Natural and synthetic SnO₂ samples were studied using scanning tunneling microscopy (STM). The SnO₂ surface flattens considerably following high temperature treatments up to 1500 K. The conductivity of the synthetic SnO₂ surface is significantly reduced following annealing at temperatures of approximately 1200-1500 K, making tunneling impossible. A decrease in conductivity was not observed for the natural SnO₂ sample following similar high temperature treatments, most likely due to impurities which act as dopants. No atomic scale images were collected on the SnO₂ surface which provided information regarding atomic positions and point defects on the surface. Water adsorption was studied on the stoichiometric Cr₂O₃ (101̲2) surface, using thermal desorption spectroscopy (TDS). Water was the only desorption product observed during TDS. Adsorption is primarily dissociative following exposure to water at 163 K. Approximately, 0.12 monolayers of water dissociate on the clean, nearly stoichiometric Cr₂O₃ (101̲2) surface. The first order kinetics observed for the recombination of dissociated water are not well understood. One possible explanation is that the rate limiting step for desorption involves the breaking of a Cr-O bond resulting in a freely diffusing OH species. The exchange of halogen and oxygen was studied on Cr₂O₃ (101̲2) using Auger electron spectroscopy (AES) and TDS. The exchange of chlorine and oxygen is completely reversible. Chlorine is removed from the Cr₂O₃ (101̲2) surface following exposure to oxygen. Exposure of CFCl₂CH₂Cl reduces the surface oxygen concentration to that of the clean, nearly stoichiometric Cr₂O₃ (101̲2) surface. The exchange of chlorine with oxygen appears to involve only chemisorbed surface oxygen, not bulk lattice oxygen. / Master of Science

In the present research a highway twin tunnel project completed in Japan in 1998 is used as a case study to verify results of numerical analyses with measurement data. Each of the tunnels had approximately 1.1 km of length. For this project a wide geometry of approximately 18.0 m was selected by the designers to facilitate three lanes per tunnel. A sequential tunneling technique known in Japan as the "TBM pilot and enlargement method" was used along with NATM principles. The tunnel was used as a reference project, involving performance testing and extensive monitoring in order to verify and standardize support requirements for other tunnels excavated under similar geologic conditions in the Tomei II expressway. The tunnel was excavated in a region consisting mainly of soft sedimentary rocks, such as locally weathered sandstone, underlain by interbedded sandstone and mudstone. Due to observed non symmetric deformations and loads in the tunnel, the distinct element and the convergence-confinement methods were used during the numerical simulations. A parametric analysis was performed initially in a pseudo-continuum approach to study the behavior of the wide tunnel geometry under various conditions. The effects of rock mass elastic modulus, in situ Ko ratio and boundary conditions are discussed. More complex parametric studies were performed in a stochastically generated model by using joint spatial data from geotechnical investigations. The Barton-Bandis constitutive law was assumed for the behavior of the joints. The sensitivity of the ground "characteristic curves" was examined by statistical variation of the joint shear strength parameters. A final simulation using the code UDEC and the convergence-confinement method yields interesting results which are comparable to the monitored data. / Master of Science

Demand for new underground transportation systems and utility networks has increased the use of tunneling in soft ground. Many of these tunnels have to be constructed in difficult soil conditions, with strict constraints on ground movement control. Technological advances, such as the pressurized shield or the New Austrian Tunneling Method (NATM), have, to some extent, overcome these difficulties. But the complex interaction between tunneling procedure, ground response, and liner support is still not fully understood. In this dissertation, the three aspects of tunneling, face stability, liner design, and ground surface settlement are analyzed for conditions that might be experienced on current projects. The study is intended to clarify some of the phenomena associated with the use of advanced tunneling techniques in soft grounds, and help improve the current design practice. The NATM generally uses "hand-mining" equipment for excavation, and shotcrete as temporary support of the tunnel wall. The amount and timing of support is optimized by continuously adapting the construction procedure to the conditions found at the tunnel face. In the present study, the applications of the finite element method to tunneling are reviewed, and it is used to model NATM tunneling projects. Using parametric studies, a simplified design method is proposed which allows an estimate of the liner forces and settlements associated with NATM tunneling to be obtained. Pressurized shields are used in soils with little to zero stand-up time to support the tunnel face during excavation. In this work, the face stability of shield tunnels in cohesionless soils is examined using limit analysis principles. Upper bound estimates of the critical face pressure are found in good agreement with results from centrifuge model tests. Empirical correlations for settlement estimates are re-examined, in view of case history data for shield driven tunnels. The ground movements observed on the F3 and F4 contracts of the Washington Metro are analyzed. Earth pressure balance shields were used on these projects. It is shown that difliculties were common in mixed face conditions, unless adequate techniques were used to prevent ground collapse to occur. / Ph. D.

One of the challenges facing condensed matter physics nowadays is to understand the electronic structure of high temperature superconductors. This dissertation compiles our contribution to the experimental information concerning this subject. Tunneling conductance spectroscopy a technique capable of probing the electronic density of states in hybrid structures was used to study the current and spin transport properties across junctions between metallic counterelectrodes and Bi2Sr2CaCu2O8- (BSCCO) crystals. Since in these structures the transport is mediated by transmission channels depending on superconductive characteristics, the energy resolved density of states is a signature of the mechanism of superconductivity. For instance, one can observe the superconductive energy gap and the behavior of subgap bound states due to phase sensitive Andreev reflections at the junction interface. In particular, tunneling spectroscopy makes possible the observation of the LOFF state characterized by the coexistence of superconductivity and magnetism. Cuprates like BSCCO are highly anisotropic materials and their superconductivity is almost two dimensional, being confined in the CuO2 planes. Therefore, our junctions combine monocrystals of underdoped samples of BSCCO with various thin film counterelectrodes normal metal (Ag), conventional superconductor (Pb) and ferromagnetic metal (Fe) deposited perpendicular onto the cuprate ab-plane (CuO2 plane). We performed measurements on Ag/BSCCO junctions for two current injection directions into the same crystal. We observed that, near the 110 crystal surface, the conductance spectra show a high zero bias peak (ZBCP) which is a manifestation of zero energy Andreev bound states due to an anisotropic superconductive order parameter. Near the 100 surface, the ZBCP is largely suppressed. This is consistent with a predominantly 2 2 x y d - -wave pairing symmetry. In some cases, the ZBCP splits or decreases in amplitude at low temperatures. This is consistent with the existence of a subdominant s-wave (or xy d ) resulting in a mixed d is + state which breaks time reversal symmetry (BTRS). Since we observe this phenomenon in the underdoped case, we do not confirm the possibility of a quantum critical point close to the optimal doping. Our Pb/BSCCO spectra contradict the theory explaining the BTRS by proximity effect. The Fe/BSCCO junctions measure the effect of spin polarization. We explain the recorded 4-peak asymmetric structure by the combined effect of a spin independent BTRS state and a spin filtering exchange energy in the barrier responsible for a large ZBCP splitting. The LOFF state was observed in the proximity region induced on the ferromagnetic side of multilayered-Fe/Ag/BSCCO structures. As expected for the LOFF order parameter, the spectra develops coherent damped oscillations with the Fe layer thickness probing different regions. The magnitude and sign of the oscillation depends on the energy. The conductances at energy zero or equal to the superconductive gap are modulated in antiphase proving that the order parameters takes successively positive and negative values. Changing the junction orientation with 4 p , results in an opposite behavior for the same distance. The maximal amplitudes in one direction is replaced by minima, showing that, besides space, the LOFF state modulation depends on the phase of the high temperature order parameter inducing the proximity

The present dissertation seeks to deepen our understanding of hydrogen transfers and especially C-H bond activations in enzymes. Hydrogen transfers are ubiquitous in chemistry and biology and a thorough understanding of how they occur and what factors influence them will facilitate developments in biomimetic catalysis, rational drug design, and other fields. A particular difficulty with H-transfers is the importance of nuclear quantum effects to the reaction, particularly tunneling. The overall scope of the work here aims to examine how experimental kinetic isotope effects (KIEs) can be interpreted with a particular type of tunneling model, referred to as Marcus-like models, to yield a semi-quantitative picture of the physical mechanisms of H-transfers. Previous work had used this kind of model to qualitatively interpret experimental data using a combination of intuition and generalized theories. The work here examines these theories in quantitative detail, testing and calibrating our intuition in the context of several experimental systems. The first chapter of research (ch. II) focusses on the temperature dependence of primary KIEs and how these experiments can be quantitatively interpreted as a probe for certain kinds of enzyme or solvent dynamics. The subsequent chapters (ch. III-VI) focus on the use of secondary KIEs to determine the detailed structures of tunneling ready states (TRSs) and how the dynamics of H-tunneling affect those structures. These chapters focus primarily on the TRS of the enzyme alcohol dehydrogenase, but by examining an uncatalyzed analogue to that reaction (ch. VI), the work gains some insight about similarities and differences between catalyzed and uncatalyzed reactions. In summary, the work uncovers some principles of catalysis, not just the mechanism of a catalyzed reaction. The mechanism of C-H activation presented here provides an elegant solution to problems that have been vexing to accommodate within traditional models. This work constitutes some initial steps in making Marcus-like models quantitatively useful as a supplement or even replacement for traditional models of reactivity.

The electronic structures of Mn £_-doped epitaxial GaN films grown on sapphire substrates are studied by scanning tunneling microscopy in this work. Local structural information and the corresponding electronic properties of Mn £_-doped GaN films are probed by the combination of scanning tunneling microscopy and atomic-scale scanning tunneling spectroscopy measurements. According to the electronic local density of states analysis indicates that Mn ions develop an acceptor level in GaN, revealing a gap state located at ~ 1.4 eV above the valence band edge of GaN. Furthermore, the energy position of the charge transfer levels of substitutional MnGa within GaN energy gap is also elucidated and discussed in the work.

A completed part of the Bolu Tunnel at Elmalik side collapsed during the 1999 D&uuml / zce earthquake. In order to by-pass the collapsed section, a new tunnel route was determined. 474 meters of the new route, including two portals and double tubing, crossed through the weak to very weak rock units with intersecting fault gouge, excavated from Elmalik side. In this study, the characteristics of the rock masses and support classes are determined for new route of the Elmalik Side. Then, during the tunnel excavation, the deformations of temporary and permanent support systems were precisely measured and recorded. The support system properties as determined from NATM were analyzed by two dimensional convergence confinement method using the numerical RocSupport software. As a result of this study, for weak ground tunneling, duration of primary support installation should be kept at minimum. Besides that, temporary support measures such as forepoling, face sealing and temporary invert have an important role in controlling deformations before the primary support installation. With the application of temporary supports, loading on the permanent support, and hence the final deformation of the excavation, was found to be reduced significantly. Application of rigid lining was found to be necessary in order to prevent long-term deformations in weak ground tunnels, even though it is contradictory to the NATM philosophy.

Superconductivity is a remarkable quantum phenomenon in which a macroscopic number of electrons form a condensate of Cooper pairs that can be described by a single quantum wave function. According to the celebrated Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, there is a minimum length scale (the coherence length) below which the condensate has a rigid quantum phase. The fate of superconductivity in a system with spatial dimensions smaller than [the coherence length] has been the subject of intense interest for decades and recent studies of superconductivity in ultra-thin epitaxial metal films have revealed some surprising behaviors in light of BCS theory. Notably, it was found that superconductivity remains robust in thin lead films with thicknesses orders of magnitude smaller than the coherence length (i.e. in the extreme two dimensional limit). Such studies raise the critical question: what happens to superconductivity as all dimensions are reduced toward the zero dimensional limit? By controlling the lateral size of ultra thin 2D islands, we systematically address this fundamental question with a detailed scanning tunneling microscopy/spectroscopy study. We show that as the lateral dimension is reduced, the strength of the superconducting order parameter is also reduced, at first slowly for dimensions larger than the bulk coherence length, and then dramatically at a critical length scale of ~ 40nm. We find this length scale corresponds to the lateral decay length of the order parameter in an island containing regions of different heights and different superconducting strength. Overall, our results suggest that fluctuation corrections to the BCS theory are important in our samples and may need to be systematically addressed by theory. / text

This research work is aimed at understanding the electronic properties of Bi(110) nanostructures. This study chiefly uses Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS) and Non Contact Atomic Force Microscope (NCAFM) to investigate the geometric and electronic structure of Bi(110) islands on highly oriented pyrolytic graphite (HOPG) substrate. STM measurements are the primary focus of the thesis which involves imaging the bismuth islands and study of its atomic structure. STM images of the Bi(110) islands reveal a ‘wedding cake’ profile of the bismuth islands that show paired layers on top of a base. I(V) (Current vs voltage) data was acquired via STS techniques and its first derivative was compared to DFT calculations. The comparison implied the presence of a dead wetting layer which was present only underneath the bismuth islands. We observed bilayer damped oscillations in the surface energy that were responsible for the stability of paired layers in Bi(110) islands. Interesting Moiré pattern arising out of misorientation between the substrate and the overlayer are also observed in STM images on some bismuth islands. Bright features pertaining to enhanced LDOS (local density of states) were observed on the perimeter of the bismuth islands and stripes in the STM images and STS dI/dV maps which appear at energies around the Fermi level. The bright features which we termed as ‘bright beaches (BB)’ are also observed on grain boundaries and defects that suggest that they are related to termination of the chain of bismuth atoms. The Bi(110) islands and stripes were observed to form preferred widths with a well defined periodicity. This peculiar phenomenon was attributed to a lateral quantum size effect (QSE) that results from a Fermi wave vector with appropriate shifts in Fermi energy. The widths of the islands prefer to adjust themselves at the nodes of this in-plane Fermi wavelength. NaCl deposited on a HOPG substrate forms cross shaped islands which were used as spacers to limit the interaction between the bismuth films and the underlying HOPG substrate. The NaCl islands are transparent to the flow of tunneling current and allow STS measurements. The LDOS of Bi/HOPG was very similar to the LDOS of Bi deposited on NaCl/HOPG which suggests that the wetting layer underneath the bismuth islands plays an important role in decoupling the film from the underlying substrate.

Organic electronics are becoming an increasingly important part of the semiconductor industry, with myriad applications enabled by their low cost, solution processability, and electrical conductivity. Charge transport in electronic applications involving organic semiconductor materials depends strongly on the electronic properties of nanoscale interfaces. Local variations in molecular environments can have a significant impact on the interfacial electronic properties, and subsequently the organic semiconductor electronic structure. Here, we use scanning tunneling microscopy and spectroscopy, supported by theoretical calculations, to investigate the impact of the local adsorption environment on the local density of states of oligothiophenes, carbon nanohoops, and carbon nanotubes. First, we present work showing that, for alkyl-substituted quaterthiophenes, molecular packing and electronic structure at interfaces differ substantially from the bulk, and a significant degree of structural and electronic variation occurs even in this relatively simple system. Then, we report on investigations of longer alkyl-substituted oligothiophenes, were we found a variety of planar molecular conformations that surprising exhibited similar, particle-in-a-box-like progressions of unoccupied molecular orbitals. Next, we share our research that found, for the first time, metal surface electrons confined within single adsorbed molecules. Finally, we study the impact of electrostatic defects in both metal and dielectric substrates on single-walled carbon nanotubes. The research presented in this dissertation increases our understanding of organic semiconductor interfaces and the impact of said interfaces on local molecular electronic structure, thereby aiding future organic semiconductor technological development. / 10000-01-01