Angular dependence of the bulk nucleation field Hc2 of aligned MgB2 crystallites

Studies on the new MgB2 superconductor, with a critical temperature Tc ~ 39 K, have evidenced its potential for applications although intense magnetic relaxation effects limit the critical current density, Jc, at high magnetic fields. This means that effective pinning centers must be added into the material microstructure, in order to halt dissipative flux movements. Concerning the basic microscopic mechanism to explain the superconductivity in MgB2, several experimental and theoretical works have pointed to the relevance of a phonon-mediated interaction, in the framework of the BCS theory. Questions have been raised about the relevant phonon modes, and the gap and Fermi surface anisotropies, in an effort to interpret spectroscopic and thermal data that give values between 2.4 and 4.5 for the gap energy ratio. Preliminary results on the anisotropy of Hc2 have shown a ratio, between the in-plane and perpendicular directions, around 1.7 for aligned MgB2 crystallites and 1.8 for epitaxial thin films. Here we show a study on the angular dependence of Hc2 pointing to a Fermi velocity anisotropy around 2.5. This anisotropy certainly implies the use of texturization techniques to optimize Jc in MgB2 wires and other polycrystalline components.Comment: 10 pages + 4 Figs.; Revised version accepted in Phys. Rev.

Adiabatic decaying vacuum model for the universe

We study a model that the entropy per particle in the universe is constant. The sources for the entropy are the particle creation and a lambda decaying term. We find exact solutions for the Einstein field equations and show the compatibilty of the model with respect to the age and the acceleration of the universe.Comment: 10 pages, 2 figure

Magnetic order of Cr thin films in Nb/Cr/Fe-nanoisland hybrid: a comparative study between magnetic and superconducting properties

Shifted hysteresis loops characteristic of the exchange bias effect between a ferromagnet and an antiferromagnet are demonstrated in structures formed by a 2.5 nm Cr layer deposited on top of an array of Fe nanoislands (Cr/Fe-nanoislands). This effect evidences the persistence of antiferromagnetic (AF) order for Cr layers much thinner than the thickness reported in the literature. The field shift measured is found to increase for the smallest island sizes, which can be related with the enhancement of the Fe-nanoisland surface to volume ratio. The comparative study between superconducting proximity effects in Nb/Cr/Fe-nanoislands and Nb/normal metal/Fe-nanoisland hybrids (where the normal metals used are Al and Pt) confirms the presence of AF order in the 2.5 nm Cr spacer layer. A much shorter penetration depth of the Cooper pairs into the AF Cr layers than in the normal metal Pt and Al spacer layers is deduced

Effect of arbuscular mycorrhizal fungi on the acclimatization of micropropagated banana plantlets.

The effect of arbuscular mycorrhizal fungi (AMF) on micropropagated banana plantlets was evaluated during the acclimatization period. Plants were inoculated with Acaulospora scrobiculata, Glomus clarum or Glomus etunicatum. After cultivation in a greenhouse for 3 months, height, leaf area, fresh weight and dry matter of root and shoots, level of AMF colonization, nutrient level, photosynthesis and transpiration rate, water potential and stomatal conductance were measured. The number of AMF spores produceded in each treatment was also determined. Plantlets inoculated with Amf had greater height, leaf area and fresh weight of shoots and roots, as well as higher rates of photosynthesis and transpiration than controls. Plants inoculated with Glomus were superior in most of the evaluated parameters

Nuclear break-up of 11Be

The break-up of 11Be was studied at 41AMeV using a secondary beam of 11Be from the GANIL facility on a 48Ti target by measuring correlations between the 10Be core, the emitted neutrons and gamma rays. The nuclear break-up leading to the emission of a neutron at large angle in the laboratory frame is identified with the towing mode through its characteristic n-fragment correlation. The experimental spectra are compared with a model where the time dependent Schrodinger equation (TDSE) is solved for the neutron initially in the 11 Be. A good agreement is found between experiment and theory for the shapes of neutron experimental energies and angular distributions. The spectroscopic factor of the 2s orbital is tentatively extracted to be 0.46+-0.15. The neutron emission from the 1p and 1d orbitals is also studied

Ecophysiological responses of the winegrape Chenin Blanc under different irrigation management in the São Francisco river valley.

The So Francisco Valley has been considered a new growing region for wine grapeproduction. In this region, the wines have a typical taste and are the unique ones producedunder tropical conditions. Because the irrigation management is one of the most importantfactors related to grape and wine quality, a study was carried out by the Embrapa Semirido,in the VINIBRASIL Farm vineyards, at Lagoa Grande, State of Pernambuco, Brazil inwhich the grapevine ¡§Chenin Blanc¡¨, grafted onto the IAC 572 rootstock, and spaced by 1,0m x 3,0 m, was submitted to different irrigation managements: T1) 75% Kc; T2) 50% Kc; T3)50 % Kc until 80 days after pruning and then 75% Kc until harvest and; T4) about 30% Kc(Farm management). Irrigation water was applied by dripping and the treatments initiated 45days after pruning. The effects of treatments were observed based on the ecophysiologicalperformance of the grapevine through measurements of leaf water potential (£Zl) obtainedwith a pressure chamber and gas exchange variables (stomatal conductance, gs;transpiration rate, E; photosynthesis, A), with a portable infrared gas analyzer (IRGA ¡V Licor6200) during flowering and maturation (August to September, 2006). The results showed thatgrapevines submitted to the T1 treatment presented the highest £Zl, gs, E and A values duringthe hottest hours of day followed by T3 and T2. The T4 treatment presented the lowestvalues on these variables but higher intrinsic water use efficiency (WUE) calculated as A/gs.At harvest, this treatment showed lower bunch number per plant and yield but higher oBrixand higher pH. The results suggest that the monitoring of the ecophysiological performanceof winegrapes during the growth cycle, might be a good tool to establish an appropriatedirrigation management toward grape and wine quality

Dependence of the flux creep activation energy on current density and magnetic field for MgB2 superconductor

Systematic ac susceptibility measurements have been performed on a MgB$_2$ bulk sample. We demonstrate that the flux creep activation energy is a nonlinear function of the current density $U(j)\propto j^{-0.2}$, indicating a nonlogarithmic relaxation of the current density in this material. The dependence of the activation energy on the magnetic field is determined to be a power law $U(B)\propto B^{-1.33}$, showing a steep decline in the activation energy with the magnetic field, which accounts for the steep drop in the critical current density with magnetic field that is observed in MgB$_2$. The irreversibility field is also found to be rather low, therefore, the pinning properties of this new material will need to be enhanced for practical applications.Comment: 11 pages, 6 figures, Revtex forma

Deformed Gaussian Orthogonal Ensemble Analysis of the Interacting Boson Model

A Deformed Gaussian Orthogonal Ensemble (DGOE) which interpolates between the Gaussian Orthogonal Ensemble and a Poissonian Ensemble is constructed. This new ensemble is then applied to the analysis of the chaotic properties of the low lying collective states of nuclei described by the Interacting Boson Model (IBM). This model undergoes a transition order-chaos-order from the $SU(3)$ limit to the $O(6)$ limit. Our analysis shows that the quantum fluctuations of the IBM Hamiltonian, both of the spectrum and the eigenvectors, follow the expected behaviour predicted by the DGOE when one goes from one limit to the other.Comment: 10 pages, 4 figures (avaiable upon request), IFUSP/P-1086 Replaced version: in the previous version the name of one of the authors was omitte

Exact solution for the energy density inside a one-dimensional non-static cavity with an arbitrary initial field state

We study the exact solution for the energy density of a real massless scalar field in a two-dimensional spacetime, inside a non-static cavity with an arbitrary initial field state, taking into account the Neumann and Dirichlet boundary conditions. This work generalizes the exact solution proposed by Cole and Schieve in the context of the Dirichlet boundary condition and vacuum as the initial state. We investigate diagonal states, examining the vacuum and thermal field as particular cases. We also study non-diagonal initial field states, taking as examples the coherent and Schrodinger cat states.Comment: 10 pages, 8 figure