550 research outputs found
On the description of two-particle transfer in superfluid systems
Exact results of pair transfer probabilities for the Richardson model with
equidistant or random level spacing are presented. The results are then
compared either to particle-particle random phase approximation (ppRPA) in the
normal phase or quasi-particle random phase approximation (QRPA) in the
superfluid phase. We show that both ppRPA and QRPA are globally well
reproducing the exact case although some differences are seen in the superfluid
case. In particular the QRPA overestimates the pair transfer probabilities to
excited states in the vicinity of the normal-superfluid phase transition, which
might explain the difficult in detecting collective pairing phenomena as for
example the Giant Pairing Vibration. The shortcoming of QRPA can be traced back
to the breaking of particle number that is used to incorporate pairing. A
method, based on direct diagonalization of the Hamiltonian in the space of two
quasi-particle projected onto good particle number is shown to improve the
description of pair transfer probabilities in superfluid systems.Comment: 9 pages, 7 figure
Proton-neutron pairing in N=Z nuclei: quartetting versus pair condensation
The isoscalar proton-neutron pairing and isovector pairing, including both
isovector proton-neutron pairing and like-particle pairing, are treated in a
formalism which conserves exactly the particle number and the isospin. The
formalism is designed for self-conjugate (N=Z) systems of nucleons moving in
axially deformed mean fields and interacting through the most general isovector
and isoscalar pairing interactions. The ground state of these systems is
described by a superposition of two types of condensates, i.e., condensates of
isovector quartets, built by two isovector pairs coupled to the total isospin
T=0, and condensates of isoscalar proton-neutron pairs. The comparison with the
exact solutions of realistic isovector-isoscalar pairing Hamiltonians shows
that this ansatz for the ground state is able to describe with high precision
the pairing correlation energies. It is also shown that, at variance with the
majority of Hartree-Fock-Bogoliubov calculations, in the present formalism the
isovector and isoscalar pairing correlations coexist for any pairing
interactions. The competition between the isovector and isoscalar
proton-neutron pairing correlations is studied for N=Z nuclei with the valence
nucleons moving in the and shells and in the major shell above
Sn. We find that in these nuclei the isovector pairing prevail over the
isoscalar pairing, especially for heavier nuclei. However, the isoscalar
proton-neutron correlations are significant in all nuclei and they always
coexist with the isovector pairing correlations.Comment: 12 pages, 1 figur
Quantal corrections to mean-field dynamics including pairing
Extending the stochastic mean-field model by including pairing, an approach
is proposed for describing evolutions of complex many-body systems in terms of
an ensemble of Time-Dependent Hartree-Fock Bogoliubov trajectories which is
determined by incorporating fluctuations in the initial state. Non-linear
evolution of the initial fluctuations provides an approximate description of
quantal correlations and fluctuations of collective observables. Since the
initial-state fluctuations break the particle-number symmetry, the dynamical
description in which pairing correlations play a crucial role is greatly
improved as compare to the mean-field evolution. The approach is illustrated
for a system of particles governed by a pairing Hamiltonian.Comment: 5 pages, 2 figures, To appear in Phys. Rev. C (Rapid communication
Subtraction method in the second random--phase approximation: first applications with a Skyrme energy functional
We make use of a subtraction procedure, introduced to overcome
double--counting problems in beyond--mean--field theories, in the second
random--phase--approximation (SRPA) for the first time. This procedure
guarantees the stability of SRPA (so that all excitation energies are real). We
show that the method fits perfectly into nuclear density--functional theory. We
illustrate applications to the monopole and quadrupole response and to
low--lying and states in the nucleus O. We show that the
subtraction procedure leads to: (i) results that are weakly cutoff dependent;
(ii) a considerable reduction of the SRPA downwards shift with respect to the
random--phase approximation (RPA) spectra (systematically found in all previous
applications). This implementation of the SRPA model will allow a reliable
analysis of the effects of 2 particle--2 hole configurations () on the
excitation spectra of medium--mass and heavy nuclei.Comment: 1 tex, 16 figure
Pairing and specific heat in hot nuclei
The thermodynamics of pairing phase-transition in nuclei is studied in the
canonical ensemble and treating the pairing correlations in a
finite-temperature variation after projection BCS approach (FT-VAP). Due to the
restoration of particle number conservation, the pairing gap and the specific
heat calculated in the FT-VAP approach vary smoothly with the temperature,
indicating a gradual transition from the superfluid to the normal phase, as
expected in finite systems. We have checked that the predictions of the FT-VAP
approach are very accurate when compared to the results obtained by an exact
diagonalization of the pairing Hamiltonian. The influence of pairing
correlations on specific heat is analysed for the isotopes Dy and
Yb. It is shown that the FT-VAP approach, applied with a level
density provided by mean field calculations and supplemented, at high energies,
by the level density of the back-shifted Fermi gas model, can approximate
reasonably well the main properties of specific heat extracted from
experimental data. However, the detailed shape of the calculated specific heat
is rather sensitive to the assumption made for the mean field.Comment: 10 pages, 12 figure
Thermodynamics of small superconductors with fixed particle number
The Variation After Projection approach is applied for the first time to the
pairing hamiltonian to describe the thermodynamics of small systems with fixed
particle number. The minimization of the free energy is made by a direct
diagonalization of the entropy. The Variation After Projection applied at
finite temperature provides a perfect reproduction of the exact canonical
properties of odd or even systems from very low to high temperature.Comment: 4 pages, 3 figure
Opening the Gate to the Serism Project: From Earth to Space and Back
In the context of Space Medicine, the aim of “SERiSM” (Role of the Endocannabinoid System in Reprogramming human pluripotent Stem cells under Microgravity) project, selected by the Italian Space Agency, was to study the involvement of the endocannabinoid system (ECS) in the osteogenic differentiation under real microgravity. An innovative and easily accessible stem cell model derived from human blood (human blood-derived stem cells, hBDSCs) was used to this purpose. This model is autologous and possesses a remarkable proliferative and differentiative capacity underground gravity conditions, with high therapeutic potential for bone degenerative diseases. ECS is a fine network of proteins that interact to regulate the endogenous levels of lipid mediators, collectively termed endocannabinoids (eCBs), which in turn are involved in cell communication and in the mechanisms governing the switch between cell life and death. In the frame of the VITA mission, led by European Space Agency (ESA) astronaut Paolo Nespoli, we analyzed the differentiation process also under microgravity condition and evaluated the expression of ECS proteins through immunoassay methods. Our results demonstrate that some elements of the ECS are modulated during the differentiation process and in microgravity, supporting the idea that increased levels of anandamide are indeed need to stimulate type-1 cannabinoid receptor. In conclusion, microgravity could drive endocannabinoid signalling in the former stages of hBDSCs differentiation
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