q-thermostatistics and the analytical treatment of the ideal Fermi gas

We discuss relevant aspects of the exact q-thermostatistical treatment for an ideal Fermi system. The grand canonical exact generalized partition function is given for arbitrary values of the nonextensivity index q, and the ensuing statistics is derived. Special attention is paid to the mean occupation numbers of single-particle levels. Limiting instances of interest are discussed in some detail, namely, the thermodynamic limit, considering in particular both the high- and low-temperature regimes, and the approximate results pertaining to the case q \sim 1 (the conventional Fermi-Dirac statistics corresponds to q=1). We compare our findings with previous Tsallis' literature.Comment: v2: comparison with conventional results and validity of approximations clarified, typos corrected; accepted for publication in Physica

The orbital-motion-limited regime of cylindrical Langmuir probes

An asymptotic analysis of electron collection at high bias Fp serves to determine the domain of validity of the orbital-motion-limited regime of cylindrical Langmuir probes, which is basic for the workings of conductive bare tethers. The radius of a wire collecting OML current in an unmagnetized plasma at rest cannot exceed a value, Rmax , which is found to exhibit a minimum as a function of Fp ; atFp values of interest, Rmax is already increasing and is larger than the electron Debye length lDe . The breakdown of the regime relates to conditions far fromthe probe, at electron energies comparable to the ion thermal energy, kTi ; Rmax is found to increase with Ti . It is also found that ~1! the maximumwidth of a thin tape, if used instead of a wire, is 4Rmax ; ~2! the electron thermal gyroradius must be larger than both R and lDe for magnetic effects to be negligible; and ~3! conditions applying to the tether case are such that trapped-orbit effects are negligible

Shape and size of simple cations in aqueous solutions: A theoretical reexamination of the hydrated ion via computer simulations

The simplest representation of monoatomic cations in aqueous solutions by means of a sphere with a radius chosen on the basis of a well-defined property (that of the bare ion or its hydrate) is reexamined considering classical molecular dynamics simulations. Two charged sphere–water interaction potentials were employed to mimic the bare and hydrated cation in a sample of 512 water molecules. Short-range interactions of trivalent cations were described by Lennard-Jones potentials which were fitted from ab initio calculations. Five statistically independent runs of 150 ps for each of the trivalent spheres in water were carried out in the microcanonical ensemble. A comparison of structural and dynamical properties of these simple ion models in solution with those of a system containing the Cr3+ hydrate ([Cr(H2O)6]3+) is made to get insight into the size and shape definition of simple ions in water, especially those that are highly charged. Advantages and shortcomings of using simple spherical approaches are discussed on the basis of reference calculations performed with a more rigorous hydrated ion model [J. Phys. Chem. B 102, 3272 (1998)]. The importance of nonspherical shape for the hydrate of highly charged ions is stressed and it is paradoxically shown that when spherical shape is retained, the big sphere representing the hydrate leads to results of ionic solution worse than those obtained with the small sphere. A low-cost method to generate hydrated ion–water interaction potentials taking into account the shape of the ionic aggregate is proposed.Dirección General de Investigación Científica y Técnica PB95-054

Protein tyrosine phosphatase activity modulation by endothelin-1 in rabbit platelets

AbstractProtein tyrosine phosphorylation, modulated by the rate of both protein tyrosine kinase and protein tyrosine phosphatase activities, is critical for cellular signal transduction cascades. We report that endothelin-1 stimulation of rabbit platelets resulted in a dose- and time-dependent tyrosine phosphorylation of four groups of proteins in the molecular mass ranges of 50, 60, 70–100 and 100–200 kDa and that one of these corresponds to focal adhesion kinase. This effect is also related to the approximately 60% decrease in protein tyrosine phosphatase activity. Moreover, this inhibited activity was less sensitive to orthovanadate. In the presence of forskolin that increases the cAMP level a dose-dependent inhibition of the endothelin-stimulated tyrosine phosphorylation of different protein substrates and a correlation with an increase in the protein tyrosine phosphatase activity (11.6-fold compared to control) have been found. Further studies by immunoblotting of immunoprecipitated soluble fraction with anti-protein tyrosine phosphatase-1C from endothelin-stimulated platelets have demonstrated that the tyrosine phosphorylation of platelet protein tyrosine phosphatase-1C is correlated with the decrease in its phosphatase activity. As a consequence, modulation and regulation by endothelin-1 in rabbit platelets can be proposed through a cAMP-dependent pathway and a tyrosine phosphorylation process that may affect some relevant proteins such as focal adhesion kinase

Note on Scalar Fields Non-Minimally Coupled to (2+1)-Gravity

Scalar fields non--minimally coupled to (2+1)-gravity, in the presence of cosmological constant term, are considered. Non-minimal couplings are described by the term $\zeta R \Psi^2$ in the Lagrangian. Within a class of static circularly symmetric space-times, it is shown that the only existing physically relevant solutions are the anti-de Sitter space-time for $\zeta=0$, and the Martinez-Zanelli black hole for $\zeta=1/8$. We obtain also two new solutions with non-trivial scalar field, for $\zeta=1/6$ and $\zeta=1/8$ respectively, nevertheless, the corresponding space-times can be reduced, via coordinate transformations, to the standard anti-de Sitter space.Comment: 5 pages, RevTe

Study of the stabilization energies of halide-water clusters: An application of first-principles interaction potentials based on a polarizable and flexible model

The aim of this work is to compute the stabilization energy Estab(n) of [ X(H2O)n ]- (X=F, Br, and I for n=1 – 60) clusters from Monte Carlo simulations using first-principles ab initio potentials. Stabilization energy of [ X(H2O)n ]- clusters is defined as the difference between the vertical photodeachment energy of the cluster and the electron affinity of the isolated halide. On one hand, a study about the relation between cluster structure and the Estab(n) value, as well as the dependence of the latter with temperature is performed, on the other hand, a test on the reliability of our recently developed first-principles halide ion-water interaction potentials is carried out. Two different approximations were applied: (1) the Koopmans’ theorem and (2) calculation of the difference between the interaction energy of [ X(H2O)n ]- and [ X(H2O)n ] clusters using the same ab initio interaction potentials. The developed methodology allows for using the same interaction potentials in the case of the ionic and neutral clusters with the proviso that the charge of the halide anion was switched off in the latter. That is, no specific parametrization of the interaction potentials to fit the magnitude under study was done. The good agreement between our predicted Estab(n) and experimental data allows us to validate the first-principles interaction potentials developed elsewhere and used in this study, and supports the fact that this magnitude is mainly determined by electrostatic factors, which can be described by our interaction potentials. No relation between the value of Estab(n) and the structure of clusters has been found. The diversity of Estab(n) values found for different clusters with similar interaction energy indicates the need for statistical information to properly estimate the stabilization energy of the halide anions. The effect of temperature in the prediction of the Estab(n) is not significant as long as it was high enough to avoid cluster trapping into local equilibrium configurations which guarantees an appropriate sampling of the configurational space. Parallel tempering method was applied in particular cases to guarantee satisfactory sampling of clusters at low temperatureDirección General de Investigaciones Científicas y Técnicas BQU2002- 0221

On the halide hydration study: Development of first-principles halide ion-water interaction potential based on a polarizable model

The development of first-principles halide-water interaction potentials for fluoride and iodide anions is presented. The model adopted is the mobile charge densities in harmonic oscillator that allows for a flexible and polarizable character of the interacting particles. The set of points of the quantum mechanical potential energy surfaces are calculated up to the MP2 level. The nonadditive many-body contributions were included explicitly at the three-body terms. Structural and energetic properties of the [ X(H2O)n ]- clusters (n=1 – 6) are studied with the new interaction potentials developed. Halide aqueous solutions are also studied by means of Monte Carlo simulations. The agreement between experimental and our predicted estimations shows the good behavior of the proposed potentials. The developed potentials are able to properly describe both the microsolvation of clusters in gas phase and their hydration in aqueous solutions. The different nature of the interactions among F-, Br-, I- and water appears in the set of studied properties, thus giving a gradual change in the behavior along the group.Dirección General de Investigaciones Científicas y Técnicas BQU2002-0221

Hard photon and neutral pion production in cold nuclear matter

The production of hard photons and neutral pions in 190 MeV proton induced reactions on C, Ca, Ni, and W targets has been for the first time concurrently studied. Angular distributions and energy spectra up to the kinematical limit are discussed and the production cross-sections are presented. From the target mass dependence of the cross-sections the propagation of pions through nuclear matter is analyzed and the production mechanisms of hard photons and primordial pions are derived. It is found that the production of subthreshold particles proceeds mainly through first chance nucleon-nucleon collisions. For the most energetic particles the mass scaling evidences the effect of multiple collisions.Comment: submitted to Phys. Lett.

Electromagnetic Radiation Hardness of Diamond Detectors

The behavior of artificially grown CVD diamond films under intense electromagnetic radiation has been studied. The properties of irradiated diamond samples have been investigated using the method of thermally stimulated current and by studying their charge collection properties. Diamonds have been found to remain unaffected after doses of 6.8 MGy of 10 keV photons and 10 MGy of MeV-range photons. This observation makes diamond an attractive detector material for a calorimeter in the very forward region of the proposed TESLA detector.Comment: 19 pages, 9 figure

The Reelin Pathway Modulates the Structure and Function of Retinal Synaptic Circuitry

AbstractThe formation of synaptic connections requires the coordination of specific guidance molecules and spontaneous neuronal activity. The visual system has provided a useful model for understanding the role of these cues in shaping the precise connections from the neural retina to the brain. Here, we demonstrate that two essential genes in the Reelin signaling pathway function during the patterning of synaptic connectivity in the retina. Physiological studies of mice deficient in either reelin or disabled-1 reveal an attenuation of rod-driven retinal responses. This defect is associated with a decrease in rod bipolar cell density and an abnormal distribution of processes in the inner plexiform layer . These results imply that, in addition to its essential role during neuronal migration, the Reelin pathway contributes to the formation of neuronal circuits in the central nervous system