
nuclear equation of state, . In particular we discuss isospin effects on
the nuclear liquid-gas phase transition, {\it Isospin Distillation}, and on
collective flows. We stress the importance of a microscopic relativistic
structure of the effective interaction in the isovector channel. The
possibility of an {\it early} transition to deconfined matter in high isospin
density regions is also suggested. We finally select {\it Eleven} observables,
in different beam energy regions, that appear rather sensitive to the isovector
part of the nuclear , in particular in more exclusive experiments.Comment: 8 pages, 7 figures, ISPUN02 Conference, Halong-Vietnam, Nov.20-25
2002, to appear in Nucl.Phys.A. Elsevier Proceedings Styl
Collective Flows in a Transport Approach
We introduce a transport approach at fixed shear viscosity to entropy ratio
\etas to study the generation of collective flows in ultra-relativistic
heavy-ion collisions. Transport theory supplies a covariant approach valid also
at large \etas and at intermediate transverse momentum , where
deviations from equilibrium is no longer negligible. Such an approach shows
that at RHIC energies a temperature dependent \etas enhances significantly
the respect to the case of constant \etas. Furthermore if NJL
chiral dynamics is self-consistently implemented we show that it does not
modify the relation between and \etas.Comment: 4 pages, 4 figures, Proceedings of Hot Quarks 2010, 21-26 June 2010
Las Londe Les Maures; to appear in Journal of Physics: Conference Serie
Pulse-like and crack-like ruptures in experiments mimicking crustal earthquakes
Theoretical studies have shown that the issue of rupture modes has important implications for fault constitutive laws, stress conditions on faults, energy partition and heat generation during earthquakes, scaling laws, and spatiotemporal complexity of fault slip. Early theoretical models treated earthquakes as crack-like ruptures, but seismic inversions indicate that earthquake ruptures may propagate in a self-healing pulse-like mode. A number of explanations for the existence of slip pulses have been proposed and continue to be vigorously debated. This study presents experimental observations of spontaneous pulse-like ruptures in a homogeneous linear-elastic setting that mimics crustal earthquakes; reveals how different rupture modes are selected based on the level of fault prestress; demonstrates that both rupture modes can transition to supershear speeds; and advocates, based on comparison with theoretical studies, the importance of velocity-weakening friction for earthquake dynamics
Friction and Roughness of a Melting Rock Surface
Under extreme conditions like those encountered during earthquake
slip, frictional melt is likely to occur. It has been observed on
ancient faults that the melt is mostly extruded toward local extensional
jogs or lateral tension cracks. In the case of laboratory experiments
with a rotary shear apparatus, melt is extruded from the sample borders.
When this happens, a thin and irregular melt layer is formed whereby
the normal load is still in part supported by contact asperities under
an incipient yield condition (as in dry friction models), but also,
in the interstices between asperities, by the pressure of the viscous
fluid wetting the interface. In addition, roughness of the surface
is dynamically reshaped by the melting process of an inhomogeneous
material (polymineralic rock). In particular, we argue that the roughness
of the melting surface decreases with melting rate and temperature
gradient perpendicular to the fault. Taking into account the above
conditions, we obtain an expression for the average melt layer thickness
and viscous pressure that may be used in estimates of friction in
the presence of melt. We argue that the ratio of melt thickness to
roughness depends on sliding velocity; such a ratio may be used as
a gauge of slip-rate during fossil earthquakes on faults bearing pseudotachylite
(solidified melt). Finally, we derive an improved analytical solution
for friction in the presence of melt including the effect of roughness
evolution
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