1,852 research outputs found
Generalized Nonlinear Proca Equation and its Free-Particle Solutions
We introduce a non-linear extension of Proca's field theory for massive
vector (spin ) bosons. The associated relativistic nonlinear wave equation
is related to recently advanced nonlinear extensions of the Schroedinger,
Dirac, and Klein-Gordon equations inspired on the non-extensive generalized
thermostatistics. This is a theoretical framework that has been applied in
recent years to several problems in nuclear and particle physics, gravitational
physics, and quantum field theory. The nonlinear Proca equation investigated
here has a power-law nonlinearity characterized by a real parameter
(formally corresponding to the Tsallis entropic parameter) in such a way that
the standard linear Proca wave equation is recovered in the limit . We derive the nonlinear Proca equation from a Lagrangian that,
besides the usual vectorial field , involves an
additional field . We obtain exact time dependent
soliton-like solutions for these fields having the form of a -plane wave,
and show that both field equations lead to the relativistic energy-momentum
relation for all values of . This suggests
that the present nonlinear theory constitutes a new field theoretical
representation of particle dynamics. In the limit of massless particles the
present -generalized Proca theory reduces to Maxwell electromagnetism, and
the -plane waves yield localized, transverse solutions of Maxwell equations.
Physical consequences and possible applications are discussed
Effects of Random Biquadratic Couplings in a Spin-1 Spin-Glass Model
A spin-1 model, appropriated to study the competition between bilinear
(J_{ij}S_{i}S_{j}) and biquadratic (K_{ij}S_{i}^{2}S_{j}^{2}) random
interactions, both of them with zero mean, is investigated. The interactions
are infinite-ranged and the replica method is employed. Within the
replica-symmetric assumption, the system presents two phases, namely,
paramagnetic and spin-glass, separated by a continuous transition line. The
stability analysis of the replica-symmetric solution yields, besides the usual
instability associated with the spin-glass ordering, a new phase due to the
random biquadratic couplings between the spins.Comment: 16 pages plus 2 ps figure
A experiência da pessoa com dor oncológica na sua transcendência
Objetivo: compreender a experiência da pessoa com dor crónica do foro oncológico na sua transcendência. Método: metodologia de natureza qualitativa, utilizando a fenomenologia. Os participantes foram dez doentes do foro oncológico seguidos na consulta da dor e como instrumento de colheita de dados utilizámos a entrevista parcialmente estruturada. Resultados: a partir do agrupamento das unidades de significação, emergiram os temas centrais sendo um dos temas: a transcendência na experiência da pessoa com dor crónica do foro oncológico e, a partir do agrupamento das unidades de significação para este tema central, foram identificados os subtemas: esperança e força interior e, fé. Conclusão: a esperança é percecionada como uma força interior que conduz ao pensamento positivo. A fé, ao estabelecer uma crença com o transcendente ou o divino é concebida como conforto, como uma fonte de apoio e possibilidade de partilha
Spin-glass phase transition and behavior of nonlinear susceptibility in the Sherrington-Kirkpatrick model with random fields
The behavior of the nonlinear susceptibility and its relation to the
spin-glass transition temperature , in the presence of random fields, are
investigated. To accomplish this task, the Sherrington-Kirkpatrick model is
studied through the replica formalism, within a one-step
replica-symmetry-breaking procedure. In addition, the dependence of the
Almeida-Thouless eigenvalue (replicon) on the random fields
is analyzed. Particularly, in absence of random fields, the temperature
can be traced by a divergence in the spin-glass susceptibility ,
which presents a term inversely proportional to the replicon . As a result of a relation between and , the
latter also presents a divergence at , which comes as a direct consequence
of at . However, our results show that, in the
presence of random fields, presents a rounded maximum at a temperature
, which does not coincide with the spin-glass transition temperature
(i.e., for a given applied random field). Thus, the maximum
value of at reflects the effects of the random fields in the
paramagnetic phase, instead of the non-trivial ergodicity breaking associated
with the spin-glass phase transition. It is also shown that still
maintains a dependence on the replicon , although in a more
complicated way, as compared with the case without random fields. These results
are discussed in view of recent observations in the LiHoYF
compound.Comment: accepted for publication in PR
Reaction cross-section predictions for nucleon induced reactions
A microscopic calculation of the optical potential for nucleon-nucleus
scattering has been performed by explicitly coupling the elastic channel to all
the particle-hole (p-h) excitation states in the target and to all relevant
pickup channels. These p-h states may be regarded as doorway states through
which the flux flows to more complicated configurations, and to long-lived
compound nucleus resonances. We calculated the reaction cross sections for the
nucleon induced reactions on the targets Ca, Ni, Zr and
Sm using the QRPA description of target excitations, coupling to all
inelastic open channels, and coupling to all transfer channels corresponding to
the formation of a deuteron. The results of such calculations were compared to
predictions of a well-established optical potential and with experimental data,
reaching very good agreement. The inclusion of couplings to pickup channels
were an important contribution to the absorption. For the first time,
calculations of excitations account for all of the observed reaction
cross-sections, at least for incident energies above 10 MeV.Comment: 6 pages, 6 figures. Submitted to INPC 2010 Conference Proceeding
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