Multienergetic continuous time random walk

We have extended the CTRW theory of Montroll and Weiss including the effect of extra variables, like the energy. This MCTRW scheme can be written in a simple matrix notation, that simplifies its solution. As an example of their usefulness we have studied a two-energy-group neutron diffusion problem. This has shown the peculiarities of the transient behaviour for the variance of the probability distribution, due to the coupling between the groups.Comisión Nacional de Energiá AtómicaComisión Nacional de Energía Atómica and Universidad Nacional de Cuyo     

Two-body problem for Weber-like interactions

The problem of two moving bodies interacting through a Weber-like force is presented. Trajectories are obtained analytically once relativistic and quantic considerations are neglected. The main results are that in the case of limited trajectories, in general, they are not closed and in the case of open trajectories, the deflection angles are not the same for similar particles with given energies and angular momenta but opposite potentials. This last feature suggests the possibility of a direct verification of the validity of Weber's law of force for electromagnetic interactions.Departamento de Eletrônica Quântica/Grupo de Plasmas (on leave of absence from Comisión Nacional de Energía Atómica, Argentina).     

A comparative review of certain gauge theories of the gravitational field

A general formal analysis is made trying to obtain a better understanding and greater synthesis of the mathematical structure of the gravitational field's gauge theories. Under this approach, some misstatements appearing in current theories are detected. A theory based on the direct product groupsT(4)×GL(4) andT(4)×O(1, 3) is suggested (in contrast to those using the Poincaré group, semidirect product). Such a theory corrects the justmentioned deficiencies possessing the attributes of the preceding ones.Comisión Nacional de Energía Atómica.     

On the positive energy theorem involving mass and electromagnetic charges

A new theorem relating mass and charges is deduced, which can be applied to more general physical systems than those covered by the theorem of Gibbons and Hull [1].On leave from Instituto de Matématicas Astronomía y Física, Universidad Nacional de Córdoba, (5000) Córdoba, ArgentinaAlfred P. Sloan Fellow; supported in part by a grant from the National Science Foundation     

Continuity of the temperature and derivation of the Gibbs canonical distribution in classical statistical mechanics

For a classical system of interacting particles we prove, in the microcanonical ensemble formalism of statistical mechanics, that the thermodynamic-limit entropy density is a differentiable function of the energy density and that its derivative, the thermodynamic-limit inverse temperature, is a continuous function of the energy density. We also prove that the inverse temperature of a finite system approaches the thermodynamic-limit inverse temperature as the volume of the system increases indefinitely. Finally, we show that the probability distribution for a system of fixed size in thermal contact with a large system approaches the Gibbs canonical distribution as the size of the large system increases indefinitely, if the composite system is distributed microcanonically.Supported by The British Council and the Universidad Nacional Autónoma de México.     

Improved coupled-mode analysis of nonlinear distributed feedback structures

Coupled-mode theory is employed to describe the nonlinear behaviour of a distributed feedback grating, including intensity-dependent terms resulting from the periodic variations in the nonlinearity. The improved coupled-mode analysis leads to modified values for the on and off-switching intensities, as demonstrated numerically.On leave from Dpto. Tecnologia Fotónica, Universidad Politécnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain.     

Renormalization group calculation for cells with inequivalent spins

We have considered a real-space renormalization group transformation for a bidimensional Ising model, carrying out approximate calculations for cells where site spins do not play the same role. The dependence on the ratio between the number of intercell and intracell nearest-neighbour interactions has also been discussed.Fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina.Fellow of the Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, Argentina.     

The hard-sphere order-disorder transition in the bethe continuum

We describe a search for solidlike singlet distribution functions in a system of hard spheres. The procedure, which is based on Widom's relation between the activity and the density in a nonuniform fluid, is applied to a sequence of hardcore lattice gases with increasingly extended interactions. When the system is defined on a Bethe lattice we obtain exact solutions for arbitrary external field and size of the hard core. This includes the limit in which the number of excluded neighbors goes to infinity while the lattice spacing is made to vanish. The study of the first few members in this family of models suggests the existence of an infinite sequence, beginning with the next-nearest-neighbor problem, of first-order sublattice ordering transitions occurring before close packing and at zero field. The periodic solutions for the density originate at bifurcation points located at uniform close packing.On leave from Facultad de Química, Universidad Nacional Autónoma de México, 04510 México, Distrito Federal     

Critical behavior of the correlation function of a coulombic fluid

An extension of the Ornstein-Zernike theory of critical scattering by a simple fluid to include a type of coulombic system is suggested. The relation between the oscillations of the charge distribution predicted by the second moment condition of Stillinger and Lovett in the restricted primitive model and the critical behavior of the correlation function is also discussed.This work was supported in part by a grant from the National Institutes of Health, GM 20800-03.Contribution No. 3100 from the Department of Chemistry, Indiana University.Supported by CONACYT (Mexico) and on leave from Universidad Autónoma de Puebla, Puebla, Mexico.     

Superspace gauge fixing of topological Yang-Mills theories

We revisit the construction of topological Yang-Mills theories of the Witten type with arbitrary space-time dimension and number of shift supersymmetry generators, using a superspace formalism. The super-BF structure of these theories is exploited in order to determine their actions uniquely, up to the ambiguities due to the fixing of the Yang-Mills and BF gauge invariance. UV finiteness to all orders of perturbation theory is proved in a gauge of the Landau type.Received: 29 October 2003, Published online: 18 February 2004Clisthenis P. Constantinidis: Supported in part by the Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq - BrazilOlivier Piguet: Supported in part by the Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq - BrazilWesley Spalenza: Supported in part by the Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq - Brazil     

Rays and phases in quantum mechanics

We show how phases enter in quantum mechanics as observable entities, in spite of the fact that physical states are represented by rays; in particular the Berry phase for closed loops and the phase difference between the two paths in the two-slit experiment stem from a common origin.1. On leave from Departamento de Fisica Teórica, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain.     

Effective Lagrangians for points,strings and membranes from the BFV formalism

We derive an expression for the charge radius and anapole moment of a free fermion induced at one loop in the standard Glashow-Salam-Weinberg model of electroweak interactions. The result, despite earlier claims to the contrary, is demonstrably gauge-invariant and observable in principle.On leave from Instituto de Física, Universidad National Autónoma de México     

The effects of concentration and temperature on the EFG generated by N.N. Pd impurities in a cubic Ag host

The time differential perturbed angular correlation (TDPAC) method is used to investigate the electric field gradients (EFG) generated by Pd impurity atoms alloyed into the cubic Ag host. As the concentration of the impurity is increased from 0.25 to 2.5 at.% we observed in detail how different near neighbour (n.n.) sites to the probe nuclei are populated, creating different EFG. The temperature dependence of the different EFG for a sample with 0.5 at.% of Pd showed that all follow theT ^3/2 law found previously for other doped cubic systems. In addition we observed that the n.n. population of impurities is strongly related to the thermal treatment of the sample.Work supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico and Fianciadora de Estudos e Projectos.     

Extended feynman formula for forced harmonic oscillator

Horváthy's modification of Feynman's original method is generalized to the path integral formula of a forced harmonic oscillator. With this new formula the propagator of a harmonic oscillator with memory is evaluated exactly beyond and at caustics.Work supported in part by the Conselho Nacional de Desenvolvimento Cientifico e Tecnólogico (CNPq), Brazil.     

The Baker-Campbell-Hausdorff formula for the chiral su(2) supergroup

In the theory of supersymmetricSU(2) Yang-Mills fields described on the 8th dimensional superspace, the local gauge transformations constitute a group whose Lie algebra has as coefficients belonging to the Weyl-spinorial Grassmann algebra.We present here a Baker-Campbell-Hausdorff formula for the chiralSU(2) supergroup and using this formula we give the finite form of each element of this group in terms of the local fields emering in the infinitesimal real superscalar generator.On leave at Departamento de Física, Universidad Simón Bolivar, Apartado 80659, Caracas 108 Venezuela     

On the perturbed angular correlation attenuation factor for relaxing systems

The attenuation factor of gamma-gamma perturbed angular correlation in the isotropic random-phase approximation is deduced for an axially symmetric quadrupole interaction and spins 1, 3/2, 2 and 5/2. It is shown that modifications to the usual expression are significant. Useful expressions for fitting procedures are obtained.Work supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Financiadora de estudos e Projetos (FINEP).     

Gauge fixing of Chern-Simons <Emphasis Type="Italic">N</Emphasis>-extended supergravity

We treat N-extended supergravity in 2 + 1 space-time dimensions as a Yang-Mills gauge field with Chern-Simons action associated to the N-extended Poincaré supergroup. We fix the gauge of this theory within the Batalin-Vilkovisky scheme.Received: 26 January 2004, Published online: 25 June 2004W. Spalenza: Supported in part by the Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq, Brazil     

Composition and magnetic properties of Fe/Fe x N thin film multilayers

The structure, composition, and magnetic characteristics of thin films multilayers of iron-iron nitride with high magnetic moment and very low coercive field are investigated. The present study together with previous ones that described in detail the magnetic properties lead to the establishment of certain requirements to obtain iron-iron nitride multilayers with outstanding magnetic characteristics.Supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil)     

57Fe Mössbauer study in the ternary hydrides of ErFe3

The ternary hydride phases ErFe₃Hx (with x=1.5 and 2.7) has been studied by⁵⁷Fe Mössbauer effect. The spectra show that the spin reorientation temperature is increased by the presence of hydrogen in the ErFe₃ lattice and is independent of hydrogen concentration for the two hydrides investigated. A preferential filling of the interstitial sites in the RFe₃ structure is indicated by the data.Work supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Financiadora de Estudos e Projetos (FINEP).