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[eng] The research presented in this thesis contributes to the understanding of both titania and silicate nanosystems by providing new information on energetic stability and properties of nanometer sized particles using computational modelling. Particular emphasis is placed on the importance of two nanosized regimes: i) tens of atoms, and ii) several hundred up to thousands of atoms. We differentiate these two size regimes by naming nanoclusters the structures containing between tens up to a hundred of atoms, and using the term nanoparticles (NPs) for the structures containing hundreds to thousands of atoms. Titania (TiO2) is the most studied photocatalyst, and thus research is mostly focused on understanding the electronic properties of different morphologies of TiO2 NPs. In detail, for TiO2 the present thesis benchmarks the ability of several interatomic potentials (IPs) to reduce the computational cost of Density Functional Theory (DFT) calculations, as well as a detailed analysis of the energetic stability of three different morphologies of NPs together with an analysis of their band-gap. We show that the Anatase crystal structure becomes the most stable for particle sizes of ~2-3 nm in diameter, while for smaller sizes amorphous particles are the most stable. Within the Anatase structure, we see that Wulff construction is the most stable for large sizes (above 2 nm), but amorphous shell-crystalline core nanoparticles are within the same energy range below a radius of 2 nm. We also find that spherical particles have a band-gap consistent with the so-called black TiO2. On the other hand, research on silicates is mainly focused on calculating the properties of nanoclusters and NPs, with the objective of obtaining a better understanding of the relevance of such species in interstellar space. In detail, we propose global minima (GM) candidates for numerous nanoclusters based on extensive global optimization (GO) searches and compare their spectroscopic and chemical properties with literature values, where the later values are mostly derived from extrapolation using macroscale laboratory samples. The GO searches were done with a reparameterization of the FFSiOH where we included the Mg element. We also evaluate whether silicate nanoclusters can be the origin of the anomalous microwave emission (AME), a foreground emission in the microwave (MW) region of the spectra from an unknown source and find that indeed nano silicates have the appropriate dipole moments in order to be a strong source of the AME. We indicate that the amount of nano silicates in the interstellar medium is constrained by the AME emission. Finally, the IR spectra of large NPs of around 4 nm in diameter is compared on the basis of their crystallinity. We find that for such sizes, the IR spectra of the crystalline particle corresponds to a broad band similar to the amorphous material, which we ascribe to the large fraction of surface atoms. We conclude that the IR spectra is not sufficient to characterize the crystallinity of astronomical silicates with sizes of several nanometers in diameter. We also show that amorphous silicate nano particles with sizes of ~1 nm in diameter are more stable than their crystalline counterparts. We extrapolate the tendency and propose that the crystalline nanoparticles become more stable than amorphous particles at particle sizes of ~12 nm in diameter., [spa] En la presente tesis se estudian la estructura y propiedades de nano partículas de TiO2 y de silicatos de magnesio. Mediante cálculos de teoría del funcional de la densidad y cálculos de potenciales interatómicos se muestra, para TiO2, cómo la estructura de la nano partícula tiene un factor clave para entender la diferencia de energía entre la banda de conducción y la banda de valencia. Además, Comparamos la estabilidad energética de nano partículas de Anatasa con diferente geometría respecto a nano partículas y nano clúster amorfos, mostrando como la geometría de Wulff es energéticamente la más estable a partir de 2nm, pero para tamaños inferiores los clúster amorfos son más estables. Para silicatos de magnesio, se describe la estructura, espectro infra-rojo y microondas de clúster de MgSiO3 y Mg2SiO4, así como la similitud del espectro infra-rojo en materiales cristalinos y amorfos de tamaños ~4 nm. Los resultados se comparan con los modelos usados para entender las propiedades del medio interestelar. Se muestra que para nano partículas de hasta algunos nanómetros de radio no es posible diferenciar el material cristalino del amorfo en base a espectroscopia infraroja. Finalmente, se calcula la intensidad de emisión en microondas (10-60 GHz) de nano clústers de silicatos de ~100 átomos y se demuestra a nivel teórico la capacidad de los silicatos como fuente viable de la emisión anómala de microondas.

[eng] This thesis is divided in different sections. After a short introduction about the thesis contents, the theory chapter presents the Standard Model of Particle Physics (SM), with an emphasis on those properties of flavour physics, such as oscillations. In this chapter a brief comment on radiative decays, the main topic through the thesis, is given, to present the specific theoretical description of these processes. The European Centre of Nuclear Research (CERN) is introduced in the second chapter, where the nature of such organization and the different experiments it holds are explained. Special emphasis is given for the LHCb experiment, which is the one that collected the data used in the analysis. The third chapter describes the monitorization and re-calibration of the SPD sub- detector, which is the one in charge of the discrimination between photons and other electromagnetic particles. During the Run 1 (2011-2012), this sub-detector suffered from ageing due to the presence of radiation inside the LHCb cavern. This ageing led to a drop in the SPD cell efficiencies, which could imply a loss in the data quality. The SPD is key in the context of radiative decays due to the presence of photons in the radiative final states and due to the fact that the main sources of background for radiative decays are related to the mis-identification of other particles as photons. With the work presented in this thesis, the performance of the SPD sub-detector reached the same status than it did for the beginning of Run 1, which corresponds to an almost flat cell efficiency around 95%. This recovery in efficiencies was performed by applying a correction factor calculated through the study of cosmic ray data and collision data from 2015. The fourth chapter describes a software tool developed within the radiative decays context. This tool is aimed for a better description of the photon/neutral pion separation variable, which is built making use of the electromagnetic clusters that these particles leave in the LHCb electromagnetic calorimeter (ECAL). This tool builds a variable that separates between the two particles but discrepancies between the data and the simulation distributions are observed. Because of this, two additional tools were developed to improve the agreement between them. These tools trust that the separation variable can be binned in terms of the particle transverse momentum and pseudorapidity. The first of the tools, “efficiency table tool”, assigns the efficiency for a certain 2-dimensional bin as a weight to the simulation sample, so the distribution is corrected and matches the one for data. The “resampling tool”, however, creates a new variable from calibration histograms built from background- subtracted distributions, trusting all radiative decays to have a similar behaviour in the 2-dimensional bins. The two tools can also be applied to neutral pions selection. The main chapter of the thesis is related to the measurement of the ratio of branching fractions for Bs decaying into phi (which decays into two kaons) and a photon and Lambda_b, decay into a Lambda bayron (which decays into a kaon and a proton) and a photon with respect to the best controlled radiative decay, Bd decaying into a K* (which decays into a kaon and a pion) and a photon, as well as the measurement of the direct CP asymmetry for the Lambda_b and Bd decays. This analysis also implies the first observation of a b-baryon radiative decay and constitutes the best measurement of the observables presented above. The analysis is performed over the whole Run 1 LHCb dataset (3 inverse fb). Special care is applied to the Lambda_b decay since it had never been done before; taking into account the different resonances that may intervene (that cannot be distinguished since they overlap and interfere). A 2- dimensional binning (on the resonance mass and on the proton angle) is defined for the study of the efficiencies, as recommended by the theory approaches to this decay and trusting (and confirmed afterwards) the different resonance decays to give a similar efficiency for the offline selection for a given bin. The thesis concludes with an overview of the whole work presented, given the calculated results for the physical observables and comparing them to the previously measured ones in the LHCb., [spa] Esta tesis está dividida en varias partes. Después de una breve introducción sobre el contenido de la tesis, las dos primeras de ellas presentan el Modelo Estándard de la Física de Partículas (ME), el contexto teórico que describe la naturaleza de las partículas y sus interacciones; y el CERN, el laboratorio de física de partículas más grande del mundo. Se dedica especial énfasis a la física de las “desintegraciones radiativas”, que son aquellas en las que un fotón forma parte del estado final de la desintegración. La tercera de las secciones de la tesis describe la toma de datos por parte del sub- detector SPD así como el proceso de recuperación del funcionamiento posterior a la observación de pérdida de eficiencia debida a la presencia de radiación en la caverna del LHCb. Este sub-detector es el encargado de proporcionar la información necesaria que permita la diferenciación entre fotones y piones neutros y por tanto muy importante en procesos radiativos. La cuarta sección explica el desarrollo y funcionamiento de una herramienta de software destinada a una mejor descripción de la variable de separación entre fotones y piones neutros a partir de la forma de los clusters que estas partículas dejan en el calorímetro electromagnético del LHCb. La quinta sección explica el análisis de datos recogidos por el experimento LHCb durante los años 2011 y 2012 en términos de la medida de frecuencia de desintegración de procesos radiativos. En particular, se estudia la relación entre la frecuencia de desintegración entre el proceso de desintegración de un mesón Bs decayendo a una partícula phi (que a su vez se desintegra en dos kaones) y un fotón y un barión Lambda_b decayendo a una partícula Lambda* (que a su vez se desintegra en un protón y un kaón) y un fotón respecto de la desintegración de un mesón Bd a un K* (que a su vez se desintegra en un kaón y un pión) y un fotón así como la medida de asimetría de CP-paridad para las dos últimas desintegraciones. Este análisis supone la medida más precisa para estas magnitudes hasta la fecha y la primera observación de una desintegración radiativa de un barión con un quark b. La tesis termina con un capítulo de conclusiones donde se detallan los valores encontrados para las magnitudes físicas y el impacto del trabajo presentado en esta tesis, comparando los valores hallados con los existentes anteriormente y recordando que este trabajo supone la primera observación de una desintegración radiativa de un barión cuyo contenido de quarks incluye un quark b.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2016, Tutor: Joaquim Crusats Aliguer, On this research project it has been studied the viability of using supramoleculary chiral aggregates of sulfonated self-assembling porphyrins as chiral counter anions on Michael and Diels-Alder organocatalytic reactions on water, which proceed through a cationic reaction intermediate to determinate the feasibility of using a ACDC (Assymetric counteranion directed catalysis) strategy to transfer the supramolecular chirality of the aggregates on an organic reaction product enantiomeric excess. To do so, a method to prepare the chiral aggregates of the meso-5,10,15,20-tetrakis(4-sulfonatofenil)porphyrin on its zwitterionic form has been optimized in order to prevent the presence of other counter anions which could also interact with the cationic intermediates from the organocatalytic reactions. It has been proved that the zwitterionic porphyrin not only maintains the aggregation state of the sodium salt, also, the supramolecular chirality still remains even though the addition of the basic organocatalistys (pyrrolidine and isoindoline). Finally it has been shown for the studied Michael and Diels-Alder reactions, exists a narrow window of experimental conditions where it is possible to use this catalysts under ACDC conditions.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2016, Tutor: Joaquim Crusats Aliguer On this research project it has been studied the viability of using supramoleculary chiral aggregates of sulfonated self-assembling porphyrins as chiral counter anions on Michael and Diels-Alder organocatalytic reactions on water, which proceed through a cationic reaction intermediate to determinate the feasibility of using a ACDC (Assymetric counteranion directed catalysis) strategy to transfer the supramolecular chirality of the aggregates on an organic reaction product enantiomeric excess. To do so, a method to prepare the chiral aggregates of the meso-5,10,15,20-tetrakis(4-sulfonatofenil)porphyrin on its zwitterionic form has been optimized in order to prevent the presence of other counter anions which could also interact with the cationic intermediates from the organocatalytic reactions. It has been proved that the zwitterionic porphyrin not only maintains the aggregation state of the sodium salt, also, the supramolecular chirality still remains even though the addition of the basic organocatalistys (pyrrolidine and isoindoline). Finally it has been shown for the studied Michael and Diels-Alder reactions, exists a narrow window of experimental conditions where it is possible to use this catalysts under ACDC conditions.

The barrier height for the most asymmetric fission decay of doubly charged sodium clusters (Na_N^2+) into singly ionized fragments has been computed with use of density-functional theory and the jellium model. We have found that the barrier is sizable for large or intermediate-size clusters, but vanishes for N40 and, in this case, monomer evaporation becomes competitive. The critical cluster size, N_c=40, for the transition from one decay mode to the other is in reasonable agreement with the experimental result. Our calculations suggest that the mechanism for neutralmonomer evaporation is different from the one currently assumed.

Within density functional theory, we have obtained the structure of $^4$He droplets doped with neutral calcium atoms. These results have been used, in conjunction with newly determined {\it ab-initio} $^1\Sigma$ and $^1\Pi$ Ca-He pair potentials, to address the $4s4p$ $^1$P$_1 \leftarrow 4s^2$ $^1$S$_0$ transition of the attached Ca atom, finding a fairly good agreement with absorption experimental data. We have studied the drop structure as a function of the position of the Ca atom with respect of the center of mass of the helium moiety. The interplay between the density oscillations arising from the helium intrinsic structure and the density oscillations produced by the impurity in its neighborhood plays a role in the determination of the equilibrium state, and hence in the solvation properties of alkaline earth atoms. In a case of study, the thermal motion of the impurity within the drop surface region has been analyzed in a semi-quantitative way. We have found that, although the atomic shift shows a sizeable dependence on the impurity location, the thermal effect is statistically small, contributing by about a 10% to the line broadening. The structure of vortices attached to the calcium atom has been also addressed, and its effect on the calcium absorption spectrum discussed. At variance with previous theoretical predictions, we conclude that spectroscopic experiments on Ca atoms attached to $^4$He drops will be likely unable to detect the presence of quantized vortices in helium nanodrops., Comment: 33 pages with 11 figures

Ferralsols are characterized by a very well developed microstructure of which the fertility depends closely. This microstructure consists of highly stable microaggregates 50 to 800 µm in diameter. Under native vegetation and pasture, the analysis of the soil bulk density, the microaggregates size and assemblage, as well as the organic matter, highlight significant variations according to depth. We established a typology of microaggregate assemblage as well as a model relating structure development to porosity. The organic matter is characterized in pyrolysis by a component of high temperature, of conjectural origin probably due to a mineral matrix effect. The whole of the differences recorded between the soils studied cannot be explained a priori by soil use alone, but more probably by a difference of activity of the social insects which would be at the origin of the microaggregates., Les Ferralsols se caractérisent par une microstructure très développée dont dépend étroitement leur fertilité. Cette microstructure repose sur des microagrégats très stables, de 50 à 800 µm de diamètre. Sous végétation naturelle comme sous pâturage, la densité apparente des sols, la taille et le mode d'assemblage des microagrégats, ainsi que la caractérisation de la matière organique, ont permis de mettre en évidence d'importantes variations avec la profondeur. Nous avons établi une typologie du mode d'assemblage des microagrégats ainsi qu'un modèle mettant en relation structure et porosité. La matière organique se singularise, en pyrolyse, par une composante de haute température, d'origine conjecturale, mais très probablement due à un effet de matrice minérale. L'ensemble des variations observées ne peuvent a priori s'expliquer par des différences d'usage du sol, mais plutôt grâce à l'activité des insectes sociaux qui sont vraisemblablement à l'origine des microagrégats.

Due to the immiscibility of 3He into 4He at very low temperatures, mixed helium droplets consist of a core of 4He atoms coated by a 3He layer whose thickness depends on the number of atoms of each isotope. When these numbers are such that the centrifugal kinetic energy of the 3He atoms is small and can be considered as a perturbation to the mean-field energy, a novel shell structure arises, with magic numbers different from these of pure 3He droplets. If the outermost shell is not completely filled, the valence atoms align their spins up to the maximum value allowed by the Pauli principle.

Not available, Les études concernant l'évolution chimique et structurale d'espèces d'aluminium en présence de salicylate ont été menées en combinant plusieurs méthodes d'analyses telles que l'électrophorèse, les spectroscopies infrarouges et RMN, les méthodes thermiques et de diffusion de rayonnement. Dans les cas étudiés, les agrégats d'Al13, la gibbsite et la montmorillonite intercalée par l'Al13, ces méthodes nous ont permis de mettre en évidence des changements structuraux important, du à la dissolution de l'aluminium en fonction de la concentration en ligand organique. L’interaction entre les espèces d'aluminium hydrolyse et le salicylate révèle deux phénomènes: la complexation et la dissolution. L’étude de la complexation nous a permis de mettre en évidence la nature et le type de complexation entre l'aluminium et le salicylate. Aux faibles concentrations en ligand, la complexation se fait sur les sites les plus réactifs, les sites charges Al-OH2+. Elle a pour effet de neutraliser le système et donc de condenser la structure des agrégats. Aux fortes concentrations en ligand, la complexation se poursuit sur les sites neutres, ce qui provoque la rupture des liaisons Al-O. La nature du complexe entre l'aluminium et le salicylate est un bidentate impliquant les oxygènes du groupement carboxyl. Ce complexe observé par IR est identique pour les agrégats et pour la gibbsite. La complexation entre le salicylate et l'aluminium a pour conséquence la dissolution des espèces étudiées. La caractéristique la plus marquante, observée pour les trois cas étudiés, est le fait que la dissolution ne commence que lorsque l'on apporte un excès de charge dans le système. La dissolution provoque des modifications structurales à plusieurs échelles sur les agrégats et la montmorillonite-Al13 mais pas sur la gibbsite: échelle locale, semi-locale et à grande distance. La cinétique de dissolution est quant à elle très différente selon la nature et l'environnement de l'aluminium

The properties of aggregates generated from an off-lattice, two-dimensional, particle-cluster aggregation model with dipolar interparticle interactions have been investigated. The fractal dimension seems to be a monotonically decreasing function of the temperature, between a definite value close to 1 at T=0 and the limit T\ensuremath{\rightarrow}\ensuremath{\infty}, corresponding to diffusion-limited aggregation of particles with no interaction. Temperature and dipolar interactions are introduced by means of a Metropolis algorithm. By analyzing the orientational correlation function, and what we call the orientation probability density of the direction of the dipoles on the clusters, an ordered state is found at low temperatures. This order diminishes when the temperature increases, due to the disorder induced by the fractal geometry of the aggregates. Our study is extended to the three-dimensional case. A disordered state is found even at T=0, and a remnant order is shown when analyzing related two-dimensional correlations.

Not available, L'insertion électrochimique de cations alcalins, dans des structures d'accueil carbonées, a été entreprise en milieu tout solide, en utilisant le polyoxyde d'éthylène comme électrolyte solide. Dans le polyacétylène, l'insertion est entravée par des problèmes de surface de contact, expliquant les faibles coefficients de diffusion et les faibles taux d'insertion. La conformation du polyacétylène est importante et conduit à deux hypothèses (insertion plus facile dans le polyacétylène cis ou isomérisation électrochimique). L'insertion, en ce qui concerne, le système li/c#6#0, donne trois transformations réversibles (li#0#,#5c#6#0, li#2c#6#0, li#3c#6#0) et deux irréversibles. Le taux de désinsertion est toujours supérieur au taux d'insertion. Ce phénomène est engendré soit par l'oxydation d'impuretés, soit par la dissolution de c#6#0 intercale, ou soit par une réaction d'insertion de l'anion dans c#6#0. L'insertion est simultanée dans les deux fullerènes (c#6#0 et c#7#0). Le système na/c#6#0 conduit également à trois phases en réduction, mais présente une dissolution importante dans l'électrolyte. Celle-ci entraine une forte variation de la ligne de base ou une variation aléatoire du potentiel, ne permettant pas le calcul du taux de désinsertion. Seule la phase na#0#,#5c#6#0 est reproductible. L'insertion, dans les sites tétraédriques (phase m#0#,#5c#6#0) et dans les sites tétraédriques et octaédriques (phases li#2c#6#0 et li#3c#6#0) de la structure cubique à faces centrées, conduit à une contraction de la maille par rapport à celle de c#6#0 vierge. Les résultats, sur les composés graphite-lithium, sont comparables à ceux obtenus en milieu liquide et montrent l'importance de la morphologie du graphite. Il en ressort que l'emploi de graphite sous forme de paillettes de grande dimension est à proscrire, car les surfaces de contact sont trop faibles. L'emploi de graphite uf4, au vue des différents résultats obtenus, est envisageable, en tant qu'électrode carbonée à insertion de lithium

Using the extended Thomas-Fermi version of density-functional theory (DFT), calculations are presented for the barrier for the reaction Na20++Na20+¿Na402+. The deviation from the simple Coulomb barrier is shown to be proportional to the electron density at the bond midpoint of the supermolecule (Na20+)2. An extension of conventional quantum-chemical studies of homonuclear diatomic molecular ions is then effected to apply to the supermolecular ions of the alkali metals. This then allows the Na results to be utilized to make semiquantitative predictions of position and height of the maximum of the fusion barrier for other alkali clusters. These predictions are confirmed by means of similar DFT calculations for the K clusters.

In the framework of a finite-range density-functional theory, we compute the response of 4HeN clusters doped with a rare-gas molecule. For this purpose, the mean field for the 4He atoms, their wave functions and effective quasiparticle interaction, are self-consistently calculated for a variety of particle numbers in the cluster. The response function is then evaluated for several multipolarities in each drop and the collective states are consequently located from the peaks of the strength function. The spectra of pure droplets approach those previously extracted with a similar algorithm resorting to a zero-range density functional. The spectra of doped clusters are sensitive to the presence of the impurity and are worth a future systematic investigation.

Ferruginous pisolites, magnetic and non-magnetic, from four laterite fragments and surface horizons of two soils were studied by XRD, SEM and ТЕМ. The pisolites showed a ferruginous clay plasma where skeletal grains are embedded. The plasma is composed by kaolinite, mica-illite and iron oxides. Skeletal grains revealed a high degree of weathering and were constituted by quartz, K-feldspar and also grains of iron minerals. Hematite and goethite were present in the non-magnetic pisolites, whereas maghemite and hematite coexisted in the magnetic samples. Based on geomorphological and mineralogical characters a mechanism of the pisolite genesis is advanced ; pisolites represent Quaternary paleoenvironmental relic structures formed in sites of absolute accumulation, and poorly drained areas, under a humid tropical climate and followed by arid conditions., Pisolites ferrugineux du Sud du Portugal, formes relictuelles d'un paléoenvironnement quaternaire — Les pisolites ferrugineux, magnétiques et non magnétiques, prélevés dans quatre fragments de cuirasses ferrugineuses et dans les horizons de surface de deux sols, ont été étudiés par DRX, MEB et MET. Les pisolites présentent un plasma argilo-ferrugineux, renfermant des grains de squelette. Le plasma est composé d'un assemblage de kaolinite, mica-illite et d'oxydes de fer. Les grains de squelette sont composés de quartz, de feldspath-K et d'oxydes de fer et témoignent d'un intense degré d'altération. L'hématite et la goethite ont été identifiées dans les pisolites non magnétiques, tandis que la maghémite et l'hématite sont présentes dans les pisolites magnétiques. Selon les données géomorphologiques et minéralogiques, on propose un mécanisme de genèse des pisolites étudiés : les pisolites seraient des formes relictuelles, nées dans des sites d'accumulation absolue du fer et constitueraient des témoins d'un paléoenvironnement quaternaire, à zones mal drainées, sous climat tropical humide suivi de conditions arides., Abreu Maria Manuela. Ferruginous pisolites from South of Portugal. Paleoenvironmental Quaternary relics / Pisolites ferrugineux du Sud du Portugal, formes relictuelles d'un paléoenvironnement quaternaire. In: Sciences Géologiques. Bulletin, tome 43, n°2-4, 1990. Minéraux argileux dans les sols et les sédiments. pp. 95-102.

The He2-SF6 trimers, in their different He isotopic combinations, are studied in the framework of both the correlated Jastrow approach and the correlated hyperspherical harmonics (CHH) expansion method. The energetics and structure of the He-SF6 dimers are analyzed, and the existence of a characteristic rotational band in the excitation spectrum is discussed, as well as the isotopic differences. The binding energies and the spatial properties of the trimers, in their ground and lowest lying excited states, obtained by the Jastrow ansatz are in excellent agreement with the results of the converged CHH expansion. The introduction of the He-He correlation makes all trimers bound by largely suppressing the short range He-He repulsion. The structural properties of the trimers are qualitatively explained in terms of the shape of the interactions, Pauli principle, and masses of the constituents.

Monodispersed colloidal crystals based on silica sub-micrometric particles were synthesized using the Stöber-Fink-Bohn process. The control of nucleation and coalescence result in improved characteristics such as high sphericity and very low size dispersion. The resulting silica particles show characteristics suitable for self-assembling across large areas of closely-packed 2D crystal monolayers by an accurate Langmuir-Blodgett deposition process on glass, fused silica and silicon substrates. Due to their special optical properties, colloidal films have potential applications in fields including photonics, electronics, electro-optics, medicine (detectors and sensors), membrane filters and surface devices. The deposited monolayers of silica particles were characterized by means of FESEM, AFM and optical transmittance measurements in order to analyze their specific properties and characteristics. We propose a theoretical calculation for the photonic band gaps in 2D systems using an extrapolation of the photonic behavior of the crystal from 3D to 2D. In this work we show that the methodology used and the conditions in self-assembly processes are decisive for producing high-quality two-dimensional colloidal crystals by the Langmuir-Blodgett technique.

The Invaded Cluster (IC) dynamics introduced by Machta et al. [Phys. Rev. Lett. 75 2792 (1995)] is extended to the fully frustrated Ising model on a square lattice. The properties of the dynamics which exhibits numerical evidence of self-organized criticality are studied. The fluctuations in the IC dynamics are shown to be intrinsic of the algorithm and the fluctuation-dissipation theorem is no more valid. The relaxation time is found very short and does not present critical size dependence., Comment: notes and refernences added, some minor changes in text and fig.3,5,7 16 pages, Latex, 8 EPS figures, submitted to Phys. Rev. E

The hidden-charm pentaquark Pc(4450) observed recently by the LHCb collaboration may be of molecular nature, as advocated by some unitary approaches that also predict pentaquark partners in the strangeness S=-1 sector. In this work we argue that a hidden-charm strange pentaquark could be seen from the decay of the Λb , just as in the case of the non-strange Pc(4450) , but looking into the J/ψηΛ decay mode and forming the invariant mass spectrum of J/ψΛ pairs. In the model presented here, which assumes a standard weak decay topology and incorporates the hadronization process and final-state interaction effects, we find the J/ψηΛ final states to be populated with similar strength as the J/ψK-p states employed for the observation of the non-strange pentaquark. This makes the Λb→J/ψηΛ decay to be an interesting process to observe a possible strange partner of the Pc(4450) . We study the dependence of the J/ψΛ mass spectra on various model ingredients and on the unknown properties of the strange pentaquark.

We have studied the structure of {sup 3}He droplets at zero temperature using a density functional approach plus a configuration interaction calculation in an harmonic oscillator major shell. The most salient feature of open shell drops is that the valence atoms couple their spins to the maximum value compatible with Pauli`s principle, building a large magnetic moment. We have determined that 29 atoms constitute the smallest self-bound droplet. {copyright} {ital 1997} {ital The American Physical Society}