Optimal random sampling designs in random field sampling

A Horvitz-Thompson predictor is proposed for spatial sampling when the characteristic of interest is modeled as a random field. Optimal sampling designs are deduced under this context. Fixed and variable sample size are considered

Polydispersity Effects in the Dynamics and Stability of Bubbling Flows

The occurrence of swarms of small bubbles in a variety of industrial systems enhances their performance. However, the effects that size polydispersity may produce on the stability of kinematic waves, the gain factor, mean bubble velocity, kinematic and dynamic wave velocities is, to our knowledge, not yet well established. We found that size polydispersity enhances the stability of a bubble column by a factor of about 23% as a function of frequency and for a particular type of bubble column. In this way our model predicts effects that might be verified experimentally but this, however, remain to be assessed. Our results reinforce the point of view advocated in this work in the sense that a description of a bubble column based on the concept of randomness of a bubble cloud and average properties of the fluid motion, may be a useful approach that has not been exploited in engineering systems.Comment: 11 pages, 2 figures, presented at the 3rd NEXT-SigmaPhi International Conference, 13-18 August, 2005, Kolymbari, Cret

Maxwell Superalgebras and Abelian Semigroup Expansion

The Abelian semigroup expansion is a powerful and simple method to derive new Lie algebras from a given one. Recently it was shown that the $S$-expansion of $\mathfrak{so}\left( 3,2\right)$ leads us to the Maxwell algebra $\mathcal{M}$. In this paper we extend this result to superalgebras, by proving that different choices of abelian semigroups $S$ lead to interesting $D=4$ Maxwell Superalgebras. In particular, the minimal Maxwell superalgebra $s\mathcal{M}$ and the $N$-extended Maxwell superalgebra $s\mathcal{M}^{\left( N\right) }$ recently found by the Maurer Cartan expansion procedure, are derived alternatively as an $S$-expansion of $\mathfrak{osp}\left( 4|N\right)$. Moreover we show that new minimal Maxwell superalgebras type $s\mathcal{M}_{m+2}$ and their $N$-extended generalization can be obtained using the $S$-expansion procedure.Comment: 31 pages, some clarifications in the abstract,introduction and conclusion, typos corrected, a reference and acknowledgements added, accepted for publication in Nuclear Physics

N=1 Supergravity and Maxwell superalgebras

We present the construction of the $D=4$ supergravity action from the minimal Maxwell superalgebra $s\mathcal{M}_{4}$, which can be derived from the $\mathfrak{osp}\left( 4|1\right)$ superalgebra by applying the abelian semigroup expansion procedure. We show that $N=1$, $D=4$ pure supergravity can be obtained alternatively as the MacDowell-Mansouri like action built from the curvatures of the Maxwell superalgebra $s\mathcal{M}_{4}$. We extend this result to all minimal Maxwell superalgebras type $s\mathcal{M}_{m+2}$. The invariance under supersymmetry transformations is also analized.Comment: 22 pages, published versio

Renormalization of spin-orbit coupling in quantum dots due to Zeeman interaction

We derive analitycally a partial diagonalization of the Hamiltonian representing a quantum dot including spin-orbit interaction and Zeeman energy on an equal footing. It is shown that the interplay between these two terms results in a renormalization of the spin-orbit intensity. The relation between this feature and experimental observations on conductance fluctuations is discussed, finding a good agreement between the model predictions and the experimental behavior.Comment: 4 pages, no figures. To appear in Phys. Rev. B (Brief Report) (2004