Дисертації з теми "Interface physics"

This is a masters thesis (20p) in computer science at the University of Linköping. This thesis will give an introduction to what a physics engine is and what it consist of. It will put some engines under the magnifying glass and test them in a couple of runtime tests. Two cutting edge commercial physics engines have been examined, trying to predict the future of physics engines. From the research and test results, an interface for physics engine independency has been implemented for a company called Craft Animations in Gothenburg, Sweden.

In recent decades, research and development of organic based semiconductor devices have attracted intensive interests. One of the most essential elements is to understand the electronic structures at various interfaces involved in these devices since the interface properties control many of the critical electronic processes. It is thus necessary to study the electronic properties of the organic semiconductors with surface analytical tools to improve the understanding of the fundamental mechanisms involved in the interface formation. This thesis covers the experimental investigations on some of the most interesting topics raised in the recent development of organic electronic devices. The thesis intends to reveal the physical processes at the interface and their contribution to the device performance with photoemission and inverse photoemission investigations on the evolution of the occupied and unoccupied electronic structures. I will report a substantial difference in the electron affinity of CuPc on two substrates as the orientations of CuPc are different. I will also illustrate that the CuPc has standing up configuration on one monolayer of C60 on SiO2 while lying down on one monolayer of C60 on HOPG. Meanwhile, the CuPc on more than one monolayers of C60 on different substrates show that the substrate orientation effect vanished. Then I will propose a two-stage model to describe the bulk doping effect of C60 by molybdenum oxide. I will also demonstrate that the doping effect of C60 by ultra-thin layer molybdenum oxide is weaker than that by interface doping and bulk doping. I will demonstrate that for Au on CH3NH3PbI3, hole accumulation occurs at the vicinity of the interface, facilitating hole transfer from CH3NH3PbI3 to Au. I will show a strong initial shift of core levels to lower binding energy in C60 on CH3NH3PbI3 interface, which indicates that electrons transfer from the perovskite film to C60 molecules. I will further demonstrate that the molybdenum oxide surface can be passivated by approximately two monolayers of organic thin films against exposure to air. I will discuss the mechanism that how oxygen plasma treatment effectively recover the high work function drop of molybdenum oxide by air exposure. At the end, I will show that a small energy offset at Pentacen/C60 heterojunction makes it easy to transfer electrons from Pentacene to C60 even under a small applied bias, facilitating the occurrence of charge generation. Finally, I will summarize the thesis.

The magnetic properties of Metallo-organic heterostructure interfaces are studied. These heterostructures are built with manganese phthalocyanine (MnPc) and iron phthalocyanine (FePc). Previously, the powder of each material is reported to be an Ising-like chain magnet with Arrhenius relaxation. The relaxation is slow enough to exhibit magnetic hysteresis at low temperatures. Each layer of the heterostructure is investigated separately by depositing a thin film of either iron phthalocyanine (FePc) or manganese phthalocyanine (MnPc) on a Silicon substrate heated to 150 °C. FePc thin films show magnetic hysteresis below 5K with a typical coercivity of 1850 ± 50 Oe and moment of about 1.9 µB in agreement with values from the literature. Similarly, the MnPc thin film deposited at 150 °C shows magnetic hysteresis at 2.5 K, and no hysteresis at 5K and 10 K. A coercive field of 390 Oe is recorded at 2.5 K. The saturation magnetization is near 9 emu cm^–3, which corresponds to an effective magnetic moment per Mn ion of about 0.5 µB. For the MnPc/FePc thin film bilayer, the FePc is deposited at 150 °C onto the Silicon substrate, the sample is cooled to room temperature followed by the MnPc deposition in situ. The magnetic moment of this heterostructure is consistent with contributions from the FePc layer only, since the room temperature deposited MnPc has antiferromagnetic characteristics. This heterostructure has magnetic hysteresis with a coercivity of 910 Oe. No measurable shift of the hysteresis loops—as expected for an antiferromagnetic-ferromagnetic coupled interface—is observed in this set of bilayers.

The sensorimotor capacities of the foot are crucial to human locomotion in diverse environments, to gathering information about walking surfaces, and to interacting with objects on the ground. Locomotion is increasingly employed to allow users to control and navigate within immersive virtual environments, but, in contrast to the hand, little attention has been given to the rendering of haptic sensations for the feet. This thesis addresses several challenges motivated by the problem of realizing haptic experiences of walking on virtual ground surfaces. First, a novel family of interfaces is introduced, based on a vibrotactile display integrated in a rigid floor plate. Its structural dynamics and controller have been optimized to ensure its ability to accurately reproduce mechanical vibrations over a wide frequency band, which was instrumental to realizing the perceptual study presented in the second part of the thesis. Distributed arrays of these devices are used to simulate virtual ground surfaces and floor-based multi-touch surfaces, whose usability for human-computer interaction is empirically demonstrated. The second component of this thesis is an experimental study of the contribution of vibrotactile sensory information to the perception of ground surface compliance. A novel haptic perceptual illusion is demonstrated, in which the apparent compliance of a floor surface is increased by vibrations felt via the plantar sole of the foot. This investigation also revealed the surprising ability of the vibrotactile floor interface to overcome, in part, a core limitation: its inability to display kinesthetic force-displacement information. The third part of the thesis analyzes texture-like mechanical signals produced through inelastic physical processes in complex, disordered materials like those encountered during walking in many natural terrains. Patterns of fluctuations accompanying sliding friction and fracture processes in quasi-brittle, heterogeneous materials subjected to time-varying loads are characterized using methods from statistical physics. This analysis was used to formulate novel algorithms for the haptic synthesis of high-frequency signatures of fracture processes in fiber composites and compressed granular media. In conclusion, this thesis presents an innovative hardware interface and techniques for interacting with virtual ground surfaces. It also demonstrates a new haptic perceptual effect that lends justification to the display paradigm adopted here. Finally, it analyzes and models transient, texture-like physical phenomena associated with stepping onto complex, natural ground materials.
Les capacités sensori-motrices du pied sont essentielles à la locomotion humaine, à la collecte d'informations sur les surfaces de marche, et à l'interaction avec des objets au sol. La locomotion est de plus en plus utilisée pour interagir et naviguer dans les environnements virtuels immersifs, mais, contrairement à la main, peu d'attention a été accordée au rendu des sensations haptiques pour les pieds. Cette thèse aborde plusieurs problèmes liés à la réalisation d'expériences haptiques de marche sur des terrains virtuels. Tout d'abord, une nouvelle famille d'interfaces est présentée, fondée sur un dispositif vibrotactile intégré dans un carreau rigide. Sa dynamique structurelle et son contrôleur ont été optimisés pour assurer sa capacité à reproduire fidèlement les vibrations mécaniques dans une large bande de fréquence, ce qui était nécessaire à la réalisation de l'étude de perception présentée en deuxième partie de la thèse. Un pavage de ces dispositifs est utilisé pour simuler des terrains virtuels et des planchers tactiles multi-points, dont l'ergonomie est démontrée de manière empirique. Le deuxième volet de cette thèse est une étude expérimentale sur la contribution de l'information vibrotactile à la perception de la compliance du sol. Une nouvelle illusion perceptuelle haptique est démontrée, dans laquelle la compliance apparente du sol est augmentée par les vibrations ressenties par la plante du pied. Cette étude a également révélé l'étonnante capacité de l'interface vibrotactile à surmonter, en partie, une limitation intrinsèque : son incapacité à transmettre des informations kinesthésiques force-déplacement. La troisième partie de la thèse analyse les signaux mécaniques complexes produits par les processus physiques inélastiques dans les matériaux désordonnés tels que ceux rencontrés lors de la marche en terrain naturel. Les modèles de fluctuations accompagnant le frottement de glissement et les processus de fracture dans les matériaux hétérogènes quasi-fragiles soumis aux charges variables sont caractérisés par des méthodes de physique statistique. Cette analyse est utilisée pour formuler de nouveaux algorithmes pour la synthèse haptique des signatures à hautes fréquences des processus de fracture dans les composites de fibres et les materiaux granulaires compressés. En conclusion, cette thèse présente un dispositif vibrotactile et des techniques novateurs pour interagir avec des terrains virtuels. Elle démontre un nouvel effet perceptuel qui justifie le paradigme d'interaction haptique adopté ici. Enfin, elle analyse et modélise certains phénomènes physiques associés à la marche sur des terrains naturels complexes.

When the thickness of a magnetic layer is comparable to (or smaller than) the electron mean free path, the interface between magnetic and non-magnetic layers becomes very important factor to determine magnetic properties of the ultra-thin films. The quality of interface can enhance (or reduce) the desired properties. Several interesting physical phenomena were studied using these interface effects. The magnetic anisotropy of ultra-thin Co films is studied as function of non-magnetic underlayer thickness and non-magnetic overlayer materials using ex situ Brillouin light scattering (BLS). I observed that perpendicular magnetic anisotropy (PMA) increases with underlayer thickness and saturates after 5 ML. This saturation can be understood as a relaxation of the in-plane lattice parameter of Au(111) on top of Cu(111) to its bulk value. For the overlayer study, Cu, Al, and Au are used. An Au overlayer gives the largest PMA due to the largest in-plane lattice mismatch between Co and Au. An unusual effect was found by adding an additional layer on top of the Au overlayer. An additional Al capping layer on top of the Au overlayer reduces the PMA significantly. The possible explanation is that the misfit strain at the interface between the Al and the Au can be propagated through the Au layer to affect the magnetic properties of Co even though the in-plane lattice mismatch is less than 1%. Another interesting problem in interface interdiffusion and thermal stability in magnetic tunnel junction (MTJ) structures is studied using X-ray photoelectron spectroscopy (XPS). Since XPS is a very chemically sensitive technique, it allows us to monitor interface interdiffusion of the MTJ structures as-deposited and during post-deposition processing. For the plasma-oxidized samples, Fe only participates in the oxidation reduction process. In contrast to plasma-oxidized samples, there were no noticeable chemical shifts as-deposited and during post-deposition processing in air-oxidized samples. However, peak intensity variations were observed due to interface interdiffusion.

Strong interactions between light and atoms at the single-quantum level are an important ingredient for quantum technologies, as well as for studies of fundamental effects in quantum optics. This thesis describes the development of a novel experimental platform that allows for trapping a single rubidium atom in the evanescent mode of a nano-fabricated optical cavity with sub-wavelength dimensions. By virtue of their small size, these cavities provide extremely large atom-photon coupling strengths and good prospects for scalability and integration into complex quantum optical circuits. Positioning the atom near the nano-structure is accomplished using a scanning optical tweezer dipole trap. As a first application, we have demonstrated a coherent optical switch, where a single gate photon controls the propagation of many subsequent signal photons, with the interaction mediated by the atom and cavity. We have also shown that the optical response of the combined atom-cavity system is nonlinear at the level of one or two photons.
Physics

In this thesis we study the preparation dependence of the interface structure of graphene on SiC. We compare epitaxial graphene grown in ultra-high vacuum (UHV), in an atmosphere of argon, and in a background of 10-4 Torr of disilane. Graphene growth is studied on both the polar faces of SiC – the SiC(0001) surface, also known as the Si-face and the SiC(0001 ) surface, also known as the C-face. We find that the quality of graphene and the interface of graphene on the substrate depend on which face of SiC is used, and also what environment it was prepared in. Characterization using atomic force microscopy (AFM), low energy diffraction (LEED) and low energy electron microscopy (LEEM) reveals that on the C-face the interface structure prior to graphitization sensitively depends on the preparation conditions. On the Si-face the interface structure prior to graphitization does not change for any of the three environments, however the quality of the graphene formed shows an improvement when prepared in disilane or argon compared to UHV. When SiC is heated in vacuum the Si atoms preferentially sublimate leaving behind C atoms that rearrange to form graphene. In our prior work we found that compared to the Si-face graphitization of the C-face in vacuum results in thicker graphene films with a larger distribution of thicknesses. A reason for this could be the different nature of the graphene-substrate interface in the two cases. On the Si-face graphene growth is mediated through an interface layer that displays a 6√3×6√3-R30 LEED pattern (this interface layer is also known as the "buffer layer") which acts as a template for graphene growth, while on the C-face no such buffer layer is formed. This buffer layer on the Si-face is known to consist of basically a graphene monolayer, but with some of the carbon atoms bonded to the underlying SiC. Graphene produced on SiC in vacuum conditions is quite inhomogeneous with small domain sizes and (we refer to an area with a constant thickness of multilayer graphene as a "domain") and numerous pits. On the Si-face it has been found by many research groups including our own that graphitization in an atmosphere of argon results in large monolayer domains with an elimination of pits. The argon decreases the Si sublimation rate, thus increasing the temperature required for graphene formation. The higher graphitization temperature results in an improved morphology of the graphene film. Some researchers use a background of disilane instead of an atmosphere of argon, and in this thesis we report our results for graphitization of SiC in a disilane environment. On the Si-face we find an improvement in the morphology of disilane prepared graphene films compared to those prepared in vacuum, consistent with other researchers. In terms of the interface structure prior to graphitization no difference was found for graphene produced in UHV, argon or disilane. For all three environments the LEED pattern from the interface prior to graphitization displayed 6√3 x 6√3-R30 (6√3 for short) symmetry. In an attempt to controllably form thin layers of graphene on the C-face we previously tried graphitizing in an atmosphere of argon, however that led to inhomogeneous islands of thick graphene forming over the surface. It was found that due to an unintentional oxidation of the surface during graphitization, the surface became resistant to graphitization. In this thesis we present results for graphitization of the C-face in a background of disilane, which to our knowledge has not been attempted before. We are able to form graphene films that are thin and uniform relative to those prepared in vacuum or argon. We demonstrate that by graphitizing in a background of disilane we avoid the unintentional oxidation that inhibits graphene formation on the C-face in argon. For C-face samples prepared in disilane, prior to graphitization we observe a √43×√43±7.6° (√43 for short) in situ LEED pattern that has never been observed in vacuum prepared samples. This √43 pattern is found to disappear after air exposure. The ex situ LEEM reflectivity curves of such a disilane prepared sample show unique features not seen in any vacuum prepared sample. By analyzing the LEED pattern and the LEEM reflectivity curves we associate the unusual reflectivity curves we observe on the C-face with a buffer layer, analogous to the 63 layer that form on the Si-face. This buffer layer has the 43 symmetry due to bonding to the underlying SiC, but upon air exposure these bonds are broken (due to oxidation of the SiC) and the layer becomes "decoupled" from the SiC. This decoupling of the buffer layer on the C-face is analogous to what occurs upon oxidation or hydrogenation of the 63 layer on the Si-face. We believe that the √43 layer does not form in vacuum prepared samples due to kinetic limitations but is able to form in Si-rich environments (such as disilane or furnace grown graphene) as the graphitization takes place with the Si sublimation rate closer to equilibrium. The schematic shown below summarizes the differences between Si-face and C-face SiC/graphene interface structures, depending on preparation conditions (vacuum or Si-rich). The right most figure shows the main result of this work, which puts graphene formation on the C-face on a similar footing as for the Si-face since the "buffer" layer provides a template for the graphene growth. A separate project discussed in this thesis is scanning tunneling microscopy/spectroscopy (STM/S) on epitaxial graphene on the Si-face. Two different studies were performed. In the first study we performed STM/S on a Si-face graphene sample in which a large fraction of the area was covered by a secondary disordered phase. The disordered phase showed a graphene-like spectrum with additional features that could arise from dangling bonds or defects. On the basis of additional data from AFM and Auger electron spectroscopy we argue that this secondary phase is similar to the nanocrystalline graphite (NCG) phase that we observe on C-face samples. In the second study we performed STS on a graphene sample that was functionalized by hydrogen. Functionalizing graphene changes the nature of its bonding and can open up a band gap in it. Our STS results indicate that no band gap opened up in the graphene, however we found the presence of additional states in the spectra that indicate the nature of the bonds in graphene had changed due to the hydrogen functionalization. In the last study of this thesis we perform LEEM on graphene samples prepared on Cu foil. The samples were made in a chemical vapor deposition (CVD) chamber under different growth conditions. LEEM was used to measure the reflectivity curves and perform selected area diffraction on the samples. The reflectivity curves allowed us to determine the graphene thickness, and the selected area diffraction allowed us to determine the orientation of the graphene and whether it was single-crystal or not.

The light scattering problem of a sphere on or near a plane surface is solved using an extension of Mie theory. The approach taken is to solve the boundary conditions at the sphere and at the surface simultaneously and develop the scattering amplitude and Mueller scattering matrices. This is performed by projecting the fields in the half space region not including the sphere multiplied by an appropriate Fresnel reflection coefficient onto the half space region including the sphere. An assumption is made that the scattered fields from the sphere, reflecting off the surface and interacting with the sphere, are incident on the surface at near-normal incidence. The exact solution is asymptotically approached when either the sphere is a large distance from the surface or the conductivity of the medium behind the surface approaches infinity. The solution is greatly simplified in the asymptotic limit when the sphere is small compared with the wavelength. Comparisons are made between the experimental Mueller matrices of a contaminated surface and matrices predicted using this simplified system. Comparisons are also made between experimental BRDFs and those predicted using the sphere-surface scattering theory.

Two-dimensional (2D) materials, including graphene and graphene oxide (GO), are a subject of interest for many researchers due to their exceptional properties (strength, conductivity, etc.). These materials, comprised of atomically-thin sheets, may naturally occur stacked together like sheets of paper, but their most interesting properties emerge when separated into individual layers. However, scaling up the processes used to isolate single sheets of some of these materials, particularly graphene, has proven problematic. They can be fiercely resistant to exfoliation, difficult to disperse, and have a worrying propensity to restack. All these problems contribute to the great difficulty these fascinating materials have encountered leaving the lab and entering commercial use. Existing production methods either produce minute quantities, require huge amounts of energy, or involve chemical treatments that transform their properties, typically for the worse. Here, we investigate a method that instead harnesses these difficulties. We force the material to exfoliate itself at the interface between two immiscible solvents, stabilizing the interface and acting as a surfactant with a two-dimensional morphology. In this work we investigate this method and its results in two ways. First, we describe a method we developed using optical microscopy and free software (ImageJ and Gwyddion) that rapidly and inexpensively provides full, simultaneous characterization of thousands of sheets of these materials, yielding both flake area and thickness. We then use this technique to examine the changes induced in 2D material that was exfoliated at the oil–water interface, improving our understanding of the process at the population/production level. Second, we characterize this interaction using force spectroscopy with graphene-functionalized colloidal probes at the surface of pinned droplets of heptane in water. This provides valuable insight into the not-well-understood mechanisms underlying the exfoliation process at the interfacial level. By combining the results seen across these two length scales, our results significantly enhance the understanding of this novel exfoliation process. Additionally, we examine the interactions between another 2D material, mica, and an oil-coated probe in a salt brine using force spectroscopy at high temperature (100 °C) and high pressure (100 atm). These tests are the first demonstration of force spectroscopy in this parameter space and reveal the significant impact of both temperature and pressure on interfacial forces between oil and mineral in this regime. Taken together, our results impact a wide variety of systems including the large-scale production of nanomaterials, nanocomposites, solar cells, sensors, flexible electronics, oil recovery, and catalysis.

Organic solar cells (OSCs) are promising candidates for solar light harvesting due to their standout advantages both in material properties and manufacturing process. During past decades, remarkable progress has been achieved. Efficiency for single-junction cells over 9% and tandem cells over 10% has been reported. For high performance OSCs towards commercialization, sufficient light absorption and high quality buffer layers are still two challenges, which are addressed in this thesis by investigating the plasmonic effects on OSCs and interface engineering. Here, the mechanisms of plasmonic effects on OSC are explored by incorporating metallic Au nanoparticles (NPs) in individual anode buffer layer and active layer, respectively, and finally in both layers simultaneously. When Au NPs are incorporated into the buffer layer, surface plasmonic resonance (SPR) induced absorption enhancement due to incorporation of Au NPs is evidenced theoretically and experimentally to be only minor contributor to the performance improvement. The increased interfacial contact area between the buffer layer and active layer, together with the reduced resistance of the buffer layer due to the embedded Au NPs, are revealed to benefit hole collection and thus are main contributors to the performance improvement. When Au NPs are embedded in the active layer, Au NPs induced SPR indeed contributes to enhanced light absorption. However, when large amount of Au NPs are incorporated, the negative effects of NPs on the electrical properties of OSCs can counter-diminish the optical enhancement from SPR, which limits the overall performance improvement. When Au NPs are embedded into both layers, both advantages of incorporating NPs in individual layers can be utilized together to achieve more pronounced improvement in photovoltaic performance; as a result, accumulated enhancements in device performance can be achieved. The results herein are applicable to other metallic NPs such as Ag NPs, Pt NPs, etc. The study herein has clarified the degree of contribution of SPR effects on OSCs and revealed the mechanisms behind. It has also highlighted the importance of considering both optical and electrical effects when employing metallic NPs as strategies to enhance the photovoltaic performance of OSCs. Consequently, the study contributes both physical understanding and technological development of applying metallic NPs on OSCs. Regarding interface engineering, we first propose a simple method to modify the substrate work function for efficient hole collection by using an ultra-thin ultraviolet-ozone treated Au. The method can be used in other situations such as modifying the work function of multilayer graphene as transparent electrode. Then we propose a general method to synthesize solution-processed transition metal oxides (TMOs). Besides high material quality, desirable electrical properties, and good stability, our method stands out particular in that the synthesized TMOs can be dispersed in water-free solvents and the TMO films require only low temperature treatment, which is very compatible with the organic electronics. Our method can also be used to synthesize other TMOs other than the demonstrated molybdenum oxide and vanadium oxide. The proposed method herein is applicable in semiconductor industry.
published_or_final_version
Electrical and Electronic Engineering
Doctoral
Doctor of Philosophy

In this dissertation, we review the physics associated with surfaces and interfaces in equilibrium and non-equilibrium. Our emphasis will be on interfaces that are driven far away from equilibrium with special interest in the phenomenon of kinetic roughening. Models which describe non-equilibrium interfaces will be introduced and analyzed using techniques such as the Renormalization Group, Monte Carlo simulations, and direct integration of the equation of motion. Different interface relaxation mechanisms will be discussed with a focus on surface diffusion, which is believed to be the dominant effect in Molecular Beam Epitaxy. These interface growth models generate self-affine structures with various correlations satisfying a dynamic scaling law. We compute the scaling exponents and functions. Finally, we study the effect of quenched impurities on the dynamics of a driven interface with a conservation law. The impurity effect leads to anomalous scaling exponents and qualitatively changes the interface dynamics. Our results are summarized in two articles to be published: Refs. (Govind and Guo, 1992; Govind, Guo and Grant, 1992).

Membrane-like structure formed by surfactant molecules of didodecyldimethylammonium bromide (DDAB) on both HOPG and gold electrodes were studied with AFM and SPR techniques. The study shows that the thickness of the adsorbed layer of DDAB is strongly dependent on the concentration of the vesicle solution. We have also investigated the adsorption of redox protein, Cytochrome c, on graphite electrode with in situ tapping mode AFM. The protein adsorbs spontaneously onto the electrode covered with an adsorbed phosphate layer and forms a uniform monolayer. The adsorbed protein exhibits a reversible electron transfer at 0.17 V (Ag/AgCI) once the electrode potential has been increased to 0.75V. Using surface plasmon resonance spectroscopy we have measured subtle conformational change in protein, Cyt c, due to electron transfer of a single electron on MPA-coated gold electrode. The electron transfer induced change in the resonant angle is about 0.006 deg., which corresponds to ~ 0.2 A decreases in the thickness. This is consistent with that reduced state is more compact than the oxidized state.

This licentiate thesis deals with the electronic and geometrical properties of metal-semiconductor and organic-semiconductor interfaces investigated by photoelectron spectroscopy and scanning tunneling microscopy.

First in line is the Co-InAs interface (metal-semiconductor) where it is found that Co is reactive and upon adsorption and thermal treatment it alloys with the indium of the substrate to form metallic islands, about 20 nm in diameter. The resulting broken bonds causes As entities to form which are loosely bond to the surface and evaporate upon thermal treatment. Thus, the adsorption of Co results in a rough interface.

Secondly the metal-free phthalocyanine (H₂PC) - titanium dioxide interface (organic-semiconductor) is investigated. Here it is found that the organic molecules arrange themselves along the substrate rows upon thermal treatment. The interaction with the TiO₂ is mainly with the valence Π-electrons in the molecule causing a relatively strong bond, but this interaction is short range as the second layer of molecules retains their molecular character. This results in an ordered adsorption but limited mobility of the molecules on the surface prohibiting well ordered close packed layers. Furthermore, the hydrogen atoms inside the cyclic molecule leave the central void upon thermal treatment.

The third case is the H₂PC-InAs/InSb interface (organic-semiconductor). Here ordered overlayer growth is found on both substrates where the molecules are preferentially adsorbed on the In rows in the [110] direction forming one-dimensional chains. The InSb-H₂PC interface is found to be weakly interacting and the bulk-like molecular character is retained upon both adsorption and thermal treatment. On the InAs-H₂PC interface, however, the interaction is stronger. The molecules are more affected by the surface bond and this effect stretches up a few monolayers in the film after annealing.

The dynamics of a driven interface, with conservation of total volume under the interface, has been studied using a conserved Kardar-Parisi-Zhang-like equation. Dynamic renormalization group analyses have been performed on the nonlinear, far from equilibrium system in all practically interesting dimensions. The dynamic scaling form, which completely determines the fractal properties of the interface morphology, has been derived and found to be in extremely good agreement with numerical simulations. A new universality class of the growth regime, characterized by a novel superscaling relation, is obtained. For substrate dimension d = 1, the interface morphology is significantly less rough than that observed in the nonconserved system; at the critical dimension $d sb{c}$ = 2, the interface is found to be logarithmically rough. The dynamic roughening transition of the conserved driven interface has also been studied by taking into account a lattice pinning potential. This conserved system exhibits a true phase transition, rather than crossover behavior, as has been observed for nonconserved driven interfaces. The conserved nonlinear driving force is favorable for smoothing the interface, implying that the roughening transition shifts to higher temperatures as that driving force is increased. The nature of the phase transition remains the same as the equilibrium transition; the critical properties are controlled by a Kosterlitz-Thouless fixed point.

Nous présentons une étude des propriétés de frottement d'une interface multicontacts (IMC), formée de multiples microcontacts entre les aspérités de surface de deux solides macroscopiques. Nous nous intéressons tout d'abord à la dynamique de glissement d'une IMC soumise à une charge normale modulée. Nous montrons que l'effet de la modulation est de diminuer la force de frottement moyenne, et de stabiliser le glissement vis-à-vis du stick-slip. Le modèle de frottement ``state and rate-dependent'' (SRF), qui décrit la dynamique de glissement en termes de la compétition entre vieillissement géométrique par fluage des microcontacts, interrompu par le glissement, et rhéologie interfaciale, ne rend pas quantitativement compte de ces effets. Nous l'étendons avec succès en tenant compte de l'élasticité interfaciale, dont le rôle est essentiel dans les couplages entre charge normale et force de frottement. Dans un deuxième temps, nous étudions la réponse interfaciale à une force tangentielle alternative biaisée, afin d'obtenir des informations sur la façon dont se fait le passage du régime statique, piégé, à glissant, dissipatif. Nous mettons en évidence, pour des amplitudes d'excitation voisines du seuil statique, un régime de fluage saturant sous cisaillement. Le modèle SRF ne permet pas à lui seul de décrire cette dynamique de fluage, ce qui suggère l'existence d'un mécanisme de vieillissement structural à l'\oe{}uvre au sein de la jonction adhésive, d'épaisseur nanométrique, formant le contact entre aspérités. Pour tester cette hypothèse, nous réalisons des expériences de frottement à une interface rugueux/lisse qui permettent de sonder finement la rhéologie interfaciale. Nous obtenons ainsi la preuve directe que la jonction adhésive confinée présente un comportement analogue à celui des solides vitreux ``faibles'' cisaillés~: elle s'écoule au delà d'un seuil, vieillit à l'arrêt, et ce vieillissement est interrompu par le cisaillement.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 133-140).
Since the emergence of quantum computing as a field, interactions between two-level systems that can carry quantum information have been conceptualized in terms of quantum devices. Here, I demonstrate four such devices for optical photons traveling in two different modes: an all-optical switch and transistor, a nondestructive photon detector, a phase shifter, and a quantum state generator. These devices rely on a nonlinear optical interface comprised of a few thousand atoms inside of a high-finesse optical cavity. Signal light is sent through a mode transverse to the cavity and is stored or travels through the atoms as a collective excitation. The corresponding atomic population couples to the cavity mode's optical field. The strength of the resulting photon-photon interaction is governed by the optical depth of the atoms for the signal light, and the strong atom-photon coupling in the optical cavity. I first demonstrate an all-optical switch and transistor using blocking interactions that reduce the cavity's transmission by a factor of 11 +/- 1 in the presence of a stored signal photon. This interaction creates anticorrelations between the output light in the signal and cavity paths. I show that these anticorrelations persist in time continuous operation when the photon signal and cavity arrive at the same time. Then, I turn to photon detection. I reconfigure the system and reconceptualize it as a nondestructive, cavity-based detector for signal light. I demonstrate strong correlations between this nondestructive detection and a subsequent destructive detection with non-destructive detection efficiency of 0.5%. Next, I show that a single cavity photon can shift the phase of stored signal light by up to 1.0 +/- 0.4 rad and demonstrate entanglement between output cavity and signal photons. Finally, I present recent experiments where this entanglement is used to modify the phase and amplitude of the signal light by making a projective measurement on the cavity light.
by Kristin M. Beck.
Ph. D.

[Co/Pd]_n multilayers with different number of repetitions n were deposited on Si substrates in order to study the magnetic properties and Hall resistivity in this system. The magnetic moment dependence on applied field was studied in parallel (in-plane) and perpendicular (out-of-plane) configurations. The [Co/Pd]_n multilayers with 2 and 3 repetitions, with thickness of the Co sublayer of 0.8 and 1.0 nm and thickness of the Pd sublayer of 1.3 and 2.0 nm, were found to have perpendicular magnetic anisotropy. The Hall resistivity dependence on applied field was measured at different temperatures. The Hall hysteresis loops confirm the perpendicular anisotropy of the multilayers, and indicate that the origin of the anomalous Hall effect (AHE) in this system, with low number of repetitions, is dominated by surface scattering.

The fields of statistical physics, discrete probability, combinatorics, and theoretical computer science have converged around efforts to understand random structures and algorithms. Recent activity in the interface of these fields has enabled tremendous breakthroughs in each domain and has supplied a new set of techniques for researchers approaching related problems. This thesis makes progress on several problems in this interface whose solutions all build on insights from multiple disciplinary perspectives. First, we consider a dynamic growth process arising in the context of DNA-based self-assembly. The assembly process can be modeled as a simple Markov chain. We prove that the chain is rapidly mixing for large enough bias in regions of Z^d. The proof uses a geometric distance function and a variant of path coupling in order to handle distances that can be exponentially large. We also provide the first results in the case of fluctuating bias, where the bias can vary depending on the location of the tile, which arises in the nanotechnology application. Moreover, we use intuition from statistical physics to construct a choice of the biases for which the Markov chain M_mon requires exponential time to converge. Second, we consider a related problem regarding the convergence rate of biased permutations that arises in the context of self-organizing lists. The Markov chain M_nn in this case is a nearest-neighbor chain that allows adjacent transpositions, and the rate of these exchanges is governed by various input parameters. It was conjectured that the chain is always rapidly mixing when the inversion probabilities are positively biased, i.e., we put nearest neighbor pair x