Crossover between ionic/covalent and pure ionic bonding in magnesium oxyde clusters

An empirical potential with fluctuating charges is proposed for modelling (MgO)_n clusters in both the molecular (small n) and bulk (n->infty) regimes. Vectorial polarization forces are explicitely taken into account in the self-consistent determination of the charges. Our model predicts cuboid cluster structures, in agreement with previous experimental and theoretical results. The effective charge transferred between magnesium and oxygen smoothly increases from 1 to 2, with an estimated crossover size above 300 MgO molecules

BB Intermeson Potentials in the Quark Model

In this paper we derive quark model results for scattering amplitudes and equivalent low energy potentials for heavy meson pairs, in which each meson contains a heavy quark. This "BB" system is an attractive theoretical laboratory for the study of the nuclear force between color singlets; the hadronic system is relatively simple, and there are lattice gauge theory (LGT) results for V_BB(r) which may be compared to phenomenological models. We find that the quark model potential (after lattice smearing) has qualitative similarities to the LGT potential in the two B*B* channels in which direct comparison is possible, although there is evidence of a difference in length scales. The quark model prediction of equal magnitude but opposite sign for I=0 and I=1 potentials also appears similar to LGT results at intermediate r. There may however be a discrepancy between the LGT and quark model I=1 BB potentials. A numerical study of the two-meson Schrodinger equations in the (bqbar)(bqbar) and (cqbar)(cqbar) sectors with the quark model potentials finds a single "molecule", in the I=0 BB* sector. Binding in other channels might occur if the quark model forces are augmented by pion exchange.Comment: 30 pages, 5 figures, revtex and epsfig. Submitted to Phys. Rev.

Rules, Standards, and the Internal Point of View

Large scale structure and cosmolog

UPLC-MS-ESI-QTOF analysis and antifungal activity of the spondias tuberosa arruda leaf and root hydroalcoholic extracts

The aim of this study was to identify and evaluate the chemical compositions and effects of the S. tuberosa leaf and root hydroalcoholic extracts (HELST and HERST) against different strains of Candida. Chemical analysis was performed by Ultra-Performance Liquid Chromatography Coupled to Quadrupole/Time of Flight System (UPLC-MS-ESI-QTOF). The Inhibitory Concentration of 50% of the growth (IC50) as well as the intrinsic and combined action of the extracts with the antifungal fluconazole (FCZ) were determined by the microdilution method while the minimum fungicidal concentrations (MFCs) and the effect on fungal morphological transitions were analyzed by subculture and in humid chambers, respectively. From the preliminary phytochemical analysis, the phenols and flavonoids were the most abundant. The intrinsic IC50 values for HELST ranged from 5716.3 to 7805.8 \ub5g/mL and from 6175.4 to 51070.9 \ub5g/mL for the HERST, whereas the combination of the extracts with fluconazole presented IC50 values from 2.65 to 278.41 \ub5g/mL. The MFC of the extracts, individually, for all the tested strains was 6516384 \ub5g/mL. When fluconazole was combined with each extract, the MFC against CA URM 5974 was reduced (HELST: 2048 and HERST: 4096 \ub5g/mL). Synergism was observed against standard C. albicans (CA) and C. tropicalis (CT) strains and with the root extract against the CT isolate. The leaf extract inhibited the morphological transition of all strains while the root extract inhibited only CT strains

Response of vegetation to fire disturbance: short-term dynamics in two savanna physiognomies

Fire is a constitutive ecological force in savanna ecosystems, but few studies have monitored its short-term effects on plant community dynamics. This study investigated changes in plant diversity in the South American savanna (Cerrado) after severe disturbance by fire. We monitored 30 permanent plots (10 m × 5 m) distributed in two Cerrado physiognomies (típico: more forested; ralo: grass-dominated), being 10 plots in the area disturbed by fire, and five in a preserved control area (undisturbed). From August 2010 to June 2011, we evaluated changes in species richness, abundance and composition of savanna vegetation. Monitoring started one week after the fire; disturbed plots were surveyed monthly, while control plots were surveyed every two months. We observed rapid reassembling in both physiognomies: plots affected by fire showed rapid increase in species richness and plant density during the first four months after the disturbance. Concerning species composition, disturbed plots in the cerrado típico tended to converge to control plots after one year, but each local assemblage followed particular temporal trajectories. A different pattern characterized cerrado ralo plots, which showed heterogeneous trajectories and lack of convergence between disturbed and control plots; the structure of these assemblages will likely change in next years. In conclusion, our results showed that fire significantly affected plant diversity in the two savanna physiognomies (cerrado típico and ralo), but also indicated that community reassembling is fast, with different dynamics between Cerrado physiognomies

NN Core Interactions and Differential Cross Sections from One Gluon Exchange

We derive nonstrange baryon-baryon scattering amplitudes in the nonrelativistic quark model using the ``quark Born diagram" formalism. This approach describes the scattering as a single interaction, here the one-gluon-exchange (OGE) spin-spin term followed by constituent interchange, with external nonrelativistic baryon wavefunctions attached to the scattering diagrams to incorporate higher-twist wavefunction effects. The short-range repulsive core in the NN interaction has previously been attributed to this spin-spin interaction in the literature; we find that these perturbative constituent-interchange diagrams do indeed predict repulsive interactions in all I,S channels of the nucleon-nucleon system, and we compare our results for the equivalent short-range potentials to the core potentials found by other authors using nonperturbative methods. We also apply our perturbative techniques to the N$\Delta$ and $\Delta\Delta$ systems: Some $\Delta\Delta$ channels are found to have attractive core potentials and may accommodate ``molecular" bound states near threshold. Finally we use our Born formalism to calculate the NN differential cross section, which we compare with experimental results for unpolarised proton-proton elastic scattering. We find that several familiar features of the experimental differential cross section are reproduced by our Born-order result.Comment: 27 pages, figures available from the authors, revtex, CEBAF-TH-93-04, MIT-CTP-2187, ORNL-CCIP-93-0