Resumo em Inglês:

It is suggested a general mechanism through which a lambda-type behavior is produced in the specific heat of a Bose gas near the critical temperature Tc . It is essential that the quasiparticle spectra have a gap proportional to the condensate. It works for a general class of quasiparticle spectra, and in particular, for the weakly interacting Bose gas. The introduction of a chemical potential in the theory is briefly discussed.

Resumo em Inglês:

The production of eta meson induced on carbon and tungsten targets by protons of kinetic energies from 800 to 1500 MeV has been studied at the Laboratoire National Saturne at Saclay. eta mesons of mean kinetic energies 55 MeV and 125 MeV have been detected by positioning the PINOT spectrometer in two different configurations. The results are presented and discussed in the framework of a recent theoretical model.

Resumo em Inglês:

We present the Quantum Section Method as a quantization technique to compute the eigenvalues and the eigenfunctions of quantum systems. As an instructive example we apply this procedure to quantize the annular billiard. The method uses the symmetry of the system to determine an auxiliary section separating the system into partial regions and computes the Green's functions for Schroedinger's equation, obeying the same boundary conditions imposed on the eigenfunctions of the system. The eigenvalues are obtained as zeroes of a finite real determinant and the eigenfunctions are also determined. The present analytical and numerical results are in total agreement with those obtained by other procedures, which shows the efficiency of the method.

Resumo em Inglês:

The synthesis of helium in the early Universe depends on many input parameters, including the value of the gravitational coupling during the period when the nucleosynthesis takes place. We compute the primordial abundance of helium as function of the gravitational coupling, using a semi-analytical method, in order to track the influence of G in the primordial nucleosynthesis. To be specific, we construct a cosmological model with varying G, using the Brans-Dicke theory. The greater the value of G at nucleosynthesis period, the greater the predicted abundance of helium. Using the observational data for the abundance of primordial helium, constraints for the time variation of G are established.

Resumo em Inglês:

We present a method of sample preparation for studies of ion implantation on metal surfaces. The method, employing a mechanical mask, is specially adapted for samples analysed by Scanning Force Microscopy. It was successfully tested on polycrystallyne copper substrates implanted with phosphorus ions at an acceleration voltage of 39keV. The changes of the electrical properties of the surface were measured by Kelvin Probe Force Microscopy and the surface composition was analysed by Auger Electron Spectroscopy.

Resumo em Inglês:

The occurrence of squeezing effects in coupled oscillators, and the transference between them, has been studied in various situations using Hamiltonians involving either nonlinear terms or time-dependent parameters. We consider a simplified scheme generating this effect, and discuss its origin.

Resumo em Inglês:

We present the results of several parametrizations to two different ensemble of data on pp total cross sections (<FONT FACE=Symbol>s<img SRC="http:/img/fbpe/bjp/v31n4/07eq01.gif" ></FONT>) at the highest center-of-mass energies (including cosmic-ray information). The results are statistically consistent with two distinct scenarios at high energies. From one ensemble the prediction for the LHC (<img SRC="http:/img/fbpe/bjp/v31n4/07eq02.gif"> = 14 TeV) is<FONT FACE=Symbol>s<img SRC="http:/img/fbpe/bjp/v31n4/07eq01.gif" ></FONT> = 113 ± 5 mb and from the other, <FONT FACE=Symbol>s<img SRC="http:/img/fbpe/bjp/v31n4/07eq01.gif"></FONT> = 140 ± 7 mb. From each parametrization, and making use of derivative analyticity relations (DAR), we determine rho(s) (ratio between the forward real and imaginary parts of the elastic scattering amplitude). A discussion on the optimization of the DAR in terms of a free parameter is also presented. In all cases good descriptions of the experimental data are obtained.

Resumo em Inglês:

The Heisenberg XXZ model is a chain model with nearest-neighbor interactions. Its thermodynamics is exactly obtained via Bethe ansatz. Recently, we developed a method to derive the high-temperature expansion of the grand potential per site of translationally invariant chain models, with periodic boundary conditions. Here we apply this approach to the XXZ model with periodic boundary conditions for the Ising limit case (t = 0) and the free fermion case (delta = 0 and h = 0), obtaining results in agreement with the literature. In this new way of obtaining the coefficients of the high-temperature expansion of the grand potential, the coefficients are derived from an auxiliary function written only in terms of open connected sub-chains.

Resumo em Inglês:

Old high-z galaxies are important tools for understanding the structure formation problem and may become the key to determine the ultimate fate of the Universe. In this paper, the inferred ages of the three oldest galaxies at high redshifts reported in the literature are used to constrain the first epoch of galaxy formation and to reanalyse the high-z time scale crisis. The lower limits on the formation redshift z f depends on the quantity of cold dark matter in the Universe. In particular, if omegam > or = 0.37 these galaxies are not formed in FRW cosmologies with no dark energy. This result is in line with the Supernovae type Ia measurements which suggest that the bulk of energy in the Universe is repulsive and appears like an unknown form of dark energy component. In a complementar analysis, unlike recent claims favoring the end of the age problem, it is shown that the Einstein-de Sitter model is excluded at high-z by 3sigma.

Resumo em Inglês:

Superconducting Quantum Interference Devices coupled to gradiometers were used to detect flaws in metals. We detected flaws in aluminum samples carrying current, measuring fields at lift-off distances up to one order of magnitude larger than the size of the flaw. Configured as a susceptometer we detected surface-braking flaws in steel samples, measuring the distortion on the applied magnetic field. We also used spatial filtering techniques to enhance the visualization of the magnetic field due to the flaws. In order to assess its severity, we used the generalized inverse method and singular value decomposition to reconstruct small spherical inclusions in steel. In addition, finite elements and optimization techniques were used to image complex shaped flaws.

Resumo em Inglês:

We present a review of the Optical Parametric Oscillator (OPO), describing its operation and the quantum correlation between the light beams generated by this oscillator. We show the construction of an OPO using a Potassium Titanyl Phosphate crystal (KTP), pumped by a frequency doubled Nd:YAG laser, and discuss the stability of the system and related thermal effects. We have measured the quantum correlation of signal and idler beams in a transient regime, obtaining a noise correlation level 39 % below the shot noise level.

Resumo em Inglês:

In this paper we review the crystal chemistry and structure data from the literature and our results reported in the last decade for natural and synthetic samples of the tantalite-columbite series. Order-disorder transitions and oxidation reactions on the ixiolite-columbite-wodginite system are reported for one Brazilian sample. It is shown that reports on these subjects are sometimes dubious and incomplete. In addition to the reaction atmosphere (air or vacuum), order-disorder and oxidation are strongly dependent on the physical state of the samples (powdered or crystal). Structure and chemical composition have been investigated in our laboratory using X-ray diffraction (XRD), electron probe microanalysis (EPMA) and Mössbauer spectroscopy (MS).