Scanning tunneling microscopy has been a powerful tool in expanding our understanding in the study of condensed matter physics. Many of the exotic materials of interest exhibit rich phases of matter at different temperatures and pressures. In order to probe the rich array of phases we developed a novel technique of combining scanning tunneling microscopy with tunable temperature and tunable mechanical strain in ultra high vacuum conditions. The mechanisms that give rise to high temperature superconductivity has been a long standing problem in physics. The discovered of iron-based high temperature superconductors (pnictides) have spurred much research into the mechanisms that give rise to the different exotic states observed in these new materials in hopes to better understand the underlying nature of unconventional superconductivity. Here we present a detailed study of the Nematic ordered phase in the prototypical iron- based high temperature superconductor, NaFeAs. Using our novel strain, temperature, scanning tunneling microscopy technique, we can attain an atomic-resolution view of the effects of the nematic phase on the local density of states along with the effects of anisotropic strain on the electronic structure. We further systematically study NaFeAs along both axes of the phase diagram, tuning temperature and Cu doping. We probe the material from the parent compound to beyond the supercon- ducting dome with increased Cu doping and from superconducting temperatures towell above the structural transition temperatures. Using our novel strain, temperature, scanning tunneling microscopy technique we nanoscopically identified the region of long-range nematic order and the region of nematic fluctuations in the phase diagram and find that true long range nematic order sets in at the tetragonal to orthorhombic structural transition temperature but nematic fluctuations continue at higher temperatures and also into the overdoped regime, then seemingly disappearing at the edge of the superconducting dome. We further find that our applied stain increasing the amplitude of the nematic fluctuations showing strong nonlinear coupling between strain and electronic nematicity. The power of our novel strain, temperature, scanning tunneling microscopy tech- nique in probing quasiparticle interference proves ideal for studying the topological, Weyl semimetal 1T’-MoTe 2 . In it’s orthorombic phase the material has topologically nontrivial protected surface Fermi arcs. By measuring quasiparticle interference in this material at different temperatures we can probe both topologically nontrivial phase (orthorhombic phase) and the topologically trivial phase (monoclinic phase). In the topologically nontrivial phase we see quasiparticle interference measurements in good agreement with angular resolved photoemission spectroscopy and theoretical calculations. In the topologically trivial phase we see the lack of the quasiparticle interference coming from the trivial surface state.

No description available.

Thesis. 1976. M.S.--Massachusetts Institute of Technology. Dept. of Civil Engineering. / Microfiche copy available in Archives and Engineering. / Bibliography: leaves 270-272. / M.S.

Calculamos a estrutura eletrônica do sistema formado por dois poços quânticos assimétricos (Al0.3Ga0.7As/GaAs) com dopagem delta (Si) no centro de cada poço. Resolvemos a equação de Schrödinger usando a teoria do funcional densidade na aproximação de densidade local. Obtemos o potencial efetivo, os níveis de energia, a ocupação das sub-bandas e a densidade eletrônica total em função da temperatura, espessura da barreira de potencial que separa os dois poços, concentração e difusão dos átomos doadores. Verificamos que a estrutura eletrônica e pouco influenciada pela temperatura e difusão dos doadores. Por outro lado, nossos resultados mostram que ela depende de forma significativa da concentração dos doadores. Investigamos também como o sistema e influenciado por uma tensão elétrica aplicada na direção de crescimento. Calculamos os níveis de energia e ocupação das sub-bandas em função da tensão elétrica aplicada. Determinamos então a tensão elétrica necessária para tomar ressonantes os dois primeiros níveis de energia para barreiras de potencial de espessuras 25&#197 e 50&#197. Uma possível aplicação para esse sistema e analisada. Verificamos que ele apresenta efeitos de mobilidade modulada e transcondutância negativa quando as mobilidades nos dois poços são diferentes. Consideramos ainda o caso em que um campo magnético e aplicado perpendicularmente a direção de crescimento. Nossos resultados mostram que a forma da relação de dispersão En( kx), inicialmente parabólica, e bastante modificada pelo campo magnético, surgindo estados ressonantes para kx diferente de zero. Conseqüentemente, a massa efetiva eletrônica nessa direção também e modificada, dependendo agora da energia do elétron. Para campo magnético nulo verificamos que a massa efetiva e diferente em cada sub-banda. / In this work we calculate the electronic structure of a double asymmetric quantum well of Al0.3Ga0.7As/GaAs with a delta doping at the center of the wells. The local density approximation was used in order to obtain the effective potential energy levels, population and the total electronic density as a function of the temperature, barrier thickness, impurity concentration and impurity spread. We have observed that the electronic structure is almost not sensitive to the temperature and the spread of the donors but it depends strongly of the donors concentration. The effect of a gate voltage (Vc) is also studied. The energy levels and the occupation of these levels are calculated as a function of Vc. The resonances of the two first levels are studied for a system with barrier thickness 25&#197 and 50&#197. A possible practical application for this system is analyzed. We observe that mobility modulation and negative transconductance can be achieved when the mobilities of the two wells are different. We have considered the effect of a uniform magnetic field applied perpendicular to the growth direction. The dispersion relation is strongly modified and the effective mass for each subband has been calculated.

Enzyme dynamics occur on a wide range of length and timescales. This work is focused on understanding enzyme dynamic at the fs-ps timescale as this is the dynamic range at which bonds are typically made and broken during chemical reactions. Our work focuses on enzymes that catalyze hydride transfer between two carbon atoms - a fundamental reaction in biology. Primary kinetic isotope effects and their temperature dependence have implied that fast dynamics of the enzyme are important in facilitating hydride transfer, however these experiments do not measure any such motions directly. We make use of two-dimensional infrared spectroscopy (2D IR), a technique that interrogates the vibrations of molecules to extract dynamic information from the surrounding environment with 100 fs resolution. A model system, formate dehydrogenase (FDH), is an excellent probe of dynamics at the fs-ps timescale. Azide bound to the ternary complex of FDH offers the ability to measure dynamics of an analog structure of the reactive complex using 2D IR, while also studying the reaction directly with and KIE’s and their temperature dependence. By altering various parts of the structure of FDH via mutagenesis and other techniques, we investigate the role of structure and dynamics to determine how fast dynamics of the active site influence the the kinetics of hydride transfer. These experiments are the first means of providing a dynamic interpretation of KIEs and their temperature dependence.

This thesis presents an effort to understand the C-H-C transfer in enzymatic reactions from the comparison of different variants of enzymes that have unrelated protein sequences and structures, but catalyze the same chemical transformation. I evaluated the kinetic isotope effects (KIEs) and their temperature dependences and interpreted the findings in accordance with Marcus-like models. The enzyme system studied is dihydrofolate reductase (DHFR), which catalyzes the reduction of 7,8-dihydrofolate (H2F) to 5,6,7,8-tetrahydrofolate (H4F) using reduced β-nicotinamide adenine dinucleotide 2' phosphate (NADPH) as a reducing agent. H-transfer reactions in typical enzymes from three genetically unrelated families, E. coli chromosomal DHFR (cDHFR, FolA), plasmid coded R67 DHFR (FolB), and pteridine reductase 1 (PTR1, FolM) were comparatively investigated. Chapter I provides a brief introduction to the thesis. Chapter II presents optimized procedures for a one-pot, enzymatic microscale synthesis of several NADPH isotopologues used in KIE experiments. Chapter III focuses on the application of novel competitive primary H/D KIE determinations. Chapter IV compares the H-transfer reactions between primitive R67 DHFR and the chromosomal DHFR, and Chapter V describes the investigation of H-transfer reactions at high and low ionic strengths with theoretical and experimental approaches in order to understand the unusual enhancement in H-transfer rate of R67 DHFR with increasing ionic strength. Chapter VI discusses an improved PTR1 purification procedure and comparisons of steady state kinetic parameters using PTR1 and cDHFR with H2F and dihydrobiopterin (H2B) substrates. Thus, the investigation of the H-transfer reaction catalyzed by cDHFR with an unnatural substrate, H2B is described. Finally, a summary is provided and future directions are discussed in Chapter VII.

On étudie les propriétés de spin des états évanescents d'un semi-conducteur dépourvu de centre d'inversion. La topologie particulière des bandes évanescentes qui résulte de l'interaction spin-orbite est à l'origine d'un l'effet tunnel anormal. La nature même du processus tunnel devient très dépendante de l'orientation cristallographique de la barrière. Deux cas typiques sont analysés : tunneling sous incidence oblique sur une barrière orientée selon la direction [001] et tunneling sous incidence normale au travers d'une barrière orientée dans la direction [110]. Dans le premier cas, un processus tunnel quasi-classique peut être restauré de façon assez subtile et des effets de filtres à spin sont mis en évidence. Dans le second cas, la situation est particulièrement originale. La notion de courant de probabilité, qui joue un rôle central, est réexaminée et les conditions de discontinuité aux interface de la dérivée de la fonction d'onde sont établies. Selon cette direction où la levée de dégénérescence de spin de la bande de conduction est maximum, il n'y a curieusement aucun filtrage de spin mais l'onde transmise subit un déphasage dont le signe dépend de l'orientation du spin. On prédit des effets de précession de spin autour d'un champ effectif complexe régnant dans la barrière. Ces résultats permettent de concevoir, par ingénierie spin-orbite d'hétérostructures, des dispositifs tunnel résonnant capables de manipuler le spin.

Graduation date: 1996

This dissertation is focused on understanding heterogeneous metal catalysts supported on oxides using a model catalyst system of SiO2 thin film supported metal nano-clusters. The primary technique applied to this study is scanning tunneling microscopy (STM). The most important constituent of this model catalyst system is the SiO2 thin film, as it must be thin and homogeneous enough to apply electron or ion based surface science techniques as well as STM. Ultra-thin SiO2 films were successfully synthesized on a Mo(112) single crystal. The electronic and geometric structure of the SiO2 thin film was investigated by STM combined with LEED, Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The relationship between defects on the SiO2 thin film and the nucleation and growth of metal nano-clusters was also investigated. By monitoring morphology changes during thermal annealing, it was found that the metal-support interaction is strongly dependent on the type of metal as well as on the defect density of the SiO2 thin film. Especially, it was found that oxygen vacancies and Si impurities play an important role in the formation of Pd-silicide. By substituting Ti atoms into the SiO2 thin film network, an atomically mixed TiO2-SiO2 thin film was synthesized. Furthermore, these Ti atoms play a role as heterogeneous defects, resulting in the creation of nucleation sites for Au nano-clusters. A marked increase in Au cluster density due to Ti defects was observed in STM. A TiO2-SiO2 thin film consisting of atomic Ti as well as TiOx islands was also synthesized by using higher amounts of Ti (17 %). More importantly, this oxide surface was found to have sinter resistant properties for Au nano-clusters, which are desirable in order to make highly active Au nano-clusters more stable under reaction conditions.

The recent discovery that a spin-polarized current can exert a large torque on a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of electrical current controlled manipulation of magnetic moment in nanoscale magnetic device structures. This so-called spin torque effect holds great promise for two applications, namely, spin torque oscillators (STOs) for wireless communication and radar communication, and spin transfer torque RAM (STT-RAM) for data/information storage.   The STO is a nanosized spintronic device capable of microwave generation at frequencies in the 1-65 GHz range with high quality factors. Although the STO is very promising for future telecommunication, two major shortcomings have to be addressed before it can truly find practical use as a radio-frequency device. Firstly, its very limited output power has to be significantly improved. One possibility is the synchronization of two or more STOs to both increase the microwave power and further increase the signal quality. Synchronization of serially connected STOs has been suggested in this thesis. In this configuration, synchronization relies on phase locking between the STOs and their self-generated alternating current. While this locking mechanism is intrinsically quite weak, we find that the locking range of two serially connected spin-valve STOs can be enhanced by over two orders of magnitude by adjusting the circuit I-V phase to that of an intrinsic preferred phase shift between the STO and an alternating current. More recently, we have also studied the phase-locking of STOs based on magnetic tunnel junctions (MTJ-STO) to meet the power specifications of actual application where the rf output levels should be above 0 dBm (1 mW). In addition to the spin torque terms present in GMR spin valves, MTJs also exhibit a significant perpendicular spin torque component with a quite complex dependence on both material choices and applied junction bias. We find that the perpendicular torque component modifies the intrinsic preferred I-V phase shift in single MTJ-STOs in such a way that serially connected STOs synchronize much more readily without the need for additional circuitry to change the I-V phase.   Secondly, equal attention has been focused on removing the applied magnetic field for STO operation, which requires bulky components and will limit the miniaturization of STO-based devices. Various attempts have been made to realize STOs operating in zero magnetic field. By using a tilted (oblique angle) polarizer (fixed layer) instead of an in-plane polarizer (standard STO), we show zero field operation over a very wide polarizer angle range without sacrificing output signal. In addition, the polarizer angle introduces an entirely new degree of freedom to any spin torque device and opens up for a wide range of additional phenomena.   The STT-RAM has advantages over other types of memories including conventional MRAM in terms of power consumption, speed, and scalability. We use a set of simulation tools to carry out a systematic study on the subject of micromagnetic switching processes of a device for STT-RAM application. We find that the non-zero k spin wave modes play an important role in the experimentally measured switching phase boundary. These may result in telegraph transitions among different spin-wave states, and be related to the back-hopping phenomena where the switching probability will decrease with increasing bias in tunnel junctions. / QC 20100819