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Ground state properties of large atoms and quantum dots
No full textWe investigate the ground state properties of large atoms and quantum dots described by a d-dimensional N-body Hamiltonian of confinement ZV. In atoms, d = 3 and V is the Coulomb interaction; in dots, d = 2 and V is phenomenologically determined. We express the grand-canonical partition function in a path integral approach, and evaluate its expansion in Z-1. The problem can be seen as that of field theory possessing a saddle point. This saddle point results in a mean-field contribution to the energy, while the fluctuations result in the correlation energy. The mean-field contribution to the energy is self-consistently determined by the Hartree potential and contains an exchange term. Its smooth contribution is evaluated by a semiclassical method, with ε = Z-1/d in the role of ℏ, while its oscillating contribution can be related to the periodic orbits in the corresponding classical Hamiltonian. In the case of atoms, the leading order in ε of the correlation energy contains a term in Z ln Z1/3, which is essential in reproducing the behaviour shown by reference values, and a term in Z. While we have evaluated the contribution to the Z-term provided by the leading fluctuation order, the numerical evaluation of the contributions provided by higher order fluctuations remains an open problem. The self-consistent contribution to the energy corresponds to the statistical atom, composed of Thomas-Fermi and its corrections, comprehensively analysed, including oscillations, by Schwinger and Englert. In the case of dots, the leading order in ε of the correlation energy is a universal contribution of order Z, which we obtain in closed form. We then determine the expansion in ε of the smooth contributions down to this correlation order. We apply the approach to dots of quadratic and quartic confinement, including the oscillating contribution in the case of a chaotic quartic confinement.GCDS
Novel applications of poly(ethylene glycol)-bl-poly(propylene sulfide) block copolymers
No full textBlock copolymers are comprised of repeating chemical groups (blocks) which commonly contain at least one hydrophilic block coupled to at least one hydrophobic block, forming an amphipathic macromolecule. This molecular arrangement drives the self assembly of these materials in water similar to lipids and low molecular weight surfactants. Unlike these conventional materials, however, block copolymers can typically form a wide variety of morphologies in water such as vesicles, worm like micelles, y-junctions, blackberry micelles, and micelles. This is due to the fact that the large hydrophobic blocks generally display a low mobility inside the core of the aggregate and the initially formed morphologies can be considered as "frozen" structures. Thus, while lipids and other low molecular weight surfactants are largely controlled by thermodynamics, block copolymers are governed by kinetic effects. A large body of work has been produced regarding the physical behavior of block copolymers over the past few decades. Various parameters have been explored including the composition of the dispersing media, the chemical composition of the repeating units of the blocks, the molar fraction of the blocks in solution, the relative and absolute block composition, the polydispersity of the block copolymer, temperature, pressure, and the compatibility of the polymer to the dispersing media. These have been extensively explored both theoretically and empirically for a wide variety of block copolymer systems. Block copolymers of poly(ethylene glycol)-bl-poly(propylene sulfide,) (PEG-PPS,) have recently emerged as a new and interesting block copolymer. This is due both to the low Tg, Tm, and relatively high hydrophobicity of the PPS block. These block copolymers can form polymeric vesicles (polymersomes,) worm like micelles, micelles, or hybrid structures, which are dependent on the relative block lengths of PEG and PPS (∞PEG.) However, the stability of the formed morphologies is directly related to the absolute PPS degree of polymerization, as the hydrophobic effect is the main driving factor towards self assembly. So while morphology is determined by the relative block composition, the stability of the formed aggregates on dilution is determined by the absolute molecular weight of the hydrophobic block. In order to exploit these materials for use as drug delivery vehicles to encapsulate hydrophilic compounds, we have explored the behavior of PEG-PPS by modifying the various parameters described above. We discovered that block copolymers displaying relatively low ∞PEG values are able to form micelles from solvent dispersion using tetrahydrofuran, a good solvent for both blocks, into water. These frustrated micelles, in which the PPS block is tightly packed, can then relax when heated in the presence of a solvent which is capable of swelling the hydrophobic block. This transition, highly dependent on temperature, the nature of the solvent, and the time exposed at elevated temperatures, allows the hydrophobic block to relax in the aqueous solution, and stretch to a more thermodynamically favorable morphology. In this case, we have observed a micelle to vesicle transition, with a wide variety of intermediate structures formed during the relaxation time. There are distinct phases during the transition which are related to critical aggregation numbers, and represent an Ostwald like ripening where vesicles are formed at the cost of micelles and other morphologies. These transitions, elucidated using cryo-TEM and optical density measurements, can be considered to be either 2D, where micelles fuse to form worm like micelles, or 3D where the fusion of micelles leads to inversion or "flip-flop" and directly form vesicles. After a certain period of time, we observe a final, stable state in which all initial and transitional morphologies become an isotropic dispersion of polymeric vesicles. We have exploited this phenomena to encapsulate a biologically active hydrophilic peptide at a high concentration of PEG-PPS micelles (10% wt/vol,) where we observed high encapsulation efficiencies (> 50%.) In a somewhat similar manner, we have also explored the formulation of PEG-PPS with excipients of an amphipathic salt (1,8-Diazabicyclo[5.4.0]undec-7-ene HCl,) (DBU-HCl,) and polyethylene glycol dimethyl ether Mw 500. We discovered that upon heating, PEG-PPS forms a melt with the excipient. When water or a protein solution is added, the mixture forms a milky polymersome dispersion in solution. In the case of adding a protein solution, we observed high encapsulation efficiencies owing to the very high concentration of the block copolymer in solution, and resultant reduced free aqueous volume outside of the polymersomes compared to the thin film hydration method. By characterizing this system using a wide variety of techniques including differential scanning calorimetry, optical density, dynamic light scattering, cryo-TEM, and others, we have been able to show that the direct hydration method is a type of modified solvent dispersion technique where the PEG-PPS is dissolved into the excipient at elevated temperature. When the mixture is allowed to cool to room temperature, and when water is slowly added, the molar fraction of the salt or PEG excipient changes and the mixture becomes more polar. When this occurs the block copolymer is driven towards self assembly where the PPS block is in a relaxed state in the matrix, forming polymeric vesicles. During this process, a substantial portion of the added aqueous solution is subsequently trapped in the core of the vesicles. Because we are forming the vesicles close to the optimal concentration of the block copolymer, a value defined as the sponge or hexagonal packing phase, we observe elevated encapsulation efficiencies compared to conventional methods such as thin film hydration. We also observed that the activity of an enzyme, β-Galactosidase, was not affected by the processing steps. In addition to exploring the behavior and exploitation of PEG-PPS as drug delivery vehicles, we have also produced a set of functionalized micelle forming PEG-PPS block copolymers for either targeted drug delivery, or as adjuvants for vaccine development. By forming mixed micelles with the similar poly(ethylene glycol)-bl-poly(propylene oxide)-bl-poly(ethylene glycol,) or Pluronic block copolymers, we were able to display a variety of surface chemistries by modifying the Pluronic block copolymer. The modification of Pluronic block copolymers is relatively straightforward, as they display PEG terminal primary hydroxyl groups. We have demonstrated that sulfated Pluronic F-127, when blended with PEG-PPS, can target Collagen I, a primary component of the extracellular matrix. Although Pluronics are known to display relatively high critical micellar concentrations (CMC,) the mixed micelles displayed a substantially lower value compared to the Pluronic alone. PEG-PPS has also been demonstrated to be particularly useful for the encapsulation and release of amphipathic and hydrophobic drugs. By quantifying the encapsulation efficiency and release of the immunosuppressant drug Sirolimus, we have proven the concept that mixed micelles of Pluronics and PEG-PPS could be used as novel drug delivery systems for targeting the extracellular matrix. To modify PEG-PPS itself, we have had to develop novel chemistries. We wanted to form block copolymer analogs to the Pluronic F-127 PPS core nanoparticles developed by our group previously. The Pluronic F-127 PPS core nanoparticles have been demonstrated to act as adjuvants for vaccine development. This was accomplished by producing nanoparticles small enough to be transported into the lymph nodes passively via interstitial flow into the lymphatic capillaries. Dendritic cells in the lymph node then are able to internalize these nanoparticles in large numbers, leading to maturation, and processing of antigens on the nanoparticle surface via the alternative complement pathway. To mimic these nanoparticles, we developed block copolymers of PEG-PPS which display either HO or H3CO on the PEG chain terminus and either SH, H2N, phthalimide, or benzyl on the PPS chain terminus. In this way we have fluorescently labeled the block copolymer covalently, and performed a wide variety of experiments to explore the relationship of surface chemistry to complement activation both in vitro and in vivo. In this last chapter, we have shown the transport of PEG-PPS micelles to the lymph nodes and subsequent uptake into lymph node dendritic cells in vivo, and the release of C3a and the maturation of bone marrow dendritic cells in vitro in a surface chemistry dependent manner. In conclusion, PEG-PPS represents an interesting new block copolymer system which we have only begun to explore. The following chapters attempt to contribute to the growing body of knowledge regarding block copolymers and their application as therapeutic drug carriers. The final chapter provides for both a look back at the accomplishments in this thesis, and possible directions for future research.LMR
Desiccation cracking of soils
No full textDesiccation cracking of soils is of great importance in geotechnical and geoenvironmental engineering. In many circumstances it is the cause of damages in earthen and soil supported structures. Desiccation cracking affects clayey soil barriers for waste storage, causing a dramatic increase of the barrier permeability; late experimental results also reveal that desiccation cracking is a burning issue for underground nuclear waste storage. Yet the mechanisms of drying shrinkage and associated cracking in soils, and the ways to control or avoid such cracking, are still elusive. This study offers a better understanding of such mechanisms, and proposes a modelling of the related processes, in order to predict the occurrence of damage. The thesis includes an experimental and phenomenological study of desiccation, characterizing the drying shrinkage and the cracking of soils (silts and silty clays) and a theoretical and numerical study, including a constitutive approach and some boundary value problem simulations. The experimental study consists in (i) desiccation tests on soil samples with controlled mechanical and drying boundary conditions, and (ii) measurement of tensile strength. Results of the desiccation tests (i) reveal that the stresses that lead to cracking clearly result from the presence of restraining boundary conditions and/or moisture gradient. Desiccation cracking of remoulded and initially saturated soils invariably occurs in a domain of drying for which saturation ratio is almost one and suction is non-zero, close to air entry value. In such a domain, a large part of the deformations are irreversible, while stresses are built up, until a critical point at which tensile strength is reached. The tensile strength clearly depends on suction. The processes related to crack propagation and patterns geometry are also discussed, particularly the effect of energy redistribution through the body, after crack initiation. For the tensile strength experimental determination (ii), two tests method are designed, both performed in the triaxial apparatus, with control of suction. The methods avoid the connexion of the specimen to the traction system during the test, and allow the control of stress field and drainage conditions. The dependence of tensile strength on suction is quantified for a certain suction range. A constitutive approach is subsequently proposed, able to reproduce the essential experimental features. The model ACMEG-DC is developed, on the basis of the reference constitutive model ACMEG, developed by L. Laloui and co-workers. The model is based on the Bishop's generalized effective stress concept, and developed in the frame of isotropic hardening elasto-plasticity. On the basis of test results, a new tensile strength criterion which enables in particular to predict desiccation cracking occurrence is added to this model. The criterion takes into account the evolution of tensile strength arising during desiccation due to suction changes. Such an approach is new indeed; it is original as it unifies the description of drying shrinkage process (viewed as the consequence of change in effective stress, in a consistent elasto-plastic framework) and tensile cracking (predicted through an adapted effective stress criterion). The model is validated on several experimental desiccation studies. Several modelling approaches of desiccation as a boundary value problem are also proposed, based on finite element and discrete element simulations. Such simulations address in particular the modelling of desiccation crack propagation and the related challenges.LM
A high-performance distributed hash table for peer-to-peer information retrieval
No full textThis thesis describes our research results in the context of peer-to-peer information retrieval (P2P-IR). One goal in P2P-IR is to build a search engine for the World Wide Web (WWW) that runs on up to hundreds of thousands or even millions computers distributed all over the world. The idea is not only to distribute the content, e.g., web pages, but also an index for searching this content. The main focus of this thesis lies on designing an overlay network that is capable of transporting data between the different parts of such a distributed search engine. We built a Distributed Hash Table (DHT) that is able to sustain and efficiently handle high traffic loads, which are typically generated by a distributed IR application. We first analyze the behavior of a state-of-the-art DHT under heavy load and show that a DHT can suffer a so-called "congestion collapse" if it does not have a congestion control mechanism. We propose different ways of integrating congestion control into DHTs to achieve stable behavior of the system under heavy load. We then look into mechanisms for increasing the throughput of a DHT by adapting its routing function to perceived congestion. We propose an algorithm that avoids congested parts of the DHT and thus increases the throughput by exploiting underutilized resources. We evaluate our fully operational DHT prototype using a ModelNet cluster and the PlanetLab testbed to assess the performance of the proposed algorithms. Furthermore, we describe an architecture of a P2P search engine for the WWW. We propose mechanisms to create a highly distributed document index. The main idea is to split the index into very small parts by using so-called highly discriminative keys. We thus achieve an extremely distributed storage of the index, which allows for high parallelism during indexing and querying. We evaluate the performance of our indexing approach with a P2P-IR prototype, which is built on top of our high-performance DHT.LSI
Exotic phases of quantum frustrated magnets : magnetization plateaus, nematic order and supersolid phases
No full textIn this work, we study two examples of frustrated magnetic systems whose degrees of freedom are spins (or pseudo-spins) on two-dimensional lattices. The first part presents the results obtained for the quantum compass model on a square lattice. This model is a minimal model for the orbital degrees of freedom of transition metal compounds. We use exact diagonalizations, quantum Monte Carlo simulations as well as a perturbative approach in order to study the ground state degeneracy in the thermodynamic limit and the type of order in the ground state. The second part of this work is devoted to the study of the magnetic properties of SrCu2(BO3)2, which is one of the few quasi two-dimensional quantum magnets whose magnetization curve as a function of the magnetic field shows plateaus. The magnetic degrees of freedom are described by a spin 1/2 Heisenberg model on the Shastry-Sutherland lattice in the presence of an external magnetic field. In order to study the magnetic properties at zero-temperature, we use perturbative continuous unitary transformations to derive an effective Hamiltonian in which the triplets Sz = 1 are particles moving on a background of singlets. This Hamiltonian is characterized by a kinetic energy dominated by correlated hopping, which let a particle hop only if there is another particle nearby. In order to better understand the effect of the correlated hopping, we start by studying a minimal model. By using exact diagonalizations, quantum Monte Carlo and a semi-classical approximation we show that the correlated hopping can stabilize a phase characterized by a condensation of pairs of particles and strongly favor supersolid phases. Next, we determine the zero-temperature phase diagram of the effective model by using a classical approximation. A comparison to previous theoretical works and experimental measurements shows that the Shastry-Sutherland model cannot reproduce the series of magnetization plateaus measured experimentally. This suggest that the residual interactions, which were neglected, could be crucial for SrCu2(BO3)2.CTM
Deformation mechanisms of nanocrystalline nickel studied by in-situ X-ray diffraction
No full textMaterials consisting of grains or crystallites with sizes below a hundred nanometers have exhibited unique physical and mechanical properties in comparison to their coarse-grained counterparts. As a result, considerable effort has been put into uncovering the new deformation mechanisms that give rise to this outstanding response of nanocrystalline materials. Moreover, the production of nanocrystalline materials of reasonable sizes for structural applications remains a challenge. However, the size limitation is of no issue for their present application in the growing field of MEMS and NEMS devices. Ultimately, the reliability and lifetime prediction of these devices will depend on the accurate knowledge of their mechanical response. This dissertation addresses experimental and simulation procedures used to understand the fundamental deformation mechanisms operating in bulk nanocrystalline Nickel. Recent results from simulations suggested dislocations as a dominant carrier of plasticity in nanocrystalline materials. In contrast to coarse-grained materials, these dislocations are nucleated at grain boundaries and, after propagating through the nano grains, they are absorbed there as well. Deformation experiments during in-situ X-ray diffraction strengthened the predicted outcome from simulation but many open questions remained. Within this thesis a more extensive range of in-situ testing experiments are performed that aim to systematically investigate the nanocrystalline deformation mechanism in terms of both temperature and external loading conditions. The development of a low temperature tensile test set-up allowed to study temperature dependent behavior and revealed that dislocation activity in nanocrystalline Nickel is a strongly thermally activated process where propagation of dislocation seems to be as important as nucleation from dislocations at the grain boundary. This finding is further supported by strain-dip tests, which revealed that pinning points strongly influence dislocation propagation. Nanocrystalline Nickel exhibits, in its as prepared state, large internal stress variations. These stress variations and the small grain size are most likely responsible for the microplastic regime characterized by an extended macroscopic strain, making the usage of the classical definition of yield questionable. Furthermore it could be shown that upon annealing, which reduces the samples' internal stress, this extended microplastic regime was observed to be less pronounced. To study the structural stability of nanocrystalline Nickel at large strains, the material was investigated by compression experiments, revealing no changes in mean grain size. Furthermore, the three dimensional atomic probe technique was utilized for localizing impurity concentrations. The program was rounded by calculating diffraction peaks from simulated nanocrystalline structures with a single type of defect. This allowed investigating the characteristics of the diffraction pattern of nanocrystalline systems in a bottom up approach. Finally, the results of the thesis are discussed in terms of a thermally activated deformation mechanism that involves the nucleation, propagation and absorption of dislocation within the nanocrystalline environment.LM
Three essays on the acceptability of environmental policy in Switzerland
No full textThe present thesis studies the acceptability of environmental policy in Switzerland. The first part of the thesis concentrates on citizens' demand for environmental quality at ballots. The analysis is guided by the public choice framework, which is rooted in the assumptions of instrumental rationality and utility-maximization. However, it is advisable to account for socially constructed norms and preferences, and a logic of appropriateness, too when studying the demand for a public good such as the environment. This is why these elements were integrated into the analysis. Furthermore, in a rich decision-making framework which controlled for citizens' voting motivations and for contextual factors influencing the vote, it was shown that voters react sensitively to both their personal and the nation's perceived economic conditions. Thus, when they feel confident about the country's or their personal economic conditions, they are more likely to support environmental policy. The second part of the thesis is devoted to actors' policy preferences and their alliance formation behavior in the pre-parliamentary phase. Based on an in-depth analysis of actors' responses to four pre-legislative drafts, it was shown that the main conflict line in Swiss environmental policy runs along the market vs. state divide and that, thus, the possibility to engage in cross-cutting alliances remains limited. Despite hypotheses of scholars that this line of conflict may be shifting due to the emergence of post-material or left-libertarian issues on political agendas, we were not able to corroborate this claim for Switzerland.LEUR
Development of a scanning near-field microscope and investigations of solid C60 luminescence
No full textThis thesis presents the development of a scattering scanning near-field microscope (s-SNOM) and investigations on the photoluminescence study of solid C60. The first part concerns setting up of such a scattering SNOM where a tuning fork with an attached AFM tip is used as the force sensor and as a light-scattering probe. The light is coupled into the microscope and detected in a confocal arrangement. The characterization of the system and the force and optical scanning measurement of nanoparticles on glass substrates are presented and discussed. In order to know the tip vibration amplitude of quartz tuning-fork based sensors, we have developed a simple method, requiring only the measurement of a few mechanical properties of the fork (dimensions and Q factor), which can be easily obtained without changing the experimental setup in a matter of minutes. This method uses the (known) electrical energy absorbed by the system and the Q-factor to derive the elasto-mechanical energy stored in the tuning fork, and from this, the amplitude of motion through the elastic constants of the system. In order to improve the sensitivity for controlling the tip-sample distance, we have also made experimental and theoretical investigations on the performance of a different resonator, based on a tuning-fork + optical fiber mechanical scheme for use in shear-force mode. We have found that both the quality-factor and the spring constant play the main role in determining the behavior of such a force sensor. Based on this understanding, we are able to control both of these important parameters and, hence to optimize the force sensor and accurately model its response to an external force. The second part presents a time resolved local spectroscopic study of the photoluminescence of single C60 microcrystals performed with the optical microscope and with time-correlated photon counting techniques. C60 crystals prepared by the vapor sublimation method and the solution evaporation method are discussed. The photoluminescence of C60 crystals prepared by the vapor sublimation method is tentatively assigned to phosphorescence from the lowest two triplet states. The emission quantum efficiency and decay dynamics shows dependence on temperatures for C60 samples made by both methods, while the sample with a less ordered crystal structure made by the solution evaporation has a higher energy barrier for opening a non-radiative decay channel, pointing to quenching effects related to exciton migrations. This work helps clarify the origins of the C60 emitting states.LS
Etude par STM de la déposition d'agrégats d'or sur TiO2 et mesure de l'émission électronique secondaire induite par l'impact d'agrégats sur une surface
No full textThis thesis work focuses on impact and diffusion processes as well as equilibrium positions of a metal deposited on a surface. The metal is deposited in the form of clusters containing n atoms in a controlled way. Gold or silver clusters cations (Au+n and Ag+n ) are used. Their size (n ∈ [1, 9]), charge state and deposition energy (E = 10 - 4500 eV) are well defined. The surface being at room temperature during the deposition, can be annealed and transferred in a scanning tunnelling microscope (STM) after deposition. The variable temperature microscope was developed during this thesis. The approach of the tip is carried out by an axial motor, which renders the microscope very stable. The first part of this work treats of the evolution of gold deposited on a TiO2(110) surface. Position and size of gold islands created by cluster or atomic depositions are determined by STM. Compared to atomic deposition at the same energy (10 < E < 100 eV), the cluster deposition produces smaller islands with a large islands density after annealing at 800 K. Regardless the deposition conditions, gold atoms diffuse on the surface already at 300 K. The impact energy is the key parameter to reduce the diffusion by cluster or atomic implantation or defect creation. Upon a high energy deposition, part of the deposited gold is invisible to the STM. The fate of these gold atoms is unclear : they are either buried under the surface or ejected in vacuum at the impact. The control of the islands size should permit the creation of a stable system with an optimum catalytic activity (i. e. CO combustion). Research groups have shown that this activity is strongly influenced by the islands size. It is equal to zero if the diameter is greater than 5 nm. The second part of this work is devoted to the electron emission γ induced by the impact of silver clusters Ag+n on a Pt and HOPG surface. Measurements are made by varying the impact energy (and so the speed v) of size-selected clusters on a defined surface. Impact velocity ranges from 104 to 105 m/s, which is lower than the classical threshold. Nevertheless every cluster size produces an electron emission. A potential emission (γ(v=0) ≠ 0) is observed for the monomer on both surfaces, which is caused by excited ions in a metastable state. The γ(v) curves are increasing, and no mesurable oscillation are observed. This does not confirm earlier work by Meiwes-Broer et al. These authors found oscillations in γ on similar systems relating them to the electronic structure of the clusters and the surface. A model based on heating of the electron gas is developed. This model gives good agreements with the γ(v) curves. The electronic temperature is estimated to 3000-8000 K. Similar behavior on both surfaces (Pt and HOPG) depending of the cluster size is shown. This behavior is probably related to the geometrical structure of the clusters. Finally, a more pronounced molecular effect is observed for the HOPG : the number of electrons emitted by the impact of a Ag+n is up to 7 times higher than the number of electrons emitted by impacts of n independent atoms. This value is smaller than 2 for the Pt surface.GPA