Single attosecond pulse generation in He+ by controlling the instant ionization rate using attosecond pulse trains combined with an intense laser pulse

High-order harmonics and single attosecond pulse generation by using an infrared laser pulse combined with attosecond pulse trains (APT) interacting with He+ have been investigated. We show that the ionization for different instant time intervals can be controlled by altering the time delay between the APT and the infrared pulse. Consequently, APT can be used as a tool to control the efficiency of high-order harmonics emitted at different times. By choosing appropriate APT and time delay, the driving pulse width for single attosecond pulse generation can be extended up to six optical cycles. (c) 2007 Optical Society of America.

Phase control of cross-phase modulation with electromagnetically induced transparency

We investigate a four-level double-Lambda atomic scheme interacting with four laser fields, a weak probe field, a weak signal field and two driven fields, in a closed-loop configuration. We study the Kerr nonlinearity associated with cross-phase modulation based on electromagnetically induced transparency. Our results show, in this closed-loop system, that the strength of cross-phase modulation and two-photon absorption are dependent critically on the relative phase between the excitation paths. By choosing the parameters appropriately, large cross-phase modulation can be achieved within a wide transparency window, while two-photon absorption is cancelled completely. The strength of cross-phase modulation can be enhanced much more by decreasing the intensities of two driven fields.

Studies of phonon anharmonicity in solids

Today our understanding of the vibrational thermodynamics of materials at low temperatures is emerging nicely, based on the harmonic model in which phonons are independent. At high temperatures, however, this understanding must accommodate how phonons interact with other phonons or with other excitations. We shall see that the phonon-phonon interactions give rise to interesting coupling problems, and essentially modify the equilibrium and non-equilibrium properties of materials, e.g., thermodynamic stability, heat capacity, optical properties and thermal transport of materials. Despite its great importance, to date the anharmonic lattice dynamics is poorly understood and most studies on lattice dynamics still rely on the harmonic or quasiharmonic models. There have been very few studies on the pure phonon anharmonicity and phonon-phonon interactions. The work presented in this thesis is devoted to the development of experimental and computational methods on this subject.

Modern inelastic scattering techniques with neutrons or photons are ideal for sorting out the anharmonic contribution. Analysis of the experimental data can generate vibrational spectra of the materials, i.e., their phonon densities of states or phonon dispersion relations. We obtained high quality data from laser Raman spectrometer, Fourier transform infrared spectrometer and inelastic neutron spectrometer. With accurate phonon spectra data, we obtained the energy shifts and lifetime broadenings of the interacting phonons, and the vibrational entropies of different materials. The understanding of them then relies on the development of the fundamental theories and the computational methods.

We developed an efficient post-processor for analyzing the anharmonic vibrations from the molecular dynamics (MD) calculations. Currently, most first principles methods are not capable of dealing with strong anharmonicity, because the interactions of phonons are ignored at finite temperatures. Our method adopts the Fourier transformed velocity autocorrelation method to handle the big data of time-dependent atomic velocities from MD calculations, and efficiently reconstructs the phonon DOS and phonon dispersion relations. Our calculations can reproduce the phonon frequency shifts and lifetime broadenings very well at various temperatures.

To understand non-harmonic interactions in a microscopic way, we have developed a numerical fitting method to analyze the decay channels of phonon-phonon interactions. Based on the quantum perturbation theory of many-body interactions, this method is used to calculate the three-phonon and four-phonon kinematics subject to the conservation of energy and momentum, taking into account the weight of phonon couplings. We can assess the strengths of phonon-phonon interactions of different channels and anharmonic orders with the calculated two-phonon DOS. This method, with high computational efficiency, is a promising direction to advance our understandings of non-harmonic lattice dynamics and thermal transport properties.

These experimental techniques and theoretical methods have been successfully performed in the study of anharmonic behaviors of metal oxides, including rutile and cuprite stuctures, and will be discussed in detail in Chapters 4 to 6. For example, for rutile titanium dioxide (TiO₂), we found that the anomalous anharmonic behavior of the B_1g mode can be explained by the volume effects on quasiharmonic force constants, and by the explicit cubic and quartic anharmonicity. For rutile tin dioxide (SnO₂), the broadening of the B_2g mode with temperature showed an unusual concave downwards curvature. This curvature was caused by a change with temperature in the number of down-conversion decay channels, originating with the wide band gap in the phonon dispersions. For silver oxide (Ag₂O), strong anharmonic effects were found for both phonons and for the negative thermal expansion.

The logic of receptor-ligand interactions in the Notch signaling pathway

The Notch signaling pathway enables neighboring cells to coordinate developmental fates in diverse processes such as angiogenesis, neuronal differentiation, and immune system development. Although key components and interactions in the Notch pathway are known, it remains unclear how they work together to determine a cell's signaling state, defined as its quantitative ability to send and receive signals using particular Notch receptors and ligands. Recent work suggests that several aspects of the system can lead to complex signaling behaviors: First, receptors and ligands interact in two distinct ways, inhibiting each other in the same cell (in cis) while productively interacting between cells (in trans) to signal. The ability of a cell to send or receive signals depends strongly on both types of interactions. Second, mammals have multiple types of receptors and ligands, which interact with different strengths, and are frequently co-expressed in natural systems. Third, the three mammalian Fringe proteins can modify receptor-ligand interaction strengths in distinct and ligand-specific ways. Consequently, cells can exhibit non-intuitive signaling states even with relatively few components.

In order to understand what signaling states occur in natural processes, and what types of signaling behaviors they enable, this thesis puts forward a quantitative and predictive model of how the Notch signaling state is determined by the expression levels of receptors, ligands, and Fringe proteins. To specify the parameters of the model, we constructed a set of cell lines that allow control of ligand and Fringe expression level, and readout of the resulting Notch activity. We subjected these cell lines to an assay to quantitatively assess the levels of Notch ligands and receptors on the surface of individual cells. We further analyzed the dependence of these interactions on the level and type of Fringe expression. We developed a mathematical modeling framework that uses these data to predict the signaling states of individual cells from component expression levels. These methods allow us to reconstitute and analyze a diverse set of Notch signaling configurations from the bottom up, and provide a comprehensive view of the signaling repertoire of this major signaling pathway.

Cosmological consequences of dark matter interactions and vacuum fluctuations

This thesis is divided into two parts: interacting dark matter and fluctuations in cosmology. There is an incongruence between the properties that dark matter is expected to possess between the early universe and the late universe. Weakly-interacting dark matter yields the observed dark matter relic density and is consistent with large-scale structure formation; however, there is strong astrophysical evidence in favor of the idea that dark matter has large self-interactions. The first part of this thesis presents two models in which the nature of dark matter fundamentally changes as the universe evolves. In the first model, the dark matter mass and couplings depend on the value of a chameleonic scalar field that changes as the universe expands. In the second model, dark matter is charged under a hidden SU(N) gauge group and eventually undergoes confinement. These models introduce very different mechanisms to explain the separation between the physics relevant for freezeout and for small-scale dynamics.

As the universe continues to evolve, it will asymptote to a de Sitter vacuum phase. Since there is a finite temperature associated with de Sitter space, the universe is typically treated as a thermal system, subject to rare thermal fluctuations, such as Boltzmann brains. The second part of this thesis begins by attempting to escape this unacceptable situation within the context of known physics: vacuum instability induced by the Higgs field. The vacuum decay rate competes with the production rate of Boltzmann brains, and the cosmological measures that have a sufficiently low occurrence of Boltzmann brains are given more credence. Upon further investigation, however, there are certain situations in which de Sitter space settles into a quiescent vacuum with no fluctuations. This reasoning not only provides an escape from the Boltzmann brain problem, but it also implies that vacuum states do not uptunnel to higher-energy vacua and that perturbations do not decohere during slow-roll inflation, suggesting that eternal inflation is much less common than often supposed. Instead, decoherence occurs during reheating, so this analysis does not alter the conventional understanding of the origin of density fluctuations from primordial inflation.

Yang-Mills theory in six-dimensional superspace

The superspace approach provides a manifestly supersymmetric formulation of supersymmetric theories. For N= 1 supersymmetry one can use either constrained or unconstrained superfields for such a formulation. Only the unconstrained formulation is suitable for quantum calculations. Until now, all interacting N>1 theories have been written using constrained superfields. No solutions of the nonlinear constraint equations were known.

In this work, we first review the superspace approach and its relation to conventional component methods. The difference between constrained and unconstrained formulations is explained, and the origin of the nonlinear constraints in supersymmetric gauge theories is discussed. It is then shown that these nonlinear constraint equations can be solved by transforming them into linear equations. The method is shown to work for N=1 Yang-Mills theory in four dimensions.

N=2 Yang-Mills theory is formulated in constrained form in six-dimensional superspace, which can be dimensionally reduced to four-dimensional N=2 extended superspace. We construct a superfield calculus for six-dimensional superspace, and show that known matter multiplets can be described very simply. Our method for solving constraints is then applied to the constrained N=2 Yang-Mills theory, and we obtain an explicit solution in terms of an unconstrained superfield. The solution of the constraints can easily be expanded in powers of the unconstrained superfield, and a similar expansion of the action is also given. A background-field expansion is provided for any gauge theory in which the constraints can be solved by our methods. Some implications of this for superspace gauge theories are briefly discussed.

A influência do discurso médico no poder punitivo

Esta dissertação discute a relação entre a medicina, a psiquiatria, a psicologia, o poder punitivo e o Direito Penal, bem como a influência que o discurso de uma produziu no outro, e vice versa. Defende a idéia de que a medicina é um espetáculo de poder que, acasalado com o poder punitivo, e interagindo com, e sobre, o indivíduo, invade e se apropria do seu corpo para, usando-o como instrumento de dominação política, discipliná-lo de acordo com a conveniência, sobretudo, da higiene e, naquela sua relação espúria com o poder punitivo, diferenciá-lo e controlá-lo social e penalmente. Sustenta, ainda, que, malgrado o acasalamento não tenha sido intencional, o Estado via na medicina o instrumento para reforçar o seu poder, enquanto essa via naquele o apoio para o seu espraiamento, embora Medicina e Estado tenham convergido, mas também divergido, por vezes tática e estrategicamente, porquanto nem sempre os dois poderes reconheceram o valor da aliança que haviam estabelecido. Então, defende a tese de que o Estado acatou a medicalização das suas ações políticas e admitiu o valor político das ações da medicina, e com vantagens para ambos que, dividindo o poder, conquistaram. É que, a medicina, mais rápida e mais adequada aos problemas salutares apresentados, ajudava-o a se imiscuir no corpo para a permanência parasitária daquela. E, para manter seu direito ao discurso, sustenta que a medicina reinventou constantemente uma necessidade para, diante dela, apresentar-se como única solução, tendo conseguido isso mediante a apresentação de uma retórica dominial eloquente, mas, sobretudo, tecnificada, é dizer, inacessível ao dominado. Com isso, a disciplina, o controle e a repressão do indivíduo, penal e medicamente, estavam prontas, pois, Direito e Medicina, aquele com a lei, esta com o remédio, juntos, dominaram e dominam os destinos do indivíduo, e da coletividade. Demonstrou, ainda, que os higienistas nunca se desocuparam de suas funções. E, por fim, que os princípios penais devem, independente da qualificação que se os dê, sempre refrear o poder punitivo.

Superconductivity, spin-glass properties, and ferromagnetism in amorphous La-Gd-Au alloys

The superconducting and magnetic properties of splat cooled amorphous alloys of composition (La_100-xGd_x)₈₀Au₂₀ (0 ≤ x ≤ 100) have been studied. The La₈₀Au₂₀ alloys are ideal type II super-conductors (critical temperature T_c = 3.5° K ). The concentration range (x less than 1) where superconductivity and spin-glass freezing n1ight coexist has been studied in detail. The spin-glass alloys (0 less than x less than 70) exhibit susceptibility maxima and thermomagnetic history effects. In the absence of complications due to crystal field and enhanced matrix effects, a phenomenological model is proposed in which the magnetic clusters are treated as single spin entities interacting via random forces using the molecular field approach. The fundamental parameters (such as the strength of the forces and the size of clusters) can be deduced from magnetization measurements. The remanent magnetization is shown to arise from an interplay of the RKKY and dipolar forces. Magnetoresistivity results are found to be consistent with the aforementioned picture. The nature of magnetic interactions in an amorphous matrix is also discussed. The moment per Gd atom (7µ_B) is found to be constant and close to that of the crystalline value throughout the concentration range investigated. Finally, a detail study is made of the critical phenomena and magnetic properties of the amorphous ferromagnet: Gd₈₀Au₂₀. The results are compared with recent theories on amorphous magnetism.

Silicide formation and the interaction of metals with polycrystalline Si

The main factors affecting solid-phase Si-metal interactions are reported in this work. The influence of the orientation of the Si substrates and the presence of impurities in metal films and at the Si-metal interface on the formation of nickel and chromium silicides have been demonstrated. We have observed that the formation and kinetic rate of growth of nickel silicides is strongly dependent on the orientation and crystallinity of the Si substrates; a fact which, up to date, has never been seriously investigated in silicide formation. Impurity contaminations in the Cr film and at the Si-Cr interface are the most dominant influencing factors in the formation and kinetic rate of growth of CrSi₂. The potentiality and use of silicides as a diffusion barrier in metallization on silicon devices were also investigated.

Two phases, Ni₂Si and NiSi, form simultaneously in two distinct sublayers in the reaction of Ni with amorphous Si, while only the former phase was observed on other substrates. On (111) oriented Si substrates the growth rate is about 2 to 3 times less than that on <100> or polycrystalline Si. Transmission electron micrographs establish-·that silicide layers grown on different substrates have different microcrystalline structures. The concept of grain-boundary diffusion is speculated to be an important factor in silicide formation.

The composition and kinetic rate of CrSi₂ formation are not influenced by the underlying Si substrate. While the orientation of the Si substrate does not affect the formation of CrSi₂ , the purity of the Cr film and the state of Si-Cr interface become the predominant factors in the reaction process. With an interposed layer of Pd₂Si between the Cr film and the Si substrate, CrSi₂ starts to form at a much lower temperature (400°C) relative to the Si-Cr system. However, the growth rate of CrSi₂ is observed to be independent of the thickness of the Pd2Si layer. For both Si-Cr and Si-Pd₂Si-Cr samples, the growth rate is linear with time with an activation energy of 1.7 ± 0.1 ev.

A tracer technique using radioactive ³¹Si (T_1/2 = 2.26 h) was used to study the formation of CrSi₂ on Pd₂Si. It is established from this experiment that the growth of CrSi₂ takes place partly by transport of Si directly from the Si substrate and partly by breaking Pd₂Si bonds, making free Si atoms available for the growth process.

The role of CrSi₂ in Pd-Al metallization on Si was studied. It is established that a thin CrSi₂ layer can be used as a diffusion barrier to prevent Al from interacting with Pd₂Si in the Pd-Al metallization on Si.

As a generalization of what has been observed for polycrystalline-Si-Al interaction, the reactions between polycrystalline Si (poly Si) and other metals were studied. The metals investigated include Ni, Cr, Pd, Ag and Au. For Ni, Cr and Pd, annealing results in silicide formation, at temperatures similar to those observed on single crystal Si substrates. For Al, Ag and Au, which form simple eutectics with Si annealing results in erosion of the poly Si layer and growth of Si crystallites in the metal films.

Backscattering spectrometry with 2.0 and 2.3 MeV ⁴He ions was the main analytical tool used in all our investigations. Other experimental techniques include the Read camera glancing angle x-ray diffraction, scanning electron, optical and transmission electron microscopy. Details of these analytical techniques are given in Chapter II.

Phase shift caused by microwave field based on light storage

We have theoretically investigated the phase shift of a probe field for a four-level atomic system interacting successively with two fields tuned near an EIT resonance of an atom, a microwave field, and a coupling field. It has been found that the phase of retrieved signal has been shifted due to the cross-phase modulation when the stored spin wave was disturbed by a microwave. Because of the low relaxation rates of the ground hyperfine state, our proposed technique can impart a large phase rotation onto the probe field with low absorption of retrieved field and very low intensity of the microwave field.

The dispersive properties of an excited-doublet four-level atomic system

We have investigated the dispersive properties of excited-doublet four-level atoms interacting with a weak probe field and an intense coupling laser field. We have derived an analytical expression of the dispersion relation for a general excited-doublet four-level atomic system subject to a one-photon detuning. The numerical results demonstrate that for a typical rubidium D1 line configuration, due to the unequal dipole moments for the transitions of each ground state to double excited states, generally there exists no exact dark state in the system. Close to the two-photon resonance, the probe light can be absorbed orgained and propagate in the so-called superluminal form. This system may be used as an optical switch.

A medicina iluminista e o vitalismo: uma discussão do Nouveaux Éléments de la Science de l‟Homme de Paul-Joseph Barthez (1734-1806).

A dissertação comenta criticamente as interpretações recentes referentes ao vitalismo no século XVIII, dedicando atenção especial aos Nouveuax Éléments de la Science de lHomme (publicado primeiramente em 1778), de Paul-Joseph Barthez (1734-1806). Até a segunda metade do século XX, como é primeiramente argumentado nesta dissertação, intérpretes do iluminismo entendiam a doutrina mecanicista como a herdeira direta da Revolução Científica, bem como a corrente dominante no mundo das ciências da vida ao longo de todo o século XVIII. Assim, na historiografia do século passado, o vitalismo era ou escassamente mencionado, ou visto como uma retrógrada corrente anti-iluminista. Mais recentemente, vários historiadores e pesquisadores da história das ciências no século XVIII (sobretudo Williams e Reill) entendem o iluminismo de um modo mais amplo e plural, considerando o vitalismo iluminista (um termo proposto por Reill) como parte integrante de um conceito mais dinâmico de iluminismo. A seguir, são apresentados a doutrina mecanicista e seus conceitos centrais, bem como as ideias de alguns dos principais representantes do mecanicismo no século XVII e início do XVIII, no caso, mais especificamente, do mecanicismo newtoniano. Em seguida, são expostos e comentados a doutrina vitalista e seus conceitos, no que é dado destaque ao vitalismo na Universidade de Montpellier. Nesse contexto, são comentados conceitos vitalistas, tal como apresentados nos Nouveuax Éléments de la Science de lHomme, no qual Barthez propõe uma nova fisiologia baseada no princípio vital; nisso são apresentados sua metodologia de pesquisa, o conceito de princípio vital, as forças sensitivas e motrizes do princípio da vida, além dos conceitos de simpatia, sinergia e, por fim, o conceito de temperamento. Esses conceitos ou essa terminologia , tal como é mostrado, não são originalmente concebidos por Barthez, mas foram por ele reapropriados e reformulados em debate com o newtonianismo e demais correntes filosóficas médicas desde a Antiguidade até o século XVIII, assim como com observações e experimentos próprios às investigações médico-científicas da época. Como resultado, é alcançada uma compreensão da doutrina vitalista como um esforço intelectual inovador tanto interagindo quanto integrado com o debate científico contemporâneo, ou seja, os médicos vitalistas se viam e, em geral, eram vistos como atuando segundo os padrões de cientificidade exigidos por seus pares.