Resumo em Inglês:

For certain orientations of Josephson junctions between two p x-wave or two d-wave superconductors, the subgap Andreev states produce a 4pi-periodic relation between the Josephson current I and the phase difference <FONT FACE=Symbol>f:</FONT> I <FONT FACE=Symbol>µ</FONT> sin( <FONT FACE=Symbol>f/</FONT>2). Consequently, the ac Josephson current has the fractional frequency eV/$\hbar $, where V is the dc voltage. In the tunneling limit, the Josephson current is proportional to the first power (not square) of the electron tunneling amplitude. Thus, the Josephson current is carried by single electrons, rather than by Cooper pairs.

Resumo em Inglês:

We study the structure of resonance states localized around nonmagnetic impurities in the CuO2 planes of the cuprate superconductors within a potential scattering formalism. In particular we show that strong quantum interference effects arise between several impurities. This interference can be utilized to distinguish the dwave superconducting state from the phase with d-density wave order. This is important if the origin of the pseudo-gap state in the underdoped regime of the High Tc superconductors is caused by preformed Cooper pairs or staggered orbital currents. Furthermore impurity interference can be utilized to reveal subdominant superconducting order parameters and to pose further constraints on the potential scattering scenario.

Resumo em Inglês:

The effect of nonlocal electrodynamics is considered in finite samples. It is found for such samples that nonlocality modifies not only the in-plane penetration depth lambda||, as predicted by Kosztin and Leggett, but also the out-of-plane penetration depth lambda<FONT FACE=Symbol>^</FONT>. The Sr2RuO4 data are reinterpreted taking into account the contribution from lambda<FONT FACE=Symbol>^</FONT>.

Resumo em Inglês:

Numerical simulations based on Monte Carlo dynamics are used to investigate the resistivity behavior of granular superconductors containing a random distribution of pi junctions, as in superconducting materials with d-wave symmetry. The presence of pi junctions leads to quenched in circulating currents (chiralities) and to chiral glass behavior at low temperatures, even without an external magnetic field. An XY spin glass model in the phase representation is used to determine the current-voltage characteristics and critical exponents of the resistivity transition. In two dimensions, the linear resistivity is nonzero at finite temperatures and the dynamic scaling analysis of the nonlinear resistivity is consistent with a phase transition at zero temperature. In three dimensions, we find a transition at finite temperatures below which the linear resistivity vanishes and the corresponding critical exponents are determined from the scaling analysis. The results are in good agreement with Langevin simulations in the phase representation. The dynamic exponent z is significantly different from previous results obtained in the vortex representation.

Resumo em Inglês:

We have investigated the field-temperature (H - T) diagram of the superconducting and pseudogapped states of Bi2Sr2CaCu2O8+y over a wide range of hole doping (0:10 < p < 0:225). Using interlayer tunneling transport in magnetic fields up to 60 T to probe the density-of states (DOS) depletion at low excitation energies we mapped the pseudogap closing field Hpg. We found that Hpg and the pseudogap onset temperature T* are related via a Zeeman relation gmB Hpg ~ kB T*, irrespective of whether the magnetic field is applied along the c-axis or parallel to CuO2 planes. In contrast to large anisotropy of the superconducting state, the field anisotropy of Hpg is due solely to the g-factor. Our findings indicate that the pseudogap is of singlet-spin origin, consistent with models based on doped Mott insulator.

Resumo em Inglês:

Within the quasiclassical approximation we have studied the thermodynamics and the thermal conductivity in the vortex state in nodal superconductors (sc). Recent angle dependent magnetothermal conductivity results indicate a gap function delta(k) corresponding to f- wave sc in Sr2RuO4 and d- wave sc in CeCoIn5 and k-(ET)2Cu(NCS)2 respectively. More recently it is shown that delta(k) in both YNi2B2C and PrOs4Sb12 have point nodes described by hybrid s+g wave gap function.

Resumo em Inglês:

The widely held notion that high-temperature superconductivity originates in the cuprate-planes is proven to be faulty. In the cuprates such as YBa2Cu3O7, we argue that the superconductivity resides in the BaO layers. This superconductivity is s-wave, not d-wave, in the bulk. The trio of ruthenate compounds, doped Sr2YRuO6, GdSr2Cu2RuO8, and Gd2-zCe zSr2Cu2RuO10 all superconduct in their SrO layers, which is why they have almost the same ~49 K onset temperatures for superconductivity.

Resumo em Inglês:

We have performed a systematic study of the structural, transport, and magnetic properties of polycrystalline samples of the magnetic superconductors Ru1-xIr xSr2GdCu2O8; x = 0, 0.02, 0.05, 0.10, and 0.15. The samples were prepared by solid state reaction and sintered at 1060 ºC for 72 h under O2 flow. X-ray diffraction analysis shows that all samples are nearly single phase and that the lattice parameters are independent of Ir content. Transport properties measurements revealed that the Ru substitution by Ir results in a decrease of Tc,onset from ~50 K (x = 0) to ~30 K (x = 0.10). Further addition of Ir (x > 0.10) causes an evolution from metallic to nonmetallic behavior of r(T). We have also found that magnetic order develops in the undoped Ru-1212 materials near T M ~ 130 K. This temperature decreases linearly with increasing Ir content at the rate of ~ -1.6 K / Ir at.%, suggesting that Ir effectively substitutes Ru in the RuO planes. A subtle drop in r(T) is observed close to T M, probably due to the suppression of the spin-flip scattering. The magnetoresistivity measurements revealed that the temperature Tc,zero, in which r(T) ~ 0, decreases rapidly for low applied magnetic fields (H < 2 T), and that this drop becomes much less pronounced in higher magnetic fields (2 < H < 18 T). The appreciable broadening of r(T) curves at low magnetic fields is reminiscent of the behavior in high-Tc materials showing granular behavior, as for example in Sm1.85Ce0.15CuO4-y.

Resumo em Inglês:

Alloy studies in the p - d organic conductor l-(BETS)2Fe xGa1-xCl4 have given new insight into the nature of field induced superconductivity (FISC), since the mechanism of the FISC involves cancellation of the p - d exchange field by the external field. Alloying on the Fe xGa1-x site allows tuning of the exchange field, thereby influencing the FISC phase boundary. A brief review of the low temperature phases are given, and new high magnetic field thermoelectric and mm wave results that probe the low temperature ground state are presented.

Resumo em Inglês:

That very strong coupling of the B-B bond-stretching E2g branch of phonons to the B 2psigma bonding hole states is responsible for the remarkable superconductivity in MgB2 is well established. This entirely new manner of driving the superconducting Tc to high levels requires additional analysis. Here recent findings, such as how this strong coupling is related to possible structural instability, are discussed, and investigation into the practical limits of such coupling is initiated.

Resumo em Inglês:

In this paper a brief overview of anomalous behavior resulting from the two-gap superconductivity in MgB2 is given. We focus on two characteristic effects: an anomalous enhancement of the upper critical field by nonmagnetic impurities and nonequilibrium interband phase textures which appear as a result of interband breakdown caused by electric field. Both effects distinguish MgB2 from the existing low-Tc and high-Tc superconductors.

Resumo em Inglês:

We have investigated the low-temperature (T < Tc=10) mixed-state current-voltage (IV) response of magnesium diboride films beyond the point where the superconductivity is completely destroyed and the system enters the normal state. The resistance-versus-current R(I) curves are extremely steep and featureless, with a critical current density j c, marking the onset of dissipation, that is unusually high (j c>j d/10) with respect to the depairing current density j d. At large flux densities Hc2/10 $\lessim$ B $\lessim$Hc2, the R(I) curve has a functional shape that is largely independent of B, indicating that the rise in resistivity with increasing current occurs mainly due to pair-breaking rather than flux motion. The macroscopic destruction current I*, which drives the system normal, has a B/Hc2: $\sqrt{B/H_{c2}}$ flux-density dependence, suggesting that the vortices mainly reduce the effective cross section over which a current of effective density j ~ j d flows.

Resumo em Inglês:

The first direct measurement of an anisotropic superconducting property in MgB2 was achieved for the bulk nucleation field Hc2, in a sample of aligned crystallites. It was found a ratio gamma(T) = Hab c2/ Hc c2= 1.6 - 1.9, for T varying from 32 K to 26K, between H applied parallel to the ab plane, and along the c direction. The anisotropy of the induced critical current density was evaluated through the Bean model to be ab c/ Jc c~ 1.5. We present here a brief review of these studies in connection with current results found in the literature.

Resumo em Inglês:

We propose a two band model for superconductivity. It turns out that the simplest nontrivial case considers solely interband scattering, and both bands can be modeled as symmetric (around the Fermi level) and flat, thus each band is completely characterized by its half-band width Wn (n=1,2). A useful dimensionless parameter is d, proportional to W2 - W1. The case delta = 0 retrieves the conventional BCS model. We probe the specific heat, the ratio gap over critical temperature, the thermodynamic critical field and tunneling conductance as functions of d and temperature (from zero to Tc). We compare our results with experimental results for MgB2 and good quantitative agreement is obtained, indicating the relevance of interband coupling. Work in progress also considers the inclusion of band hybridization and general interband as well as intra-band scattering mechanisms.

Resumo em Inglês:

We examine the electronic structure of Na xCoO2.yH2O within the local density approximation. The parametrization of the band which forms the largest hole-Fermi surface centered at gamma shows significant deviations from what is frequently assumed in recent sophisticated theoretical studies. In particular, the commonly used nearest neighbor approaches in the framework of single band pictures are found to be unrealistic. The special role of H2O in screening the disorder in the charge reservoir is briefly discussed and compared with the case of Y1-xCa xCu3O6+delta.

Resumo em Inglês:

Glassy carbon (GC) has been high-pressure high-temperature treated. An interesting morphology evolution from the pristine sample to the high pressure products was observed. It is found that GC can be graphitized under pressure at a temperature much lower than that at ambient condition. Furthermore the in-situ structure and electrical measurements of GC and graphitized glassy carbon (GGC) under high temperature and high pressure have been investigated up to 30 GPa. We particularly emphasize the unusual magnetic properties of GC treated under high pressures and high temperatures. A paramagnetic to ferromagnetic-like, and then to superconducting (a diamagnetic signal with hysteresis magnetic response) -like behavior, which can be observed at temperatures as high as 80 K, appears as a successive evolution from the initial GC to GGC in accordance with three regions distinguished by the graphitization temperature. This interesting evolution of magnetic properties probably evokes the new understanding of carbon element.

Resumo em Inglês:

We discuss two methods used for the study of unconventional superconductors: ultrasound attenuation and collective modes. These two methods, as well as microwave absorption, turn out to be coupled and have become very important now. Within models built by path integration technique we analyze some recent ideas concerning possible realization of the mixture of different d-wave states in high temperature superconductors (HTSC).We specifically consider the mixture of d x2-y2 and d xy states. We study the collective mode spectrum and the ultrasound attenuation in the mixed state, and show that each of the two methods allows us to distinguish a pure d-wave state from a mixed one. They also allow us to identify the type of pairing and order parameter in unconventional superconductors, including the presence and topology of gap nodes, the magnitude of the gap, and degree of admixture in the mixed state.

Resumo em Inglês:

Millimeter-sized single crystals of CaLaBaCu3O7-delta were grown by the conventional self-flux technique. In this report we present a characterization of the structural and morphological properties of our first time grown single crystals. X-Ray diffraction patterns show that the unit cell of the samples corresponds to a tetragonal structure. SEM images evidence excellent morphological properties without presence of twins. EDX measurements show the difficulty of Ca to enter in the structure. The implications of our results are extensively presented and discussed.

Resumo em Inglês:

Although a considerable amount of the research work has been done on graphite, its physical properties are still not well understood, and novel phenomena such as the magnetic-field-driven metal-insulator transition (MIT), the quantum Hall effect, ferromagnetic and superconducting correlations have recently been revealed. Theoretical analysis suggests that the MIT in graphite is the condensed-matter realization of the magnetic catalysis phenomenon known in relativistic theories of (2 + 1) - dimensional Dirac fermions (DF), i. e. that the applied field opens an insulating gap in the spectrum of DF, associated with the electron-hole pairing. On the other hand, we demonstrate in this paper that a two parameter scaling analysis proposed by Das and Doniach [D. Das and S. Doniach, Phys. Rev. B 64, 134511 (2001)] to characterize the magnetic-field-tuned Bose metal - insulator transition can be well applied to the MIT observed in graphite. We discuss the possibility that the MIT in graphite is associated with the transition between Bose metal and excitonic insulator states.

Resumo em Inglês:

We review the concept of the Bose metal state which arises in 2D superconductors in the quantum limit where vortex loops can grow spontaneously in (2+1) dimensions and, as zero point motions become larger, eventually lead to an insulating state. A nonlocal Coulomb charging term in a Josephson array type model leads to an effective transverse gauge field which can suppress the condensate of vortices in the superinsulating state leading to an intermediate Bose liquid which is not superfluid at T=0, the Bose metal state. We comment on recent work on this state and on other non-superfluid Bose liquids.

Resumo em Inglês:

Superconductivity often occurs in crystals with one active electron per site with charge density wave (CDW) or spin density wave (SDW) as 'mother state'. It is proposed that superconductivity is possible when the differences in equilibrium geometry and energy between the diabatic CDW and SDW states are so small that there is interaction between them via the zero point vibrations. Electron pairing in real space is directly related to oxidation states being different in two units. Three valence states in succession have to be stable (ground state or low-energy excited states) and we therefore refer to this mixed valence model as the MV-3 model. Examples are chosen from bismuthates, cuprates, and fullerides. The theory is simple and straightforward and offers solutions to other important problems as well, for example for A3C(6)0(A = K; Rb), that (1) there are no magnetic moments in crystal phase, and (2) that these systems are superconducting metals while A4C(6)0 are insulators.

Resumo em Inglês:

We study numerically superconductivity in a system characterized by the presence of a phenomenological ”pseudogap”, Eg, in the energy spectrum, for 0 < T < Ts.T* is a crossover temperature. As a simplification, the pseudogap and the superconducting gap have the same s-wave symmetry. We find that for<FONT FACE=Symbol>"</FONT>Eg <FONT FACE=Symbol>¹</FONT> 0 we require a critical value of the superconducting interaction, Vd, to produce a finite superconducting critical temperature, Tc and another one for deltao <FONT FACE=Symbol>¹</FONT> 0.

Resumo em Inglês:

We argue that the spatial distribution of resonant impurity states in underdoped high-Tc superconductors serves as a probe for distinguishing different theoretical models for the pseudogap state. Superconducting pairing fluctuations are characterized by off-diagonal short-range order which distinguishes them from other possible instabilities that could give rise to the pseudogap phenomena. Due to the mixture of particle and hole states in a superconductor an impurity resonant state is composed of both a particle and a hole-like component. On the contrary a state with a gap induced by a particle-hole instability, like a d-density wave (DDW) or spin-density wave (SDW), exhibits no off-diagonal short-range order and consequently a resonant impurity state consists of only one either particle or hole-like component. Furthermore, a charge-spin separated state shows no resonance state at all inside the gap region.

Resumo em Inglês:

Recently experiments on high critical temperature superconductors have shown that the doping levels and the superconducting gap are usually not uniform properties but strongly dependent on their positions inside a given sample. We show here that the large diamagnetic signal above the critical temperature Tc and the unusual temperature dependence of the upper critical field Hc2 with the temperature can be explained taking the inomo-geneities and a distribution of different local critical temperatures into account.

Resumo em Inglês:

The superconductivity of graphite-sulfur composites is highly anisotropic and associated with the graphite planes. The superconducting state coexists with the ferromagnetism of pure graphite, and a continuous crossover from superconducting to ferromagnetic-like behavior could be achieved by increasing the magnetic field or the temperature. The angular dependence of the magnetic moment m(alpha) provides evidence for an interaction between the ferromagnetic and the superconducting order parameters.

Resumo em Inglês:

We analyze how solar neutrino experiments could detect time fluctuations of the solar neutrino flux due to magnetohydrodynamic (MHD) perturbations of the solar plasma. We state that if such time fluctuations are detected, this would provide a unique signature of the Resonant Spin-Flavor Precession (RSFP) mechanism as a solution to the Solar Neutrino Problem.

Resumo em Inglês:

A secondary proton radiation belt can be observed in the equatorial region between the upper atmosphere and the interior edge of the main radiation belt. It is thought that the protons appear there in a result of ionization of energetic neutral hydrogen atoms coming from the internal area of the traditional radiation belt where they were born in charge exchange collisions of the trapped protons with the cold hydrogen of the gecorona. The process of formation of this secondary belt is numerically simulated in this paper assuming this charge exchange-re-ionization mechanism. Standard models of the trapped radiation, of the atmosphere and geocorona were used to simulate the source and the exospheric media. Experimental data were used for charge transfer cross sections. Result of simulation agrees very good with the experimental observation.

Resumo em Inglês:

A theoretical and experimental study was developed about the applicability of a double probe system consisting of two directional Langmuir probes, both probes being located separately in a plasma column. The current- voltage characteristic of the double probe was obtained considering a plasma with a drifting maxwellian electron velocity distribution function and stationary ion background. In deriving the characteristic of the double probe, the plasma parameters, namely, electron temperature (Te), electron density (Ne), electron drift velocity (Vde) and plasma potential (Vp) are assumed to be non-uniform. The double probe characteristic is also dependent on the angle between the axial direction of the electron drift and the normal to the collecting area of the probe. Each probe can be rotated such that this angle can be varied between zero and 180 degrees. Various probe characteristics were simulated using plasma parameters obtained by independent single probe measurements in the positive column of a low-pressure arc discharge in mercury vapor. Typical parameters of the positive column, used in the simulation, are: Te = 5 eV, Ne = 10(17) m-3, v de = 8x10(5) ms-1. Experimental characteristics of the double probe were obtained and compared with the simulated results, showing good agreement. It is concluded that this directional probe system can be a reliable diagnostic tool especially for studies of non-uniform plasmas.

Resumo em Inglês:

We have run electron optics simulations and determined the tolerance in the control voltages of all elements (retarding input lens, analyzer, accelerating exit lens) of the La Trobe University photoelectron analyzer, recently redesigned to reach a spectral resolution of 5000, and which will be installed at LNLS (Campinas Brasil) and BESSY II (Berlin, Germany).

Resumo em Inglês:

Structural and thermodynamic properties of hydrogen molecular clusters formed around an atomic or molecular cation are examined. The shell distribution of H2 molecules and the size of the clusters are discussed. The Bloom-Margenau model for predicting the number of neutral molecules that could bind to a cation core is investigated and its limitations are illustrated using the Li+(H2)k clusters as test case. Finally, results for the entropy of the H+n clusters (n = 5 - 27, odd) and for the Gibbs free energy variations associated to the cluster formation are presented and the spontaneity of the clustering process in different conditions is examined.

Resumo em Inglês:

In this work we present experimental results of electron plasma waves excited in a beam plasma system. Based on our experimental results we determine the transition from the quasi-linear to non-linear regime. We present the space evolution of the electron beam distribution function for both regimes. The spectrum of the electron plasma wave in the non-linear regime shows a component with frequency larger than the plasma frequency besides the plasma frequency itself. We show that the higher frequency component is strongly affected by Landau damping, indicating a dissipation region. The measured experimental power spectrum of this wave shows a dependence on wave number k given by Wk <FONT FACE=Symbol>µ</FONT> k-7/2 as theoretically predicted.

Resumo em Inglês:

In the present paper we discuss stochastic diffusion of energetic ions by a set of lower hybrid waves with frequencies close to each other and random phases which change along the time evolution of the system. We obtain efficient long term diffusion in velocity space, which is more representative of the diffusion produced by a continuous wave packet than the diffusion produced by a set of waves with random phases which are constant along the time evolution.

Resumo em Inglês:

We present a review of experimental data on the nonlinear optical properties of thermotropic and lyotropic liquid crystals probed by the Z-scan technique. Depending on the time scale, different processes originate a nonlinear optical response. Particularly, at nanosecond range, the physical processes associated with the nonlinear response in thermotropics are not completely understood. In lyotropics, the nonlinear response at ms time scale is from thermal origin and depends on the particular mesophase, relative concentration of the components, and temperature.

Resumo em Inglês:

The absence of ghost-negative energy radiative modes in a proposed generalized theory of gravitation based on a non-symmetric metric is reanalyzed. The missing contribution of the symmetric sector of the Lagrangian is calculated and shown to be null. The ghost free character of the theory is then firmly established.

Resumo em Inglês:

Explaining codon evolution in the standard genetic code is a remarkable subject in Molecular Biology. There are many works which try to develop a model to represent this evolution, sometimes using a certain amount of mathematical tools. The present work has as its main objective to explain one possible dynamical evolution model, which is based in the algebraic approach proposed in 1993 by Hornos and Hornos. This model made an analogy between the evolution of elementary particles and evolution of codons. As a result, the symmetry group that better adjusts degeneracy of the genetic code is the simplectic group Sp(6). The dynamical model present here is based in the 3 dimensional weight space of Sp(6), called Codon Space. This space is invariant under the action of the Octahedral Group. A map, <FONT FACE=Symbol>Â</FONT>³ -> <FONT FACE=Symbol>Â</FONT>³, was constructed using an equivariance principle and its action in the Codon Space was observed. The results were based in the analysis of the attractors and their preserved symmetries. The dynamical system action reproduces the main aspects of the labeling proposed in the algebraic approach. Moreover, the map separates the codons in distinguished sets, coupling codons from the same aminoacid in the same attractor. These dynamical results indicate that the algebraic model proposed in 1993 may be an important contribution to explain codon evolution, both in the algebraic and in the dynamical aspects.

Resumo em Inglês:

We consider the process m -> nue e$\bar{\nu}$mu in the framework of a two Higgs doublet model with flavor changing neutral currents (FCNC). Since FCNC generates in turn flavor changing charged currents in the lepton sector, this process appears at tree level mediated by a charged Higgs boson exchange. From the experimental upper limit for this decay, we obtain the bound |ximue/mH+| < 3.8 x 10-3 GeV-1, where ximue refers to the mixing between the first and second lepton generations, and mH± denotes the mass of the charged Higgs boson. This bound is independent on the other free parameters of the model. In particular, for mH± ~ 100 GeV we get |ximue| $\lessim$0.38.

Resumo em Inglês:

A fluid of domain walls may have an effective equation of state p w = -<img id="_x0000_i1026" src="../../../img/revistas/bjp/v33n4/a39img01.gif" align=absbottom>rhow. This equation of state is qualitatively in agreement with the supernova type Ia observations. We exploit a cosmological model where the matter content is given by a dust fluid and a domain wall fluid. The process of formation of galaxies is essentially preserved. On the other hand, the behaviour of the density contrast in the ordinary fluid is highly altered when domain walls begin to dominate the matter content of the Universe. This domain wall phase occurs at relative recent era, and its possible consequences are discussed, specially concerning the Sachs-Wolfe effect.

Resumo em Inglês:

We study the $^{6}_{\Lambda\Lambda}$He and $^{10}_{\Lambda\Lambda}$Be hypernuclei in the three- and four-body models, respectively, employing harmonic oscillator bases and presently most realistic alpha-alpha, alpha-lambda and lambda-lambda interactions. In order to improve convergence we use correlation functions in the case of $^{10}_{\Lambda\Lambda}$Be. Comparison is made with results obtained using similar interactions and other methods, and it is performed an analysis concerning the possibility of a unified description of both hypernuclei.

Resumo em Inglês:

This paper describes the general characteristics of spherical tokamaks, or spherical tori, with a brief overview of work in this area already performed or in progress at several institutions worldwide. The paper presents also the steps in the development of the ETE (Experimento Tokamak Esférico) project, its research program, technical characteristics and operating conditions as of December, 2002 at the Associated Plasma Laboratory (LAP) of the National Space Research Institute (INPE) in Brazil.

Resumo em Inglês:

We discuss the Casimir effect for a massive bosonic field with mixed (Dirichlet-Neumann) boundary conditions. We use the zeta-function regularization prescription to obtain our physical results. Particularly, we analyse how the Casimir energy varies with the mass of the field and compare this mass dependence with those obtained for other boundary conditions. This is done graphically. Some other graphs involving a massive fermionic field are also included.

Resumo em Inglês:

The conditions required for efficient current drive in a weakly resistive plasma column, subject to a double helix traveling magnetic field, are examined in detail by using a simple non-linear Ohm's law for the plasma. In agreement with previous numerical work on the subject by Bertram [5], it is shown that field penetration is indeed strongly limited when large external bias longitudinal magnetic fields are used. However, there is a range of small external bias fields that allow for significant penetration when reasonable driving fields are used. This should be of interest for sustaining reversed-field pinches and toroidal screw pinches.

Resumo em Inglês:

A class of bound-state problems which represent the interaction between a system of N two-level atoms and an electromagnetic radiation via an m-photon process is studied, with m = 1; 2; 3;.... We also evaluate some nonlinear effects usually related with the polarisability of the medium and with the dependence on the intensity of the radiation field on the matter-radiation interaction. We obtain exact values for the eigenstates and eigenvalues for all values of total angular momentum of the system and all possible number m of photons involved in the interaction. We give explicit analytic expressions for small numbers of atoms and discuss some aspects of the composition of the eigenstates and of the spectra obtained in these cases.

Resumo em Inglês:

The A²pi-X²sigma+ emission band system of the MgCl molecule has been studied by means of high resolution Fourier Transform Spectroscopy (FTS). The MgCl species were produced by mixing Mg vapor with a gaseous flow of a He/Cl2 and excited in a ''heated'' Schüller's type discharge tube. Rovibrational analysis of the 0-0 and 0-1 bands was performed and the values of the vibrational constant <FONT FACE=Symbol>we</FONT> of the ground state and the spin-orbit constants A0 and A J of the A2pi state were determined. For the first time transitions of the isotopic species 24Mg37Cl could be assigned and included in the analysis presented here.

Resumo em Inglês:

The entropy of the Viana-Bray model at zero temperature and external field is calculated within the solution which takes into account only delta functions for the global order parameter P(h). It is shown that such solution is unsatisfactory both from the viewpoint of stability analysis and for not reproducing the well known Sherrington-Kirkpatrick result in the large connectivity limit thus pointing out the relevance of considering solutions with continuous part in P(h) for such model and possibly related models.

Resumo em Inglês:

In this work the properties of minority games containing agents which try to winning all the time are studied by means of computational simulations. We have considered several ways of introducing above the rules clever players using ''strategies'' which try to outdo the others endowed with statistically equivalent strategies and compared the resulting behaviors of the ensemble. It is shown that by introducing such agents the overall performance of the system gets significantly poorer. While the introduction of a very small fraction of these never-loosing-players may not destroys the unordered / ordered phase transition of the standard minority game we find that even for a low concentration of their presence only a state ''worse'' than random coin toss choices sets in. These special agents/players have the role of impurities or vacancies in spin systems and their presence may lead to a critical concentration where the usual phases are washed out.

Resumo em Inglês:

We study the stability of the replica-symmetric solution of a two-sublattice infinite-range spin-glass model, which can describe the transition from an antiferromagnetic to a spin-glass state. The eigenvalues associated with replica-symmetric perturbations are in general complex. The natural generalization of the usual stability condition is to require the real part of these eigenvalues to be positive. The necessary and sufficient conditions for all the roots of the secular equation to have positive real parts is given by the Hurwitz criterion. The generalized stability condition allows a consistent analysis of the phase diagram within the replica-symmetric approximation.