Resultados do Grupo de Renormalização Numérico (GRN) para a condutância linear dependente da temperatura associada a corrente de tunelamento através de uma ponta de prova nas proximidades de uma impureza magnética são apresentados. Nós usamos o Modelo de Anderson de uma impureza para descrever o metal hospedeiro e um Hamiltoniano livre para simular a ponta de prova do MVT (Microscópio de Varredura por Tunelamento). O cálculo da condutância é obtida a partir da fórmula de Kubo com o Hamiltoniano de tunelamento tratado como uma perturbação com dois canais de tunelamento, ponta-impureza e ponta-substrato, com o objetivo de descrever esse sistema que está totalmente fora do equilíbrio. Esse cálculo é guiado pelo GRN de Wilson para determinar a fórmula da condutância em termos de densidades espectrais: a densidade local da impureza e a densidade relativa ao primeiro sítio de condução da rede tight-binding do GRN. Esse resultado para o operador do GRN transforma esse objeto teórico em uma quantidade mensurável. Mostramos sob condições especiais, que o gráfico da condutância em função da temperatura é uma curva universal. Como função da posição ponta-impureza, as correntes de tunelamento mostram oscilações de Friedel, que determinam o tamanho da nuvem Kondo. Finalmente, mostramos como função da energia da impureza, a corrente da impureza para a ponta mostra um platô de Kondo. A interferência entre essa corrente e a que flui da banda de condução para a ponta exibe anti-ressonâncias de Fano como as observadas em medidas espectroscópicas. / Numerical Renormalization Group (NRG) results for the temperature dependent linear conductance associated with the scanning-tunneling current through a probe near a magnetic impurity are reported. We used the Single Impurity Anderson Model to describe the host metal and a free electron Hamiltonian to simulate a STM (Scanning Tunneling Microscope) biased tip. The calculation of the conductance is obtained from the Kubo Formula with the Tunneling Hamiltonian treated as a perturbation with two tunneling channels, STM tip-impurity and STM tip-host metal, with the objective to describe this fully nonequilibrium system. This calculation is guided by Wilson\'s NRG to determine a conductance formula as a funciton of spectral densities: the local impurity density and the density relative to the first conduction site of the NRG tight-binding chain. This result for the NRG operator transforms this theoretical object into a measurable quantity. We show that, under special conditions, plotted as a function of temperature, this zero-bias conductance follows a universal curve. As a function of tip-impurity separation, the tunneling currents display Friedel Oscilations, which determine the size of the Kondo cloud. Finally, plotted as a function of impurity energy, the current from the impurity to the tip displays a Kondo plateau. The inferference between this current and that flowing from the conduction band to the tip displays Fano anti-ressonances analogous to those seen in spectroscopic measurements.

A theoretical framework for calculating electron transport through molecular junctions is presented. It is based on scattering theory using a Green's function formalism. The model can take both elastic and inelastic scattering into account and treats chemical and physical bonds on equal footing. It is shown that it is quite reliable with respect to the choice of functional and basis set. Applications concerning both elastic and inelastic transport are presented, though the emphasis is on the inelastic transport properties. The elastic scattering application part is divided in two part. The first part demonstrates how the current magnitude is strongly related to the junction width, which provides an explanation why experimentalists get two orders of magnitude differences when performing measurements on the same type of system. The second part is devoted to a study of how hydrogenbonding affects the current-voltage (I-V) characteristics. It is shown that for a conjugated molecule with functional groups, the effects can be quite dramatic. This shows the importance of taking possible intermolecular interactions into account when evaluating and comparing experimental data. The inelastic scattering part is devoted to get accurate predictions of inelastic electron tunneling spectroscopy (IETS) experiments. The emphasis has been on elucidating the importance of various bonding conditions for the IETS. It is shown that the IETS is very sensitive to the shape of the electrodes and it can also be used to discriminate between different intramolecular conformations. Temperature dependence is nicely reproduced. The junction width is shown to be of importance and comparisons between experiment as well as other theoretical predictions are made. / QC 20101118

Radio frequency (RF) cavities are the foundational infrastructure which facilitates much of the fundamental research conducted in high energy particle physics. These RF cavities utilize their unique shape to produce resonant electromagnetic fields used to accelerate charged particles. Beside their core role in fundamental physics research, RF cavities have found application in other disciplines including material science, chemistry and biology which take advantage of their unique light sources. Industry has been keen on taking advantage of accelerator technology for a multitude of applications. Particle accelerators like the one found at Jefferson Lab’s Continuous Electron Beam Accelerator Facility must produce stable beams of high energy particles which is an incredibly costly endeavor to pursue. With the gargantuan size of these facilities, the cost of high-quality beam production is a matter of great importance. The quest to find highly efficient RF cavities has resulted in the widespread use of superconducting radio frequency (SRF) cavities which are the most efficient resonators that exploit a superconductor’s incredibly low AC surface resistance. While metals like Cu are up to the demanding job of RF cavity particle acceleration, their efficiency at transferring RF power to the particle beam is low when they are compared with SRF Nb cavities. Nb is the standard material for all SRF cavity technology particularly for its reproducibly low surface resistance, comparatively high transition temperature and thermodynamic critical field. Using superconducting Nb is not without its drawbacks. Keeping hundreds of Nb cavities in their superconducting state under extreme RF conditions is quite a daunting task. It requires the normal state not nucleate during operation. This is achieved by producing high-quality cavities with as few defects and impurities as possible while also keeping the cavities at low temperature, usually 2K. Again, due to the sheer scale of the facilities, hundred million-dollar cryogenic plants are required to handle the heat loads during SRF cavity operation. This means even small increases in maximum accelerating gradients or decrease in cavity surface resistance results in a sizably reduced operation cost. Considerable effort has been put forth to increase the efficiency of Nb cavities toward and even beyond the theoretical maximum accelerating gradients and quality factor for a clean superconductor. Recently, a new method to produce high quality factor cavities has emerged that involves nitrogen doping the cavity. The mechanism by which N doping causes the improvement is still not well understood, but the experimental research described in this dissertation shines some light into the mechanisms behind such a drastic improvement. These insights are universal for all superconductors and may prove useful for SRF cavities beyond Nb. With Nb approaching its fundamental limits, new materials are being proposed to increase the performance of future SRF cavities which MgB2 finds itself among. MgB2 is a two-band superconductor that possesses many properties that are very attractive for the next generation of SRF cavities. One of the most important properties is MgB2’s comparatively large critical temperature which in part predicts it will have a lower surface resistance than Nb at higher operating temperatures. Such behavior of MgB2 may unlock the possibility of using cryocoolers instead of costly liquid helium plants for large scale industrial use. This dissertation starts with an introduction to superconductivity, its theory, and application to SRF cavities as well as the open questions that can be addressed in Nb and the next generation of SRF materials. A description of the experimental techniques of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy is presented. Our experimental investigation into Nb SRF cavity cutouts starts with a discussion of the material’s limitations for SRF applications with an emphasis on the proximity effect which arises at the surface of this material due to its myriad of naturally forming oxides. The results of our scanning tunneling microscopy measurements for typically prepared Nb and nitrogen doped Nb follows and comparisons are made which show that the surface oxides are fundamentally different between these samples likely resulting in the profound enhancement of the cavity’s quality factor. Experimental investigation into the native oxide of hot spot nitrogen doped Nb shows a degraded oxide and superconducting properties as compared with the cold spot. The dissertation continues with a brief introduction to MgB2, followed by our scanning tunneling and electron tunneling insights into MgB2. The dissertation is concluded with a summary of our investigations and broader impact of our research on the SRF community. / Physics

No description available.

Polymer bonded explosives (PBXs) are composites comprised of energetic crystals with a very high energy density surrounded by a polymer binder. The formation of hotspots within polymer bonded explosives can lead to the thermal decomposition and initiation of the energetic material. A frictional heating model is applied at the mesoscale to assess the potential for the formation of hotspots under low velocity impact loadings. Monitoring of the formation and growth of damage at the mesoscale is considered through the inclusion of a piezoresistive carbon nanotube network within the energetic binder providing embedded strain and damage sensing. A coupled multiphysics thermo-electro-mechanical peridynamics framework is developed to perform computational simulations on an energetic material microstructure subject to these low velocity impact loads. With increase in impact energy, the model predicts larger amounts of sensing and damage thereby supporting the use of carbon nanotubes to assess damage growth and subsequent formation of hotspots. The framework is also applied to assess the combined effects of thermal loading due to prescribed hotspots with inertial effects due to low velocity impact loading. It has been found that the present model is able to detect the presence of hotspot dominated regions within the energetic material through the piezoresistive sensing mechanism. The influence of prescribed hotspots on the thermo-electro-mechanical response of the energetic material under a combination of thermal and inertial loading was observed to dominate the lower velocity impact response via thermal shock damage. In contrast, the higher velocity impact energies demonstrated an inertially dominated damage response. Quantifying the piezoresistive effect derived from embedding carbon nanotubes in polymers remains a challenge since these nanocomposites exhibit significant variation in their electro-mechanical properties depending upon factors such as CNT volume fraction, CNT dispersion, CNT alignment and properties of the polymer. Of interest is electrical percolation where the electrical conductivity of the CNT/polymer nanocomposite increases through orders of magnitude with increase in CNT volume fraction. Estimates and distributions for the electrical conductivity and piezoresistive coefficients of the CNT/polymer nanocomposite are obtained and analyzed with increasing CNT volume fraction and varying barrier potential, which is a parameter that controls the extent of electron tunneling. The effect of CNT alignment is analyzed by comparing the electro-mechanical properties in the alignment direction versus the transverse direction for different orientation conditions. Estimates of piezoresistive coefficients are converted into gage factors and compared with experimental sources in literature. The methodology for this work uses automated scripts which are used in conjunction with high performance computing to generate several 5 μm ×5 μm realizations for different CNT volume fractions. These realizations are then analyzed using finite elements to obtain volume averaged effective values, which are then subsequently used to generate measures of central tendency (estimated mean) and variability (standard deviation, coefficient of variation, skewness and kurtosis) in a descriptive statistical analysis. / Doctor of Philosophy / Carbon nanotubes or CNTs belong to a class of novel materials known as nanomaterials which are materials with length scales on the order of nanometers. CNTs have been widely studied due to their unique mechanical, electrical and thermal properties in comparison to traditional materials such as metals or plastics. Often times, research and applications concerning the use of CNTs involves embedding the CNTs as a filler within a larger composite material system. In the present work, CNTs are considered to be embedded within a polymer. It is known that the electrical properties of such a CNT/polymer composite change in response to the application of a mechanical force. This change in electrical properties is caused due to the presence of CNTs and is used as a means of sensing the mechanical state of the composite, i.e. real time structural monitoring. The extent of the change in electrical properties, also known as sensing, depends upon a number of different factors such as the amount of CNTs used per unit volume of the polymer, how well dispersed or clumped together the CNTs are within the polymer and the type of polymer material used, among other factors. A statistical analysis is performed with several case studies where these factors are varied and the resulting change in the sensing response is monitored. Several important conclusions were made from the statistical analysis with some of the results providing new insights into the sensing behavior of CNT/polymer composites. For example, it was found that a key parameter known as barrier potential, which directly influences the extent of sensing achieved through a mechanism known as electrical tunneling, needs to be several orders of magnitude lower than previously reported values to accurately capture the sensing effects. Key metrics quantifying the extent of sensing from the analysis were found to be in agreement with previously reported experimental results. The significance of such a statistical study lies in the fact that CNT embedded composites are increasingly being proposed and used for sensing applications. The use of CNT embedded polymers to encase explosive crystalline grains such as HMX or RDX is one such example. These explosive grains are used in a number of different civil and military applications such as fuel rocket propellants, industry explosives, military munitions etc. The grains possess extremely high energy densities and are susceptible to undergo violent chemical reaction if a trigger is provided through thermal or mechanical means. As such, the monitoring of the structural state of these explosives is crucial for their safe handling and processing. In this work, the sensing response of a composite material comprising of explosive grains surrounded by polymer material containing CNTs is studied in response to different types of mechanical loads, ranging from mild stimuli to impact. It was found that the sensing mechanism was capable of tracking mechanical damage as well as the resulting temperature increases interior to the composite. In addition to its application to safety and preventative measures, the use of CNTs in this context also provided insight into the mechanisms related to the sudden release of energy in these explosive grains which is of significant interest since this is an active area of research as well.

No description available.

Dans un contexte de besoin grandissant en précision dans la mesure des faibles courants pour les instituts nationaux de métrologie, l’industrie, les fabricants d’instruments et la physique fondamentale, l’étude des dispositifs à un électron (SET) capables de générer un courant continu directement proportionnel à une fréquence et la charge élémentaire, couplés à un amplificateur de courant très performant, le comparateur cryogénique de courant (CCC), devient pertinente pour réaliser un étalon quantique de courant. Dans ce contexte, les travaux ont été poursuivis au LNE sur l’étude de nouveaux dispositifs SET et le développement de nouveaux CCC. Durant cette thèse, un montage expérimental a été mis en place afin d’évaluer les performances d’un nouveau CCC, constitué d’une conception originale et de 30 000 tours. Les résultats expérimentaux obtenus sont satisfaisant par rapport aux objectifs fixés, que ce soit en termes de résolution en courant, d’erreurs, de stabilité des mesures et de reproductibilité. Le CCC développé durant la thèse peut donc être utilisé pour quantifier de manière métrologique les dispositifs à un électron. De plus, une modélisation réalisée à partir d’un schéma électrique équivalent a été mis en place afin de simuler le comportement réel du CCC en prenant en compte les aspects magnétiques et électriques mis en jeu. Cette simulation a permis la quantification de l’erreur due aux fuites de courants au travers des capacités parasites entourant les enroulements. Les résultats de la simulation indiquent que cette erreur atteint 10 10 à la fréquence de travail, ce qui est inférieure de deux ordres de grandeurs à l’erreur maximale tolérable : 10-8. Les résultats expérimentaux et de modélisation fournissent de nouveaux éléments d’amélioration de la conception de CCCs de grand gain. Enfin, la modélisation développée, une fois insérée dans une routine d’optimisation, pourra aussi être un outil de conception des CCCs très utile. / In a context of growing need of precision in measuring low currents for national metrology institutes, industry, instrument manufacturers and fundamental physics, study of single-electron tunneling (SET) devices capable of generating a direct current directly proportional to the frequency and the elementary charge, coupled with a high performance current amplifier, the cryogenic current comparator (CCC), becomes relevant to realize a quantum current standard. In this framework, at LNE, study of new SET devices and the development of CCCs continues. In this thesis, an experimental setup was implemented to evaluate the performance of a new CCC, consisting of a new design and 30 000 turns. The experimental results fulfill our goals, whether in terms of current resolution, errors, measurement stability and reproducibility. The CCC developed during the thesis can thus be used to metrologically quantify SET devices. In addition, a model based on an equivalent circuit diagram has been developed to simulate the actual behavior of the CCC, taking into account the magnetic and electrical aspects involved. This simulation allows the quantification of the error due to currents leakage through parasitic capacitances surrounding the windings. Results of the simulation indicate that this error reaches 10 10, which is less, by two orders of magnitude, than the maximum tolerable error: 10 8. Results obtained experimentally and by simulation provide new improvement elements in the design of high ratio CCCs. The developed model, once inserted into an optimization routine, can also be a very useful design tool of CCCs.

Al–O monolayers embedded in ultrathin SiO₂ were shown previously to contain Al-induced acceptor states, which capture electrons from adjacent silicon wafers and generate a negative fixed charge that enables efficient Si-surface passivation. Here, we show that this surface passivation is just in part attributed to field-effect passivation, since the electrically active interface trap density Dit itself at the Si/SiO₂ interface is reduced by the presence of the acceptor states. For sufficiently thin tunnel-SiO₂ films between the Si-surface and the Al–O monolayers, Dit is reduced by more than one order of magnitude. This is attributed to an interface defect deactivation mechanism that involves the discharge of the singly-occupied dangling bonds (Pb0 defects) into the acceptor states, so that Shockley-Read-Hall-recombination is drastically reduced. We demonstrate that the combined electronic and field-effect passivation allows for minority carrier lifetimes in excess of 1 ms on n-type Si and that additional H₂-passivation is not able to improve that lifetime significantly.

No description available.

Exploiter des effets dépendants du temps pour induire et contrôler des courants à travers des conducteurs mésoscopiques et nanoscopiques est un enjeu majeur dans le domaine du transport quantique. Dans cette thèse, nous considérons deux systèmes de taille nanométrique pour lesquels un courant est induit grâce au couplage entre champs extérieurs dépendants du temps et le transport d'électrons. Nous étudions d'abord un problème de pompage quantique au sein d'un système à trois sites en configuration d'anneau, en considérant la possibilité d'induire un courant continu par modulation temporelle des paramètres de contrôle. Nous nous intéressons en particulier à la transition entre régime adiabatique et antiadiabatique en présence d'un mécanisme de dissipation modélisé par un couplage entre le système et un bain extérieur.Nous montrons que le modèle dissipatif admet une solution analytique complète valable pour la composante DC du courant à fréquence arbitraire. Ceci nous permet de bien comprendre comment le courant induit dépend de la fréquence de pompage. Nous nous concentrons ensuite sur un autre système de contrôle du courant exploitant le phénomène des oscillations tunnel à un électron (SETOs). Contrairement au cas précédent, ici la circulation d'un courant continu à travers un circuit comportant une jonction tunnel produit, pour le régime approprié, un courant quasi-périodique d'électrons. On étudie le spectre de bruit à température nulle d'une jonction tunnel dans différents environnements résistifs dans le but de déterminer les limites du régime des SETOs et de quantifier leur degré de périodicité. Nous généralisons par la suite les résultats à température finie et discutons des effets des fluctuations quantiques. / Exploiting time-dependent effects to induce and control currents through mesoscopic and nano\-scopic conductors is a major challenge in the field of quantum transport. In this dissertation we consider two nanoscale systems in which a current can be induced through intriguing mechanisms of coupling between excitations by external fields and electron transport.We first study a quantum pumping problem, analyzing the possibility to induce a DC response to an AC parametric driving through a three-site system in a ring configuration. We are interested in particular in the crossover between adiabatic and antiadiabatic driving regimes and in the presence of dissipation, which is accounted for by coupling with an external bath. We show that for a clever choice of this coupling the dissipative model admits a full analytical solution for the steady state current valid at arbitrary frequency, which allows us to fully understand the pumping-frequency dependence of the induced current. We then focus on a different current-controlling scheme exploiting the phenomenon of single-electron tunneling oscillations (SETOs). In this case, opposite to what happens for pumping, an AC effect, an almost periodic current of single electrons, arises through a tunnel junction circuit as a consequence of a DC bias. We study the zero-temperature noise spectrum of a tunnel junction in different resistive environments with the aim to determine the boundaries of the SETOs regime and quantify their quality in terms of periodicity. We then discuss the finite-temperature generalization and the possibility to account for the effects of quantum fluctuations.