Coupled continuous-time random-walk: Pseudo diffusion with anisotropic scattering

A generalization of pseudo-diffusion processes in media with anisotropic scattering, described by a coupled RW, is done by means of the CTRW theory. The behaviour of the probability distribution has been studied. A kind of phase breaking in such a behaviour arises as a function of the anisotropic parameter. Some examples, with different models for the waiting time density, are presented. In the limit of strong forward or backward scattering, we have a new way to analyze quasi-unidirectional or quasi-collapsed processes respectively.Also at Universidad Nacional del Comahue, Centro Regional BarilocheComisión Nacional de Energia 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.     

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.     

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     

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     

The instrumental complexity of states

We apply a formalism developed previously to study the notion of the complexity of states in a general statistical theory. We identify the extreme points of the instrument sets as those instruments that view an intrinsic complexity of states and are uncontaminated by stochastic contributions of the experimenter. Various notions of entropy of a state are investigated.This work was financially supported by CNP_q (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and FINEP (Financiadora de Estudos e Projetos) of the Brazilian government.     

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.     

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)     

New NH2H4 far-infrared laser lines and their frequencies

We report 25 new far-infrared laser lines and 26 heterodyne frequency measurements in hydrazine. The frequencies range from 1.0 to 5.5 THz with most of the frequencies between 2.5 and 4.0 THz. The lines were generated in a high frequency, far-infrared Fabry-Perot laser cavity pumped by a CO₂ laser. The cavity has a high Q for wavelengths below 150 µm and uses variable coupling to optimize the power for each line.This work has been financed by the Brazilian Government - Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and by the United States Government - NASA grant W-18, 623.Contribution of NIST, not subject to U.S. copyright.     

A new approximation for the dynamics of the transverse ising model

An improved mean field theory, the Reaction Field Approximation, which was previously used to obtain the static properties of model systems of localized spins (Heisenberg, XY, Ising and also more complicated interaction Hamiltonians) is extendend so that dynamical properties of the Ising model in presence of a transverse field can also be calculated. Theq- and -dependent susceptibility is obtained as function of field and temperature, from which the dynamic correlation functions can also be obtained. The dependence of the critical temperature on the transverse field is calculated and a phase diagram is presented.Work partly supported by Conselho Nacional de Desenvolvimento Cientifico e Tecnológico, CNPq, Brazil and by Alexander von Humboldt Stiftung, FRG     

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).     

Temperature transform of the Boltzmann equation

We define an integral transform of the energy distribution function for an isotropic and homogeneous diluted gas. It may be interpreted as a linear combination of equilibrium states with variable temperatures. We show that the temporal evolution features of the distribution function are determined by the singularities of this temperature transform. We compare the relaxation to the equilibrium process for Maxwell and very hard-particle interaction models, finding many basic discrepancies. Finally, we formulate an alternative approach, which is given by anN-pole approximation with a clear physical meaning.Fellow of the Conselho Nacional de Desenvolvimento Cientifico e Tecnólogico, Brazil.     

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     

The spiked harmonic oscillatorV(r)=r 2+λr −4 as a challenge to perturbation theory

The standard weak- and strong-coupling perturbation series are interpreted as extreme special cases of expansions obtainable within the framework of Rayleigh-Schroedinger perturbation theory with non-diagonal propagators and unspecified zero-order energies. The formalism of the latter type is then tested by our strongly singular example. It proves suitable for applications in the domain of virtually arbitrary couplings. A few related technicalities and especially the quadruple problem of convergence are also discussed.Dedicated to the memory of M. Gmitro.RG acknowledges support from the Comisión Interministerial de Ciencia y Tecnología, Spain, under contract 969/87. MFF acknowledges support via a fellowship from the Ministerio de Educación y Ciencia, Spain, under the program Estancias Temporales de Científicos y Tecnólogos Extranjeros. MZ acknowledges a partial support from the former resource during his short stay in Valencia, and a partial support from the Grant Agency of the Czechosl. Acad. Sci.     

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.