Modified gravity on the brane and dark energy

We analyze the dynamics of an AdS₅ braneworld with matter fields when gravity is allowed to deviate from the Einstein form on the brane. We consider exact five-dimensional warped solutions which are associated with conformal bulk fields of weight –4 and describe on the brane the following three dynamics: those of inhomogeneous dust, of generalized dark radiation, and of homogeneous polytropic dark energy. We show that, with modified gravity on the brane, the existence of such dynamical geometries requires the presence of non-conformal matter fields confined to the brane.     

Zn-doping effect on the energy barrier to magnetization reversal in superparamagnetic nickel ferrite nanoparticles

Measurements of ac-susceptibility and dc-magnetization were carried out on samples of Ni_1-xZn_xFe₂O₄ nanoparticles (x=0, 0.25, 0.5, 0.75) with average diameters 〈D〉≈7 nm. Values of the superparamagnetic blocking temperature T̄_B were obtained from the characteristic temperature behavior of the imaginary susceptibility χ_imag. An Arrhenius-type law, which accurately describes the relationship between the observation time τ_obs and the blocking temperature, was used to determine the effective energy barrier to magnetization reversal U_eff. A Zn-content dependence of the energy barrier is observed, where U_eff changes little for 0≤x≤0.25, it peaks at x=0.5, and decreases back upon further Zn-doping. The large increase of U_eff at x=0.5 is attributed to an enhanced magnetic anisotropy induced by the crossover between two spatial arrangements of spins in the A and B sub-lattices of the ferrimagnetic inverse spinel. PACS 75.50.Bb; 75.50.Gg; 75.30.Et     

One-loop counterterms in 2d dilaton-Maxwell quantum gravity

The renormalization structure of two-dimensional quantum gravity is investigated in a covariant gauge. One-loop divergences of the effective action are calculated. All the surface divergent terms are taken into account thus completing previous one-loop calculations of the theory. It is shown that the on-shell effective action contains only surface divergences. The off-shell renormalizability of the theory is discussed and classes of renormalizable dilaton and Maxwell potentials are found.     

Generation of Molecular Fields, Quantum Similarity Measures and Related Questions

A straightforward discussion on how to generate molecular fields is developed within the postulates of quantum mechanics. The theoretical formalism points towards the generalization and extension of the well-known molecular field forms, associated to density function and electrostatic molecular potential (EMP), including another category of fields associated to quantum molecular similarity measures. The results show that the new formalism can be easily applied to obtain an unlimited number of new information about molecular behavior.     

Current theories in the calculation of activity coefficients-II. Specific interaction theories applied to some equilibria studies in solution chemistry

The adequation of the current theories for calculating activity coefficients has been studied for a series of ionic equilibria in analytical chemistry: the water ionic product as well as two reactions of complex formation and oxidation-reduction. In all the cases, the specific interaction theory (SIT) has proved to be adequate and simple to apply in a restricted interval (0.5-3M), whereas Pitzer's theory is outstanding for its high precision in reproducing experimental behaviour and to correlate values of equilibrium constants for different conditions and media.     

On the automatic restricted-step rational-function-optimization method

The rational function optimization algorithm is one of the widely used methods to search stationary points on surfaces. However, one of the drawbacks of this method is the step reduction procedure to deal with the overstepping problem. We present and comment on a method such that the step obtained from the solution of the rational function equations possesses the desired correct length. The analysis and discussion of the method is mainly centered on the location and optimization of transition states. Received: 18 June 1998 / Accepted: 17 September 1998 / Published online: 23 November 1998     

Completely soft molecular electrostatic potentials (CoSMEP) and total density functions

Soft molecular electrostatic potentials (SMEP or SEMP) have been recently defined substituting the point-like proton by a Gaussian positive charge distribution. In the present paper an additional step is taken forward, transforming SMEP into a completely soft MEP (CoSMEP). Such transformation is carried out using a charge distributed proton as in SMEP and also a Gaussian positive nuclear charge distribution, instead of the classical point-like nuclear charges. The general form of MEP is roughly preserved, but new features can be noticed. Such new point of view is also associated to the possibility to redefine the molecular charge density. Definition of CoSMEP is thus connected to the definition of total molecular density functions (DF), where to the negative electronic DF is summed up the soft nuclear DF, made of linear combinations of Gaussian distributions of nuclear charges.     

On the nature of atomic shell approximation (ASA) electrostatic molecular potentials (EMP)

General properties of the electrostatic molecular potential (EMP) when obtained by means of the atomic shell approximation (ASA) density function (DF) are discussed. In the present study it is proved that in general and without exceptions promolecular ASA DF provide EMP which are everywhere repulsive. The polarized ASA DF version, providing an alternative enhanced kind of EMP, behaves in such a way that in the neighborhood of negative charged atoms, attractive regions appear resembling the ab initio results.