Damping and frequency shift in the oscillations of two colliding Bose-Einstein condensates

We have investigated the center-of-mass oscillations of a Rb87 Bose-Einstein condensate in an elongated magneto-static trap. We start from a trapped condensate and we transfer part of the atoms to another trapped level, by applying a radio-frequency pulse. The new condensate is produced far from its equilibrium position in the magnetic potential, and periodically collides with the parent condensate. We discuss how both the damping and the frequency shift of the oscillations are affected by the mutual interaction between the two condensates, in a wide range of trapping frequencies. The experimental data are compared with the prediction of a mean-field model.Comment: 5 RevTex pages, 7 eps figure

Tetrad gravity, electroweak geometry and conformal symmetry

A partly original description of gauge fields and electroweak geometry is proposed. A discussion of the breaking of conformal symmetry and the nature of the dilaton in the proposed setting indicates that such questions cannot be definitely answered in the context of electroweak geometry.Comment: 21 pages - accepted by International Journal of Geometric Methods in Modern Physics - v2: some minor changes, mostly corrections of misprint

Effects of interaction on the diffusion of atomic matter waves in one-dimensional quasi-periodic potentials

We study the behaviour of an ultracold atomic gas of bosons in a bichromatic lattice, where the weaker lattice is used as a source of disorder. We numerically solve a discretized mean-field equation, which generalizes the one-dimensional Aubry-Andr\`e model for particles in a quasi-periodic potential by including the interaction between atoms. We compare the results for commensurate and incommensurate lattices. We investigate the role of the initial shape of the wavepacket as well as the interplay between two competing effects of the interaction, namely self-trapping and delocalization. Our calculations show that, if the condensate initially occupies a single lattice site, the dynamics of the interacting gas is dominated by self-trapping in a wide range of parameters, even for weak interaction. Conversely, if the diffusion starts from a Gaussian wavepacket, self-trapping is significantly suppressed and the destruction of localization by interaction is more easily observable

Localized Asymmetric Atomic Matter Waves in Two-Component Bose-Einstein Condensates Coupled with Two Photon Microwave Field

We investigate localized atomic matter waves in two-component Bose-Einstein condensates coupled by the two photon microwave field. Interestingly, the oscillations of localized atomic matter waves will gradually decay and finally become non-oscillating behavior even if existing coupling field. In particular, atom numbers occupied in two different hyperfine spin states will appear asymmetric occupations after some time evolution.Comment: 4 pages, 4 figure

Spatial interference of coherent atomic waves by manipulation of the internal quantum state

A trapped 87Rb Bose-Einstein condensate is initially put into a superposition of two internal states. Under the effect of gravity and by means of a second transition, we prepare two vertically displaced condensates in the same internal state. These constitute two coherent sources of matter waves with adjustable spatial separation. Fringe patterns, observed after free expansion, are associated with the interplay between internal and external degrees of freedom and substantially agree with those for a double slit experiment

Sensitive measurement of forces at micron scale using Bloch oscillations of ultracold atoms

We show that Bloch oscillations of ultracold fermionic atoms in the periodic potential of an optical lattice can be used for a sensitive measurement of forces at the micrometer length scale, e.g. in the vicinity of dielectric surface. In particular, the proposed approach allows to perform a local and direct measurement of the Casimir-Polder force which is, for realistic experimental parameters, as large as 10^-4 gravity

Exponential localization in one-dimensional quasiperiodic optical lattices

We investigate the localization properties of a one-dimensional bichromatic optical lattice in the tight binding regime, by discussing how exponentially localized states emerge upon changing the degree of commensurability. We also review the mapping onto the discrete Aubry-Andre' model, and provide evidences on how the momentum distribution gets modified in the crossover from extended to exponentially localized states. This analysis is relevant to the recent experiment on Anderson localization of a noninteracting Bose-Einstein condensate in a quasiperiodic optical lattice [G. Roati et al., Nature 453, 895 (2008)].Comment: 13 pages, 6 figure

Correlation function of weakly interacting bosons in a disordered lattice

One of the most important issues in disordered systems is the interplay of the disorder and repulsive interactions. Several recent experimental advances on this topic have been made with ultracold atoms, in particular the observation of Anderson localization, and the realization of the disordered Bose-Hubbard model. There are however still questions as to how to differentiate the complex insulating phases resulting from this interplay, and how to measure the size of the superfluid fragments that these phases entail. It has been suggested that the correlation function of such a system can give new insights, but so far little experimental investigation has been performed. Here, we show the first experimental analysis of the correlation function for a weakly interacting, bosonic system in a quasiperiodic lattice. We observe an increase in the correlation length as well as a change in shape of the correlation function in the delocalization crossover from Anderson glass to coherent, extended state. In between, the experiment indicates the formation of progressively larger coherent fragments, consistent with a fragmented BEC, or Bose glass.Comment: 16 pages, 8 figure

Influence of reaction conditions on hydrothermal carbonization of monosaccharides

The grim perspective of a near future when out-of-control global warming caused by C02 emissions will threaten to put human lives in serious danger is pushing the scientific community to seek for alternatives to fossil fuels to counteract this negative trend. At the moment, fossil fuels are the main source of energy and chemical building blocks for the synthesis of plastics. Hydrothermal carbonization is a process that aims to replace fossil fuels with renewable biomass as source of energy (biofuels) and materials (platform chemicals and hydrothermal carbon). The process of hydrothermal carbonization has been known for a little more than a century as a way to mimic the natural process of coalification of biomass. It consists in a conversion of wet biomass in water, at subcritical temperatures (180-250°C) and autogenous pressure. Biomass is made of lignin, a polymer of alkylphenol derivatives, cellulose and hemicellulose (polysaccharides). These materials, in hydrothermal conditions, undergoes a series of reaction: hydrolysis of large polymer chains, solubilisation of monomers in water, dehydration, fragmentation and ring opening reaction, oxidation and formation of organic acids and re-polymerization to amorphous carbonaceous materials. This process is extremely interesting because some of its products have been recognised as strategic for a future emancipation from fossil fuels: furan derivatives like furfural, 5-hydroxymethylfurfural and levulinic acid can be a source for the synthesis a great variety of chemicals, including biofuels. The amorphous carbonaceous materials (hydrothermal carbon) has been successfully employed as a starting material for the development of electrode in batteries, supercapacitors and fuel cells, or gas capture. However, a thorough understanding of the underlying mechanisms of hydrothermal carbonization still needs to be achieved. The aim of this research project is to evaluate the effect of the chosen parameters on the sugar conversion, the change of the product yields and the morphological and chemical properties of HT carbon; to highlight the correlation between chemicals in the liquid phase and HT carbon; to get a deeper understanding on the chemical structure of HT carbon. The attention was focused on three monosaccharides: fructose, glucose and xylose. Hydrothermal conversion of fructose was tested by varying the reaction time (2-12h), acid catalysis (H2SO4, HNO3, HCl, HBr, HI) and headspace feed gas (air, N2, CO2). The soluble and insoluble products were collected and the results discussed. Fructose proved to be a very reactive substrate for hydrothermal conversion also in plain water and absence of catalyst, leading to a maximum HMF yield of HMF of 52% after 3 h. Strong acids strongly accelerate fructose conversion to carboxylic acids but they have a less pronounced effect on HT carbon formation. A pressurized system has also a positive effect in terms of conversion. Morphological and chemical analysis of HT carbon produced showed that the alkylfuran skeleton evolves through time to a more condensed and cross-linked structure. The presence of a family of oligomers formed by units with a mass of 211 Da suggests that HT formation proceeds via progressive polymerization of a well-defined monomer. Hydrothermal conversion of glucose was performed in conditions of increasing reaction time (2-12h) and different acid catalysis (H2SO4, HNO3, HCl, HBr, HI). In this case, glucose proved to be a less sensitive substrate to dehydration than fructose. Acid catalysis greatly increase its conversion and it is possible to distinguish the different contribution of the anions in the ability to catalyse the reaction. Morphological and chemical analysis of HT carbon produced showed similar results to those obtained from fructose but also suggest that HMF concentration throughout time plays a key role in the growth rate of carbon particles. Oligomers species were also detected in this case. Finally, the effect of reaction time (2-12h) was evaluated for the hydrothermal conversion of xylose. The structural difference between xylose and the previously studied fructose and glucose has a profound impact on its reactivity in hydrothermal conditions. Although the time scale of its conversion to FF is roughly comparable to glucose conversion to HMF, FF is notably more stable than its hexose-derived furan analogous. Its relative stability depends on the fact that there is no reaction occurring on FF that is similar to the HMF ring opening. Lower HT carbon yields also suggest that carbon formation is less efficient with FF molecules. The slight difference of the FF molecule has repercussions on the structure of carbon spheres as well as their chemical structure. HT carbon particles have a reduced tendency to aggregate as reaction time proceeds. Chemical characterization showed similarities with C6 HT carbon but also a distinctive more aromatic character that once again can be ascribed once the different chemistry of FF. In this case, a few species in the mass range between 800 Da and 1500 Da were found, whose masses increase with time, with little evidence of oligomeric nature. The kinetic modelling of the data of concentration versus reaction time allowed to find the reaction rate constants associated with glucose, fructose and xylose degradation to their dehydration products (HMF and furfural respectively) as well as the constants related to levulinic acid and hydrothermal carbon formation. These constants are in good agreement with previous studies and proves that glucose dehydration is the slowest (k=1.8 ∙10-5 s-1), followed by xylose (k=3.9 ∙10-5 s-1) and fructose (k=7.6 ∙10-5 s-1)