The first part of this thesis is an experimental study of the spin-dependent transport in magnetic point contacts. Nano-contacts are produced micromechanically, by bringing a sharpened non-magnetic (N) tip into contact with a ferromagnetic (F) film. The magnetic and magneto-transport properties of such N/F nanocontacts are studied using transport spectroscopy, spanning the ballistic, diffusive, and thermal transport regimes. Single N/F interfaces can exhibit current driven magnetic excitations, which are often manifest as peaks in the differential resistance of a point contact defining the N/F interface. Our experiments show that such surface magnetization excitations, and thus the single-interface spin torques, are observed for diffusive and thermal transport regimes where the conduction electrons experience strong scattering near the N/F interface, and are absent for purely ballistic contacts. We conclude that the single-interface spin torque effect is due to impurity scattering at N/F interfaces. Single N/F interfaces can also exhibit hysteretic conductivity, which is qualitatively similar to the spin-valve effect found in F/N/F trilayers. Based on our measurements of N/F point contacts in the size range of 1-30 nm, we propose two mechanisms of the observed hysteresis. The first mechanism relies on a non-uniform spin distribution near the contact core and is magnetoelastic in origin. This interpretation is in good agreement with some of our experiments on larger point contacts as well as with a numerical micromagnetic model we have developed, where a stress-induced anisotropy creates a non-uniform, domain-wall-like spin distribution in the contact core. The second mechanism we propose is a surface effect which relies on a difference between the surface and interior spins in the ferromagnet in terms of their exchange and anisotropy properties. The surface spin-valve mechanism is in good agreement with the hysteretic magnetoresistance observed for our smallest contacts (~1 nm) and for contacts to nanometer thin ferromagnetic films. This interpretation means that the surface magnetization can be reduced and weakly coupled to the interior spins in the ferromagnet. We find that this surface spin layer can be affected by both external fields and the spin torque of a transport current. The surface magnetization can even form nano-sized spin vorticies at the interface. The nature of the magnetic excitations induced by by nominally unpolarized currents through single N/F interfaces was probed directly using microwave irradiation. We observed two characteristic high-frequency effects: a resonant stimulation of spin-wave modes by microwaves, and a rectification of off-resonant microwave currents by spin-wave nonlinearities in the point contact conductance. These experiments demonstrate that the effects observed are spin-dynamic in nature. In the second part of the thesis we study the spin-dynamics in spin-flop tunnel junctions used in toggle magnetic random access memory. Current pulses in the range of 100 ps used to excite the magnetic moments of the two coupled Py free layers into an oscillatory state, in both the antiparallel and scissor states of the cell. These oscillations are detected directly by measuring the junction resistance in real time with a 6 GHz measurement bandwidth. The junctions had the shape of an ellipse, with lateral size ranging from 350x420 to 400x560 nm. The optical and acoustical precession modes of the the spin-flop trilayer are observed in experiment, as expected from single-domain model. The experimental spectra contain additional features, which are explained using numerical micromagnetic simulations, as originating from magnetic state transitions between different magnetization states with non-uniform spin distributions. / QC 20100818

Les cadhérines sont des récepteurs d'adhésion intercellulaires permettant d'établir un lien entre la cellule adjacente et le cytosquelette. Pendant la formation des contacts, l'association des cadhérines avec l'actine via le recrutement des caténines alpha et bêta a été largement étudié. Pourtant la contribution des microtubules (MT) commence seulement à être abordée. Mon travail de thèse s'est focalisé sur l'étude de la relation entre la N-cadhérine (Ncad) et les MT dans deux contextes particuliers : l'adhésion et la migration cellulaires. En utilisant des substrats de Ncad recombinante, j'ai pour la première fois montré que dans un contexte d'adhésion cellulaire (lignée myogénique, souris), que (i) l'engagement de la Ncad inhibe le recrutement et la dynamique des MT à travers la régulation de l'actine et (ii) que la stabilisation des MT mène à une accumulation de la Ncad aux contacts. Dans ce contexte la Ncad et les MT établiraient donc une boucle de rétrocontrôle négatif. Ensuite j'ai démontré que pendant la migration cellulaire (modèle de neurones d'hippocampe de rat), la Ncad stimule la dynamique des MT. En conclusion, ces résultats révèlent un lien fonctionnel Ncad/MT dépendant de l'actine, dont la nature est différente selon le contexte cellulaire. Mon travail met à jour un nouveau mécanisme moléculaire impliqué dans la formation des contacts et la migration cellulaires. J'ai également participé à l'étude du rôle de la Ncad au cours de la survie, travail publié dans PlOS ONE. Nous avons montré que la Ncad promeut la survie neuronale par stimulation de la voie de signalisation activant les kinases Mek1/2, menant à la dégradation de la protéine pro-apoptotique Bim

Nickel based coatings were deposited on copper substrates by two different sputtering techniques from a nickel alloy based target. The substrates used were commercially available copper based substrates for low duty electrical contacts. The coatings were analyzed and evaluated as copper diffusion barriers for electrical contact applications. In addition two types of commercially available electroplated nickel coatings (referred to as type A electroplated coatings and type B electroplated coatings) were characterized for comparison. The Technique I sputtered coatings were deposited using three different substrate bias voltages and two different working gas pressures. The Technique II coatings were deposited using two different substrate bias voltages and two different working gas pressures. All sputtered coatings were deposited at a temperature of 200° C. The quality of the barriers was investigated by analyzing their composition, microstructure, stress, mechanical properties , and surface roughness. The results show that sputtered coatings have polycrystalline structures while the two plated films had (200) orientation and (111) orientation. Both plated coatings contained impurities that originate from chemicals used in the plating baths. The surface of the sputtered coatings reflects the substrate surface, while the electroplated samples on the same substrate (type A coatings) show a smooth mirror like surface and the type B electroplated coatings show a rough surface. Technique II sputtered coatings showed the highest hardness in the amount of 13 GPa, followed by electroplated type A coatings with a hardness of about 9 GPa while the Technique I coatings showed hardness of 6-8 GPa. All sputtered coatings exhibited compressive stress while the electroplated type A coatings exhibited tensile stress of almost twice the magnitude. In this study it is shown that sputtered nickel based coatings sputtered nickel based coatings are a promising more environmental friendly alternative to electroplated nickel coatings.

As a result of manufacturing processes, real surfaces have roughness and surface curvature. The real contact occurs only over microscopic contacts, which are typically only a few percent of the apparent contact area. Because of the surface curvature of contacting bodies, the macrocontact area is formed, the area where microcontacts are distributed randomly. The heat flow must pass through the macrocontact and then microcontacts to transfer from one body to another. This phenomenon leads to a relatively high temperature drop across the interface. Thermal contact resistance (TCR) is a complex interdisciplinary problem, which includes geometrical, mechanical, and thermal analyses. Each part includes a micro and a macro scale sub-problem. Analytical, experimental, and numerical models have been developed to predict TCR since the 1930's. Through comparison with more than 400 experimental data points, it is shown that the existing models are applicable only to the limiting cases and none of them covers the general non-conforming rough contact. The objective of this study is to develop a compact analytical model for predicting TCR for the entire range of non-conforming contacts, i. e. , from conforming rough to smooth sphere-flat in a vacuum. The contact mechanics of the joint must be known prior to solving the thermal problem. A new mechanical model is developed for spherical rough contacts. The deformation modes of the surface asperities and the bulk material of contacting bodies are assumed to be plastic and elastic, respectively. A closed set of governing relationships is derived. An algorithm and a computer code are developed to solve the relationships numerically. Applying Buckingham Pi theorem, the independent non-dimensional parameters that describe the contact problem are specified. A general pressure distribution is proposed that covers the entire spherical rough contacts, including the Hertzian smooth contact. Simple correlations are proposed for the general pressure distribution and the radius of the macrocontact area, as functions of the non-dimensional parameters. These correlations are compared with experimental data collected by others and good agreement is observed. Also a criterion is proposed to identify the flat surface, where the influence of surface curvature on the contact pressure is negligible. Thermal contact resistance is considered as the superposition of macro and micro thermal components. The flux tube geometry is chosen as the basic element in the thermal analysis of microcontacts. Simple expressions for determining TCR of non-conforming rough joints are derived which cover the entire range of TCR by using the general pressure distribution and the flux tube solution. A complete parametric study is performed; it is seen that there is a value of surface roughness that minimizes TCR. The thermal model is verified with more than 600 data points, collected by many researchers during the last 40 years, and good agreement is observed. A new approach is taken to study the thermal joint resistance. A novel model is developed for predicting the TCR of conforming rough contacts employing scale analysis methods. It is shown that the microcontacts can be modeled as heat sources on a half-space for engineering applications. The scale analysis model is extended to predict TCR over the entire range of non-conforming rough contacts by using the general pressure distribution developed in the mechanical model. It is shown that the surface curvature and contact pressure distribution have no effect on the effective micro thermal resistance. A new non-dimensional parameter is introduced as a criterion to identify the three regions of TCR, i. e. , the conforming rough, the smooth spherical, and the transition regions. An experimental program is designed and data points are collected for spherical rough contacts in a vacuum. The radius of curvature of the tested specimens are relatively large (in the order of m) and can not be seen by the naked eye. However, even at relatively large applied loads the measured joint resistance (the macro thermal component) is still large which shows the importance of surface out-of-flatness/curvature. Collected data are compared with the scale analysis model and excellent agreement is observed. The maximum relative difference between the model and the collected data is 6. 8 percent and the relative RMS difference is approximately 4 percent. Additionally, the proposed scale analysis model is compared/verified with more than 880 TCR data points collected by many researchers. These data cover a wide range of materials, surface characteristics, thermal and mechanical properties, mean joint temperature, directional heat transfer effect, and contact between dissimilar metals. The RMS difference between the model and all data is less than 13. 8 percent.

A simple cost-effective approach was proposed and successfully employed to fabricate high-quality screen-printed (SP) contacts to high sheet-resistance emitters (100 ohm/sq) to improve the Si solar cell efficiency. Device modeling was used to quantify the performance enhancement possible from the high sheet-resistance emitter for various cell designs. It was found that for performance enhancement from the high sheet-resistance emitter, certain cell design criteria must be satisfied. Model calculations showed that in order to achieve any performance enhancement over the conventional ~40 ohm/sq emitter, the high sheet resistance emitter solar cell must have a reasonably good (120,000 cm/s) or low front-surface recombination velocity (FSRV). Model calculations were also performed to establish requirements for high fill factors (FFs). The results showed that the series resistance should be less than 0.8 ohm-cm^2, the shunt resistance should be greater than 1000 ohm-cm^2, and the junction leakage current should be less than 25 nA/cm^2. Analytical microscopy and surface analysis techniques were used to study the Ag-Si contact interface of different SP Ag pastes. Physical and electrical properties of SP Ag thick-film contacts were studied and correlated to understand and achieve good-quality ohmic contacts to high sheet-resistance emitters for solar cells. This information was then used to define the criteria for high-quality screen-printed contacts. The role of paste constituents and firing scheme on contact quality were investigated to tailor the high-quality screen-printed contact interface structure that results in high performance solar cells. Results indicated that small particle size, high glass transition temperature, rapid firing and less aggressive glass frit help in producing high-quality contacts. Based on these results high-quality SP contacts with high FFs0.78 on high sheet-resistance emitters were achieved for the first time using a simple single-step firing process. This technology was applied to different substrates (monocrystalline and multicrystalline) and surfaces (textured and planar). Cell efficiencies of ~16.2% on low-cost EFG ribbon substrates were achieved on high sheet-resistance emitters with SP contacts. A record high-efficiency SP solar cell of 19% with textured high sheet-resistance emitter was also fabricated and modeled.

Thin-film niobium mechanically controlled break junctions and resistively shunted niobium mechanically-controlled break junctions were developed and successfully microfabricated. Using these devices, high-stability atomic size contacts were routinely produced and investigated both in the normal and superconducting states. Investigations of the two-level conductance fluctuations in the smallest contacts allowed the calculation of their specific atomic structure. Embedding resistive shunts close to the superconducting atomic-sized junctions affected the coherence of the electronic transport. Finally, point contact spectroscopy measurements provide evidence of the interaction of conduction electrons with the mechanical degrees of freedom of the atomic-size niobium contacts.

In this thesis, we propose a high density non-classical unipolar CMOS width two embedded oxide (2EO) NMOS load. The words ¡§unipolar CMOS¡¨ refer to the fact that the conventional NMOS driver and the proposed 2EO NMOS load are presented in which the electron is the only carrier used. Among them, the 2EO scheme is used to isolate the inversion current. And the dominant current in the 2EO NMOS load is the punch through current which is not a destructive current mechanism. Our proposed CMOS, due to the same carrier used, does not have to compensate the layout width in load design. In addition, the shared terminal of output contacts and the elimination of N-well technique are also presented in our proposed CMOS. Therefore, the layout area can be reduced 72%, in comparison with conventional CMOS. Furthermore, the packing density can be increased and the fabrication cost can be reduced, respectively. We also find out that the delay time can be improved 39% to increase the operating frequency, as compared with the convention CMOS.

AgCdO is one of the most widely used contact materials in the world because of its outstanding performance. Nevertheless, due to environmental considerations, it will soon be completely forbidden by European environmental directives. Therefore, finding a good substitute is of crucial importance. Electrical arc erosion plays a crucial role in the reliability and life of power switching devices. Depending on the contact material's behavior in response to an electrical arc, surface damage can induce severe changes in contact material properties that will impact the power switching device's functioning. Consequently, electrical arc effects and consequences on the contact material surface are of first importance. In this context, we have focused our research activities on the following axes. First of all, in order to better understand AgCdO (Current contact material in aerospace industry) and AgSnO₂(Potential candidate to AgCdO substitution) arc erosion behaviors, arc erosion experiments, where the power switching devices have been subjected to different numbers of arc discharges, have been realized. Further, a general macroscopic electrical contact arc erosion model valid for low and high currents was developed. To compare model results to experimental data, this model describes the complete breaking process of electrical contacts and gives the total amount of material removed after one breaking operation. In parallel, arc erosion experiments on AgCdO power switching devices have been conducted at high currents (0 -> 1000 A) in order to validate the arc erosion model. Next, using the general arc erosion model, the properties having the greatest influence on the electrical arc erosion process have been determined through simulations on silver contact material. At this stage, ab initio calculations were needed to obtain ranges of variation of certain silver contact material properties. Finally, an investigation of the trends of changing local contact material on these identified material properties was performed. This study composition of AgSnO₂was based on ab initio calculations for two different oxide compositions of AgSnO₂. These will allow us to give directions to aid the design of a good substitute for AgCdO, and therefore, to complete the main objective of this research work.

<p>In this thesis, the problem of controlling a surgical masterand slave system with force reflection is studied. The problemof stiff contacts between the slave and the environment isgiven specific attention. The work has been carried out at KTHbased on an initial cooperation with Karolinska Sjukhuset. Theaim of the over all project is to study the possibilities forintroduction of a force reflective teleoperator in neurologicalskullbase operations for the particular task of bone millingand thereby, hopefully, increase patient safety, decreasesurgeon workload and cost forthe society.</p><p>The main contributions of this thesis are:</p><p>Derivation of a dynamical model of the master andoperator’s finger system and, experimental identificationof ranges on model parameter values. Based on this model, theinteraction channel controllers optimized for transparency arederived and modified to avoid the influence of the uncertainmodel parameters. This results in a three channel structure. Todecrease the influence of the uncertain parameters locally atthe master, a control loop is designed such that the frequencyresponse of the reflected force is relatively unaffected by theuncertainties, a result also confirmed in a transparencyanalysis based on the H-matrix. The developed teleoperatorcontrol structure is tested in experiments where the operatorcould alter the contact force without facing any problems aslong as the slave is in contact with the environment.</p><p>As a result of the severe difficulties for the teleoperatorto move from free space motion to in-contact manipulationwithout oscillative behaviour, a new detection algorithm basedon passivity theory is developed. The algorithm is able todetect the non-passive behaviour of the actual teleoperatorinduced by the discrete change in system dynamics occurring atthe contact instant. A stabilization controller to be activatedby the detection algorithm is designed and implemented on themaster side of the teleoperator. The detection algorithm andthe stabilization controller are shown highly effective in realexperiments.</p><p>All major research results presented in the thesis have beenverified experimentally.</p><p><b>Keywords</b>Teleoperator, Force Feedback, Passivity, StiffContacts, Control, Robustness, Transparency, Bone Milling,Uncertainty</p>

Group I introns are structured RNAs that have been used extensively as model systems for RNA folding because they are experimentally tractable, yet complex enough to have folding challenges associated with larger RNAs. The Tetrahymena group I intron consists of a set of conserved core helices and a set of peripheral elements. Peripheral elements surround the core helices and form long range tertiary contacts between each other and to the core. Interestingly, a long-lived misfolded state is populated that has the same long range tertiary contacts as the native state but differs locally within the core. Our lab showed that the intact periphery is necessary to specify the correct core structure, as mutating tertiary contacts or removing the P5abc peripheral element dramatically destabilized the native ribozyme relative to the misfolded form. However, we also showed that the thermodynamic benefit peripheral structure provided is accompanied by kinetic liability in folding, apparently because native tertiary contacts formed by peripheral elements around the misfolded core must come apart to allow refolding of the misfolded RNA to the native state. In addition to peripheral elements, proteins also play a role in stabilizing the native structures of many group I introns. The CYT-18 protein, which occupies the same binding site as P5abc, stabilizes the functional structures of certain group I introns by using a set of insertions that are absent in other related bacterial and mitochondrial aminoacyl tRNA synthetases. Using the P5abc deletion variant of the Tetrahymena ribozyme, I sought to further define CYT-18 roles in RNA folding by probing its thermodynamic and kinetic effects on the native state formation relative to the misfolded state. I demonstrated that CYT-18, like P5abc, provided thermodynamic stability to the native state. However, unlike P5abc, CYT-18 had no apparent effect on the refolding kinetics, suggesting that a protein co-factor can stabilize the functional structure without acquiring the associated costs in RNA folding kinetics. Furthermore, I found that the mechanism of CYT-18 action appears to be distinct from P5abc. Disruption of the long-range contact P14, which is formed between P5c and L2 and is part of the network of peripheral contacts, dramatically weakened P5abc binding to the native ribozyme core by ~10⁸ fold. Interestingly, CYT-18 maintained specific and tight binding to these mutants, which suggests that CYT-18 does not rely on a circular network of contacts to specifically stabilize the native state. Instead, the specificity may arise from a more direct and intimate contact of CYT-18 with the ribozyme core. This study gives insight into an evolutionary advantage of protein co-factors in RNA folding; proteins may offer thermodynamic assistance without inhibiting folding kinetics. / text