17,883 research outputs found
The nuclear pseudospin symmetry along an isotopic chain
We investigate the isospin dependence of pseudospin symmetry in the chain of
tin isotopes (from Sn until Sn). Using a Woods-Saxon
parametrization of the nuclear potential for these isotopes we study in detail
the effect of the vector-isovector and Coulomb potentials in the energy
splittings of neutron and proton pseudospin partners in the isotopic chain. We
conclude that the realization of nuclear pseudospin symmetry does not change
considerably with the mass number, and is always favored for neutrons. We also
find that the potential accounts for essentially all the pseudospin
isospin asymmetry observed and that the Coulomb potential plays a negligible
role in this asymmetry. This can be explained by the dynamical nature of
pseudospin symmetry in nuclei, namely the dependence of the pseudospin
splittings on the shape of the nuclear mean-field potential.Comment: 4 pages, 4 figures, to be published in Brazilian Journal of Physic
Dynamical nature of the nuclear pseudospin and its isospin asymmetry
Pseudospin symmetry in nuclei is investigated by solving the Dirac equation
with Woods-Saxon scalar and vector radial potentials. We relate the pseudospin
interaction with a pseudospin-orbit term in a Schroedinger-like equation for
the lower component of the Dirac spinor. We show that this term gives a large
contribution to the energy splittings of pseudospin partners, so that the near
pseudospin degeneracy arises from a significant cancellation among the
different terms in that equation. This is a manifestation of the dynamical
character of this symmetry in the nucleus. We analyze the isospin dependence of
the pseudospin symmetry in a nuclear isotope chain by including a
vector-isovector potential V_rho and a Coulomb potential and conclude that
V_rho gives the main contribution to the observed pseudospin isospin asymmetry.Comment: 4 pages, 2 figures, uses World Scientific style file. Contribution
presented at the VIII International Workshop on Hadron Physics, Bento
Goncalves, RS, Brazil, April 14-19, 2002. To be published by World Scientific
in the proceedings of the "International Workshop on Hadron Physics 2002
Transport in tunnelling recombination junctions: a combined computer simulation study
The implementation of trap-assisted tunneling of charge carriers into numerical simulators ASPIN and D-AMPS is briefly described. Important modeling details are highlighted and compared. In spite of the considerable differences in both approaches, the problems encountered and their solutions are surprisingly similar. Simulation results obtained for several tunneling recombination junctions made of amorphous silicon (a-Si), amorphous silicon carbide (a-SiC), or microcrystalline silicon (”c-Si) are analyzed. Identical conclusions can be drawn using either of the simulators. Realistic performances of a-Si/a-Si tandem solar cells can be reproduced with simulation programs by assuming that extended-state mobility increases exponentially with the electric field. The same field-enhanced mobilities are needed in single tunneling recombination junctions in order to achieve measured current levels. Temperature dependence of the current-voltage characteristics indicates that the activation energy of enhanced mobilities should be determined. Apparent discrepancies between simulation results and measurements are explained and eliminated making use of Gillâs law. For application in tandem and triple solar cell structures, tunneling recombination junctions made of (”c-Si) are the most promising of all examined structures.Fil: Vukadinovic, M.. University of Ljubljana; EsloveniaFil: Smole, F.. University of Ljubljana; EsloveniaFil: TopiÄ, M.. University of Ljubljana; EsloveniaFil: Schropp, R. E. .. Utrecht University; PaĂses BajosFil: Rubinelli, Francisco Alberto. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Santa Fe. Instituto de Desarrollo TecnolĂłgico para la Industria QuĂmica. Universidad Nacional del Litoral. Instituto de Desarrollo TecnolĂłgico para la Industria QuĂmica; Argentin
Challenging the weak cosmic censorship conjecture with charged quantum particles
Motivated by the recent attempts to violate the weak cosmic censorship
conjecture for near-extreme black-holes, we consider the possibility of
overcharging a near-extreme Reissner-Nordstr\"om black hole by the quantum
tunneling of charged particles. We consider the scattering of spin-0 and
spin-1/2 particles by the black hole in a unified framework and obtain
analytically, for the first time, the pertinent reflection and transmission
coefficients without any small charge approximation. Based on these results, we
propose some gedanken experiments that could lead to the violation of the weak
cosmic censorship conjecture due to the (classically forbidden) absorption of
small energy charged particles by the black hole. As for the case of scattering
in Kerr spacetimes, our results demonstrate explicitly that scalar fields are
subject to (electrical) superradiance phenomenon, while spin-1/2 fields are
not. Superradiance impose some limitations on the gedanken experiments
involving spin-0 fields, favoring, in this way, the mechanisms for creation of
a naked singularity by the quantum tunneling of spin-1/2 charged fermions. We
also discuss the implications that vacuum polarization effects and quantum
statistics might have on these gedanken experiments. In particular, we show
that they are not enough to prevent the absorption of incident small energy
particles and, consequently, the formation of a naked singularity.Comment: 9 pages; Final version to appear in PR
Spin and pseudospin symmetries in the antinucleon spectrum of nuclei
Spin and pseudospin symmetries in the spectra of nucleons and antinucleons
are studied in a relativistic mean-field theory with scalar and vector
Woods-Saxon potentials, in which the strength of the latter is allowed to
change. We observe that, for nucleons and antinucleons, the spin symmetry is of
perturbative nature and it is almost an exact symmetry in the physical region
for antinucleons. The opposite situation is found in the pseudospin symmetry
case, which is better realized for nucleons than for antinucleons, but is of
dynamical nature and cannot be viewed in a perturbative way both for nucleons
and antinucleons. This is shown by computing the spin-orbit and
pseudospin-orbit couplings for selected spin and pseudospin partners in both
spectra.Comment: 8 figures, uses revtex 4.1 macro
Tensor coupling and pseudospin symmetry in nuclei
In this work we study the contribution of the isoscalar tensor coupling to
the realization of pseudospin symmetry in nuclei. Using realistic values for
the tensor coupling strength, we show that this coupling reduces noticeably the
pseudospin splittings, especially for single-particle levels near the Fermi
surface. By using an energy decomposition of the pseudospin energy splittings,
we show that the changes in these splittings come by mainly through the changes
induced in the lower radial wave function for the low-lying pseudospin
partners, and by changes in the expectation value of the pseudospin-orbit
coupling term for surface partners. This allows us to confirm the conclusion
already reached in previous studies, namely that the pseudospin symmetry in
nuclei is of a dynamical nature.Comment: 11 pages, 5 figures, uses REVTeX macro
Glassy behaviour in short range lattice models without quenched disorder
We investigate the quenching process in lattice systems with short range
interaction and several crystalline states as ground states. We consider in
particular the following systems on square lattice:
- hard particle (exclusion) model;
- q states planar Potts model.
The system is initially in a homogeneous disordered phase and relaxes toward
a new equilibrium state as soon as the temperature is rapidly lowered. The time
evolution can be described numerically by a stochastic process such as the
Metropolis algorithm. The number of pure, equivalent, ground states is q for
the Potts model and r for the hard particle model, and it is known that for r
or q larger or equal to d+1, the final equilibrium state may be
polycrystalline, i.e. not made of a uniform phase. We find that in addition n_g
and q_g exist such that for r > r_g, or q > q_g the system evolves toward a
glassy state, i.e. a state in which the ratio of the interaction energy among
the different crystalline phases to the total energy of the system never
vanishes; moreover we find indications that r_g=q_g. We infer that q=q_g (and
r=r_g) corresponds to the crossing from second order to discontinuous
transition in the phase diagram of the system.Comment: 10 pages, 3 figure
On the relativistic L-S coupling
The fact that the Dirac equation is linear in the space and time derivatives
leads to the coupling of spin and orbital angular momenta that is of a pure
relativistic nature. We illustrate this fact by computing the solutions of the
Dirac equation in an infinite spherical well, which allows to go from the
relativistic to the non-relativistic limit by just varying the radius of the
well.Comment: LateX2e, 12 pages, 1 figure, accepted in Eur. J. Phy
Ostrogradski approach for the Regge-Teitelboim type cosmology
We present an alternative geometric inspired derivation of the quantum
cosmology arising from a brane universe in the context of {\it geodetic
gravity}. We set up the Regge-Teitelboim model to describe our universe, and we
recover its original dynamics by thinking of such field theory as a
second-order derivative theory. We refer to an Ostrogradski Hamiltonian
formalism to prepare the system to its quantization. Our analysis highlights
the second-order derivative nature of the RT model and the inherited
geometrical aspect of the theory. A canonical transformation brings us to the
internal physical geometry of the theory and induces its quantization
straightforwardly. By using the Dirac canonical quantization method our
approach comprises the management of both first- and second-class constraints
where the counting of degrees of freedom follows accordingly. At the quantum
level our Wheeler-De Witt Wheeler equation agrees with previous results
recently found. On these lines, we also comment upon the compatibility of our
approach with the Hamiltonian approach proposed by Davidson and coworkers.Comment: 11 pages, 2 figures, accepted for publication in Phys. Rev.
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