10,481 research outputs found
On the Mass to Charge Ratio of Neutron Cores and Heavy Nuclei
We determine theoretically the relation between the total number of protons
and the mass number (the charge to mass ratio) of nuclei and
neutron cores with the model recently proposed by Ruffini et al. (2007) and we
compare it with other versus relations: the empirical one, related to
the Periodic Table, and the semi-empirical relation, obtained by minimizing the
Weizs\"{a}cker mass formula. We find that there is a very good agreement
between all the relations for values of typical of nuclei, with differences
of the order of per cent. Our relation and the semi-empirical one are in
agreement up to ; for higher values, we find that the two relations
differ. We interprete the different behaviour of our theoretical relation as a
result of the penetration of electrons (initially confined in an external
shell) inside the core, that becomes more and more important by increasing ;
these effects are not taken into account in the semi-empirical mass-formula.Comment: Some misprints of the published version corrected (value of nuclear
density and eq. 7). Talk given at the 4th Italian-Sino Workshop, July 20-30
(2007), Pescara (Italy
Crystalline free energies of micelles of diblock copolymer solutions
We report a characterization of the relative stability and structural
behavior of various micellar crystals of an athermal model of AB-diblock
copolymers in solution. We adopt a previously devel- oped coarse-graining
representation of the chains which maps each copolymer on a soft dumbbell.
Thanks to this strong reduction of degrees of freedom, we are able to
investigate large aggregated systems, and for a specific length ratio of the
blocks f = MA/(MA + MB) = 0.6, to locate the order-disorder transition of the
system of micelles. Above the transition, mechanical and thermal properties are
found to depend on the number of particles per lattice site in the simulation
box, and the application of a recent methodology for multiple occupancy
crystals (B.M. Mladek et al., Phys. Rev. Lett. 99, 235702 (2007)) is necessary
to correctly define the equilibrium state. Within this scheme we have performed
free energy calculations at two reduced density {\rho}/{\rho}\ast = 4,5 and for
several cubic structures as FCC,BCC,A15. At both densities, the BCC symmetry is
found to correspond to the minimum of the unconstrained free energy, that is to
the stable symmetry among the few considered, while the A15 structure is almost
degenerate, indicating that the present sys- tem prefers to crystallize in less
packed structures. At {\rho}/{\rho}\ast = 4 close to melting, the Lindemann
ratio is fairly high (~ 0.29) and the concentration of vacancies is roughly 6%.
At {\rho}/{\rho}\ast = 5 the mechanical stability of the stable BCC structure
increases and the concentration of vacancies ac- cordingly decreases. The ratio
of the corona layer thickness to the core radius is found to be in good
agreement with experimental data for poly(styrene-b-isoprene)(22-12) in
isoprene selective solvent which is also reported to crystallize in the BCC
structure
Unexpected effect of Ru-substitution in lightly doped manganites
In this Communication we report about the unexpected effect of ruthenium
doping in sodium ligthly-doped manganites. This effect seems to be in contrast
with the usual model applied to describe the effect of this magnetic ion into
the manganite structure. We propose a possible compensation mechanism which
seems also able to describe other peculiar features encountered in these
materials.Comment: 3 pages, 2 Figures to appear in ChemCom
Quantum Pattern Retrieval by Qubit Networks with Hebb Interactions
Qubit networks with long-range interactions inspired by the Hebb rule can be
used as quantum associative memories. Starting from a uniform superposition,
the unitary evolution generated by these interactions drives the network
through a quantum phase transition at a critical computation time, after which
ferromagnetic order guarantees that a measurement retrieves the stored memory.
The maximum memory capacity p of these qubit networks is reached at a memory
density p/n=1.Comment: To appear in Physical Review Letter
Minimal Models for a Superconductor-Insulator Conformal Quantum Phase Transition
Conformal field theories do not only classify 2D classical critical behavior
but they also govern a certain class of 2D quantum critical behavior. In this
latter case it is the ground state wave functional of the quantum theory that
is conformally invariant, rather than the classical action. We show that the
superconducting-insulating (SI) quantum phase transition in 2D Josephson
junction arrays (JJAs) is a (doubled) Gaussian conformal quantum critical
point. The quantum action describing this system is a doubled
Maxwell-Chern-Simons model in the strong coupling limit. We also argue that the
SI quantum transitions in frustrated JJAs realize the other possible
universality classes of conformal quantum critical behavior, corresponding to
the unitary minimal models at central charge .Comment: 4 pages, no figure
Unlocking the deployment of spectrum sharing with a policy enforcement framework
Spectrum sharing has been proposed as a promising way to increase the efficiency of spectrum usage by allowing incumbent operators (IOs) to share their allocated radio resources with licensee operators (LOs), under a set of agreed rules. The goal is to maximize a common utility, such as the sum rate throughput, while maintaining the level of service required by the IOs. However, this is only guaranteed under the assumption that all âplayersârespect the agreed sharing rules. In this paper, we propose a comprehensive framework for licensed shared access (LSA) networks that discourages LO misbehavior. Our framework is built around three core functions: misbehavior detection via the employment of a dedicated sensing network; a penalization function; and, a behavior-driven resource allocation. To the best of our knowledge, this is the first time that these components are combined for the monitoring/policing of the spectrum under the LSA framework. Moreover, a novel simulator for LSA is provided as an open access tool, serving the purpose of testing and validating our proposed techniques via a set of extensive system-level simulations in the context of mobile network operators, where IOs and several competing LOs are considered. The results demonstrate that violation of the agreed sharing rules can lead to a great loss of resources for the misbehaving LOs, the amount of which is controlled by the system. Finally, we promote that including a policy enforcement function as part of the spectrum sharing system can be beneficial for the LSA system, since it can guarantee compliance with the spectrum sharing rules and limit the short-term benefits arising from misbehavior
Solving QCD evolution equations in rapidity space with Markovian Monte Carlo
This work covers methodology of solving QCD evolution equation of the parton
distribution using Markovian Monte Carlo (MMC) algorithms in a class of models
ranging from DGLAP to CCFM. One of the purposes of the above MMCs is to test
the other more sophisticated Monte Carlo programs, the so-called Constrained
Monte Carlo (CMC) programs, which will be used as a building block in the
parton shower MC. This is why the mapping of the evolution variables (eikonal
variable and evolution time) into four-momenta is also defined and tested. The
evolution time is identified with the rapidity variable of the emitted parton.
The presented MMCs are tested independently, with ~0.1% precision, against the
non-MC program APCheb especially devised for this purpose.Comment: version compatible with with the erratum in Acta Physica Polonic
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