Resumo em Inglês:

Some properties of the ux-line lattice in conventional and high-Tc superconductors are reviewed, with particular stress on phenomenological theories, nonlocal elasticity, irreversible magnetization curves, and inuence of the specimen shape on the electromagnetic response.

Resumo em Inglês:

Current and magnetic field profiles of bent strips in external magnetic fields and with applied currents are numerically calculated. The flat strip is correctly obtained as the limiting case of bent strips when the bend angle is zero. The Meissner response and the flux penetration are studied, the latter under the consideration of high critical currents and small bends. The dependence of these profiles on the creep exponent (by a material relation E alpha (j / jc)n) is also studied. In the presence of an external magnetic field the flux penetration is delayed in bent strips compared with at strips. In the case of an applied current the flux penetration from the strip edge is also delayed, but a flux front starting from the corner is also observed.

Resumo em Inglês:

Very long and thin superconducting cylinders with square cross section are studied using a modified Ginzburg Landau theory that incorporates the boundary conditions into the free energy expression.

Resumo em Inglês:

In the present work we study a long superconducting wire with a columnar defect in the presence of an applied magnetic field. The cross section of the cylinder is assumed to be circular. The field is taken uniform and parallel to the cylinder axis. We use the London theory to investigate the vortex lattice inside the wire. Although this theory is valid in the limit of low vortex density, that is, when the nearest neighbor vortex distance is much larger than the coherence length, we can obtain a reasonable qualitative description of lattice properties. We calculate: (1) the vortex lattice structure using the simulated annealing technique; (2) the magnetization curve as a function of the applied field.

Resumo em Inglês:

The critical behavior of zero-temperature superconducting transitions which can occur in disordered two-dimensional Josephson-junction arrays are investigated by Monte Carlo calculation of ground-state excitation energies and dynamical simulation of the current-voltage characteristics at nonzero temperatures. Two models of arrays in an applied magnetic field are considered: random dilution of junctions and random couplings with half-ux quantum per plaquette f = 1/2. Abovea critical value of disorder, finite-size scaling of the excitation energies indicates a zero-temperature transition and allows an estimate of the critical disorder and the thermal correlation length exponent characterizing the transition. Current-voltage scaling is consistent with the zero-temperature transition. The linear resistance is nonzero at finite temperatures but nonlinear behavior sets in at a characteristic current density determined by the thermal critical exponent. The zero-temperature transition provides an explanation of the washing out of structure for increasing disorder at f = 1/2 while it remains for f = 0, observed experimentally in supercondoucting wire networks.

Resumo em Inglês:

We present a percolation theory for the high-Tc oxides pseudogap and Tc dependence on the hole level. The doping dependent inhomogeneous charge structure is modeled by a distribution which may represent the stripe morphology and yield a spatial distribution of local Tc(r). The temperature onset of spatial dependent superconducting gap is identified with the vanishing of the pseudogap temperature T*. The transition to a superconducting state corresponds to the percolation threshold among regions of different Tc. As a paradigm we use a Hubbard Hamiltonian with a mean field approximation to yield a doping and temperature dependent superconducting d-wave gap. We show here that this new approach reproduces the phase diagram, explains and gives new insights on several experimental features of high-Tc oxides.

Resumo em Inglês:

We study the formation of Cooper pairs in high-Tc cuprate superconductors within a spin uctuation model for doped quantum Heisenberg antiferromagnets. The charge of the dopants (chargons) is associated to quantum skyrmion excitations of the Cu++ antiferromagnetic spin background. We then compute the quantum skyrmion e effective interaction potential as a function of doping and temperature in order to study charge pairing. It becomes clear that Cooper pair formation is determined by the competition between the spin uctuations of the Cu++ magnetic ions and the spins of the O-- doped holes (spinons). The superconducting transition occurs when the effective potential allows for skyrmion bound states. Our theoretical predictions for the superconducting phase diagram of La2-x Sr x CuO4 and YBa2 Cu3O6+x are in good agreement with experiment.

Resumo em Inglês:

We have shown that the Paramagnetic Meissner Effect (PME) is directly associated with pinning, and not necessarily related to the presence of pi-junctions. Through the study of the magnetic properties of two-dimensional Josephson junction arrays (2D-JJA) in the present work we show that, among the systems exhibiting PME, only those with suffciently low dissipation and high capacitance will show dynamics reentrance. The concept of a critical state and its use in the interpretation of AC magnetization data in terms of a critical current density were introduced to derive the magnetic properties of hard type-II superconductors. In the critical state model proposed by Bean, flux lines penetrate into the sample and, due to the presence of disorder they give rise to a steady flux gradient. Here we show that in 2D-JJA this typical picture is valid only in short-range distances. For long-range distances, the picture of uniform flux fronts, as described by a critical state model, breaks down and the penetration of the magnetic field takes place through the growth of magnetic dendrites. De Gennes originally compared the slope of a pile of vortices to a sand-pile, with the slope being proportional to the local magnitude of the critical current. Dynamical properties of the sand-pile problem have attracted new attention since it consists of a marginally stable system displaying self-organized criticality (SOC). In this case, when a superconductor is in the Bean critical state, the addition of vortices occurs by increasing the external magnetic field. This procedure is analogous to the introduction of new grains to a sand-pile and is expected to produce an avalanche of grains of sand (or, equivalently, vortices) of all sizes to maintain a constant gradient in the grain (or, magnetic ux) density. We show in this work strong evidences pointing out that, for some specific conditions, magnetic field penetrates 2D-JJA in ux avalanches.

Resumo em Inglês:

Polycrystalline samples of PrBa2Cu3O7– y were prepared through two different routes: (a) mixing Pr6O11, BaCO3, and CuO; (b) sol-gel precursors. The samples were heat-treated in the temperature range 700 <= T <= 930 ºC and under different atmospheres, (Argon and air). Oxidation of these compounds, based on the results of thermogravimetric analysis, TGA, was performed at low temperatures, from 300 to 400 ºC, for time intervals as long as 120 h. The crystal structure was determined by measurements of X-ray powder diffraction and revealed that samples heat-treated at 810 ºC in Ar and for 130 h have a tetragonal structure with lattice parameters c <FONT FACE=Symbol>~</FONT> 11.88 Å. Oxygenated PrBa2Cu3O7-y samples were found to have an orthorhombic structure with lattice parameter c ranging from 11.72 to 11.75 Å. Electrical resistivity rho(T) measurements performed on these compounds revealed a semiconductor-like behavior in the temperature range 77 <= T <= 300 K. The magnetic susceptibility chi(T) data showed a paramagnetic-like behavior from 5 to 300 K and a clear contribution of the crystalline electrical field (CEF) below 150 K. We have also found an antiferromagnetic ordering associated with the Pr sub-lattice at T N <FONT FACE=Symbol>~</FONT> 17 K. Evidence of super-conductivity was not observed on our samples, in agreement with most of the results reported in the literature.

Resumo em Inglês:

Temperature dependent Mössbauer spectroscopy on 57Fe doped (1 at % of Ni) RNi2B2C and RNiBC provided clear evidence of a pair-breaking field at the Ni site for non-superconducting compounds. This field is not present in the superconducting collinear AF DyNi2B2C, however it appears when this compound is diluted with non-magnetic Lu (Dy1-xLu xNi2B2C). Important local information on the spin structure of the R magnetic moments is obtained for both systems. The local symmetries of RNi2B2C and RNiBC compounds, measured through the deltaE Q, were found to be linearly correlated with the c<FONT FACE=Symbol>¢/</FONT>a ratio (c´ is the distance of the R-C layers between which the Ni2-B2 layers are sandwiched). A scaling of Tc with the local symmetry (c<FONT FACE=Symbol>¢/</FONT>a) was found to be the same for all the RNi2B2C and RNiBC (except for Y) compounds.

Resumo em Inglês:

This work presents a study of structural and magnetic properties of the Ba2Cu3O4Cl2 (Ba2342) compound. Doped samples with K or Sr in the Ba site and Zn or Mg in the Cu site of the Ba2342 were prepared using a solid state diffusion method. They were characterized by X-ray powder diffractometry, scanning electron microscopy and dc-magnetization. Samples with Ba2342 composition presented tetragonal structure of the I4/mmm space group when heat-treated at 750ºC. A structural transition was observed when the samples were doped with Zn or Mg or heat-treated at 700ºC. dc-Magnetization measurements performed in SQUID showed the existence of diamagnetic signal. A difference of the magnetic moments measured at zero field cooled (ZFC) and field-cooled (FC) procedures was observed suggesting the existence of localized superconductivity in this oxychloride system.

Resumo em Inglês:

This paper presents a review on reported anisotropic properties of MgB2. The first direct measurement of an anisotropic superconducting property in MgB2was achieved for the bulk nucleation field Hc2, in samples of aligned crystallites. A ratio <IMG SRC="http:/img/fbpe/bjp/v32n3/a13img01.gif">/ <IMG SRC="http:/img/fbpe/bjp/v32n3/a13img02.gif">~ 1:7 was found between the critical field parallel to the ab plane and parallel to the c axis direction. Further, detailed study of the Hc2 angular dependence confirmed its bulk origin, in contrast with what would be expected for the surface nucleation field Hc3. A Fermi velocity anisotropy was evaluated to be <IMG SRC="http:/img/fbpe/bjp/v32n3/a13img03.gif">~1:6 <IMG SRC="http:/img/fbpe/bjp/v32n3/a13img04.gif">, assuming an isotropic order parameter. For an anisotropic s-wave pairing symmetry it has been estimated that <IMG SRC="http:/img/fbpe/bjp/v32n3/a13img03.gif"> > or = 2:5 <IMG SRC="http:/img/fbpe/bjp/v32n3/a13img04.gif">. Different Hc2 anisotropy has been found by different authors, using different samples, measured in varied temperature ranges. Other reported anisotropic properties of MgB2 in the superconducting state are the field penetration depth <IMG SRC="http:/img/fbpe/bjp/v32n3/a13img05.gif">, coherence length xi, and energy gap delta; in the normal state are the magnetoresistance, compressibility, and thermal expansion. So far, most of the reported results have been obtained using aligned crystallites, c-axis oriented thin films and sub-millimeter crystals.

Resumo em Inglês:

X-ray diffraction, ac susceptibility and 57Fe Mössbauer spectroscopy measurements have been performed in the MgC(Ni1-xFe x )(0 <= x <= 0.5) series. The superconducting transition temperature Tc of the undoped sample, determined by ac susceptibility, is <FONT FACE=Symbol>~</FONT> 7K. The Fe doping quickly depresses Tc, completely destroying the superconducting state for x <FONT FACE=Symbol>~</FONT> 0.04. The establishment of an ordered magnetic state is only observed for Fe concentration (x > or = 0.3) far above the concentration for which the superconducting state has completely disappeared. An expansion of the lattice with the increase of the Fe content was observed.

Resumo em Inglês:

This work presents a mean field estimation of JC as a bulk characteristic of YBCO blocks. That average JC allows a good fitting of the finite-element-method (FEM) simulation of the levitation forces to experimental results. That agreement is quite enough for levitation requirements of device projects, at short gaps and zero field cooling process, within the Bean model. The physical characterization for that estimation was made measuring the interaction force between the PM and one YBCO block in 1-D and mapping the trapped magnetic field in those blocks in 2-D.

Resumo em Inglês:

Effects of grain-domain-size (GDS) on levitation force have been directly investigated and identified in well-textured YBCO bulks. A single-grain-domain YBCO bulk (phi=30mm) was prepared by a top seeded melt growth process, then divided into two, three and four grain-domain to acquire the levitation forces of samples with different GDS. It is found that the levitation force of the samples monotonously decreases with the decreasing of the average GDS (or with the increasing of the total length grain-boundaries of the sample). The maximum levitation force for the single-grain-domain sample is about 1.68, 2.05 and 2.4 times higher than that of the samples with two, three, and four grain-domains. It is concluded that the levitation force of a single-grain-domain YBCO bulk is higher than that of samples with multi-grain-domains. It is also found that the levitation force is proportional to the average grain-domain radius, but inverse proportional to the total length of grain-boundaries of the sample, a simple physical model has been provided and well interpret the experiment results.

Resumo em Inglês:

Recent results obtained by the author for the dynamical phase diagrams for vortices in clean films, driven by an uniform force, and interacting with periodic pinning resulting from a columnar defect lattice are discussed. Using numerical simulations of a simple model and other considerations, the dynamical phase diagrams are obtained as a function of the driving force magnitude and direction, the temperature, and the vortex density. The following dynamical phases and dynamical phase transitions are found. Moving vortex lattices at low temperatures, with spatial order that can be commensurate or incommensurate with the periodic pinning, moving vortex liquids and moving smectics. Dynamical melting of moving vortex lattices into moving vortex liquids takes place and transverse pinning of moving commensurate vortex lattices and smectics occurs. It is found that the dynamical phase diagrams in the theoretical limit of infinite driving force magnitudes play a central role in determining the whole dynamical phase diagram: each dynamical phase originates from an infinite-drive limit phase with the same spatial symmetry that evolves continuously into finite-drive regions of the dynamical phase diagram. It is argued that this conclusion also applies for a large class of periodic pinning potentials.

Resumo em Inglês:

The magnetic response of MgB2 samples prepared by solid state reaction method, were studied. Important features revealing the granular character of the specimens were verified, as (i) depressed magnetization versus field response, when compared with the corresponding curve for unlinked powder, (ii) the paramagnetic Meissner effect and (iii) the occurrence of a magnetic remanence in a limited interval of temperatures. Experimental evidence is added to indicate that these effects are related to flux pinning.

Resumo em Inglês:

The anisotropic effects on the dynamical phases of a vortex lattice driven by a uniform Lorentz force are investigated by numerical simulations. A square array of columnar defects is represented by a periodic pinning potential. We assume that ow is elastic, so that the vortex-vortex interactions can be represented by elastic forces. V-I characteristics and mean-square displacements and velocities for different orientations of the driving force are calculated. It is found that, for a wide range of driving force magnitude and orientation, the vortex lattice is transversely pinned in high symmetry directions of the pinning potential. In addition, the transverse pinned vortex lattice is thermally depinned at high enough temperatures.

Resumo em Inglês:

An electrostatic model was presented for the calculation of the capacitance-voltage characteristics of a semiconductor structure where quantum dots were embedded. The model was based on the linear coupling between the contributions of the quantum dots and the bulk host. We further applied this model to an InAs/GaAs self-assembled quantum dots system. The calculated capacitance was found in good agreement with the experimental curves, providing parameters of the dots ensemble, as the excitation energy of the confined electrons.

Resumo em Inglês:

A description is made of electron trajectories in a magnetic field consisting of helical wiggler and a uniform axial guide field. The explicit motion of particles for which the quadratic approximation to the Hamiltonian was carried out in some previous publications is now presented. The transverse position as a function of time is essentially the superposition of three circular motions of different frequency; one corresponds to the fixed point, the second to the cyclotronic motion, while the third is a very slow motion of the center of gyration. Electrons in this model swept out the x - y plane with elliptical and circular orbits that characterize the group I, the normal and reversed-field group-II configurations. Several illustrative examples are discussed in detail.

Resumo em Inglês:

We consider the effects of homogeneous Dirichlet's boundary conditions on two infinite parallel plane surfaces separated by some small distance a. We find that although spontaneous symmetry breaking does not occur for the theory of a massless, quartically self-interacting real scalar field, the theory becomes a theory of a massive scalar field.

Resumo em Inglês:

In this work we present a theoretical study on electron scattering by both polar and nonpolar polyatomic molecules in the low-energy range. More specifically, we report calculated elastic and rotationally inelastic differential cross sections for electron scattering by CH4, H2O, and H2S in the (2.14-30)-eV range. Exact static-exchange plus model correlation-polarization potentials are used to represent the electron-molecule interaction. The Schwinger variational iterative method is used to solve the scattering equations. In addition, the adiabatic-nuclei-rotation approximation is applied to calculate rotational cross sections. The comparison of our calculated results with experimental and other theoretical data available in the literature is encouraging.

Resumo em Inglês:

We evaluate the quantum propagator for the motion of a particle in a linear potential via a recently developped formalism [A.B. Nassar et al., Phys. Rev. E56, 1230, (1997)]. In this formalism, the propagator comes about as a type of expansion of the wave function over the space of the initial velocities.

Resumo em Inglês:

We investigate the spectrum of small amplitude oscillations of the sine-Gordon vortex-antivortex pair in layered ferromagnetic systems described by the anisotropic three-dimensional XY-model. In the case of a small interlayer coupling constant, vortex-pairs can be formed independently in each plane. Analytical expressions for two discrete modes with zero frequency are obtained. These zero modes are associated with the two-dimensional motion of the excitation on an individual plane and are important because the interference between them and a vortex-pair gives rise to contributions to the central peak. The continuum states, which contribute to EPR linewidth, are derived using the Born approximation.