Atomic oxygen effects on metals

The effect of specimen geometry on the attack of metals by atomic oxygen is addressed. This is done by extending the coupled-currents approach in metal oxidation to spherical and cylindrical geometries. Kinetic laws are derived for the rates of oxidation of samples having these geometries. It is found that the burn-up time for spherical particles of a given diameter can be as much as a factor of 3 shorter than the time required to completely oxidize a planar sample of the same thickness

Experimental results on atomic oxygen corrosion of silver

The results of an experimental study of the reaction kinetics of silver with atomic oxygen in 10 degree increments over the temperature range of 0 to 70 C is reported. The silver specimens, of the order of 10,000 A in thickness, were prepared by thermal evaporation onto 3 inch diameter polished silicon wafers. There were later sliced into pieces having surface areas of the order of 1/4 to 1/2 square inch. Atomic oxygen was generated by a gas discharge in a commercial plasmod asher operating in the megahertz frequency range. The sample temperature within the chamber was controlled by means of a thermoelectric unit. Exposure of the silver specimens to atomic oxygen was incremental, with oxide film thickness measurements being carried out between exposures by means of an automated ellipsometer. For the early growth phase, the data can be described satisfactorily by a logarithmic growth law: the oxide film thickness increases as the logarithm of the exposure time. Furthermore, the oxidation process is thermally activated, the rate increasing with increasing temperature. However, the empirical activation energy parameter deduced from Arrhenius plots is quite low, being of the order of 0.1 eV

Investigation of electrical conductivity in amorphous semiconductors Semiannual report

Electric conductivity and electron transport mechanisms in amorphous semiconductor

Surface analysis of space telescope material specimens

Qualitative and quantitative data on Space Telescope materials which were exposed to low Earth orbital atomic oxygen in a controlled experiment during the 41-G (STS-17) mission were obtained utilizing the experimental techniques of Rutherford backscattering (RBS), particle induced X-ray emission (PIXE), and ellipsometry (ELL). The techniques employed were chosen with a view towards appropriateness for the sample in question, after consultation with NASA scientific personnel who provided the material specimens. A group of eight samples and their controls selected by NASA scientists were measured before and after flight. Information reported herein include specimen surface characterization by ellipsometry techniques, a determination of the thickness of the evaporated metal specimens by RBS, and a determination of trace impurity species present on and within the surface by PIXE

Chemical reaction of atomic oxygen with evaporated films of copper, part 4

Evaporated copper films were exposed to an atomic oxygen flux of 1.4 x 10(exp 17) atoms/sq cm per sec at temperatures in the range 285 to 375 F (140 to 191 C) for time intervals between 2 and 50 minutes. Rutherford backscattering spectroscopy (RBS) was used to determine the thickness of the oxide layers formed and the ratio of the number of copper to oxygen atoms in the layers. Oxide film thicknesses ranged from 50 to 3000 A (0.005 to 0.3 microns, or equivalently, 5 x 10(exp -9) to 3 x 10(exp -7); it was determined that the primary oxide phase was Cu2O. The growth law was found to be parabolic (L(t) varies as t(exp 1/2)), in which the oxide thickness L(t) increases as the square root of the exposure time t. The analysis of the data is consistent with either of the two parabolic growth laws. (The thin-film parabolic growth law is based on the assumption that the process is diffusion controlled, with the space charge within the growing oxide layer being negligible. The thick-film parabolic growth law is also based on a diffusion controlled process, but space-charge neutrality prevails locally within very thick oxides.) In the absence of a voltage measurement across the growing oxide, a distinction between the two mechanisms cannot be made, nor can growth by the diffusion of neutral atomic oxygen be entirely ruled out. The activation energy for the reaction is on the order of 1.1 eV (1.76 x 10(exp -19) joule, or equivalently, 25.3 kcal/mole)

Innovative milieu and social capital - exploring conceptual complementarities (example of the Aachen region, Germany)

Since local systems of relationships (e.g. networks) have been recognized as crucial promoters of regional economic success and restructuring through industrial innovativeness, several concepts try to capture the essence of relevant developments. Much discussed notions such as industrial district, learning region, or innovative milieu each aim at highlighting certain facets of areas that prosper on the base of interacting agents. They represent specific, however complementary concepts, which are linked by the common central idea of the importance of intraregional collaboration. This paper picks up two approaches whose complementarities in explaining regional development and restructuring deserve to get further explored: innovative milieu and social capital. They both put into terms the benefits of trustful, information rich personal relationships in creating supportive externalities for innovative firms. In this regard the notion of social capital - originating from sociological discourse - has recently gained importance for explaining successful economic development as well. The following conceptual questions emerge and will be dealt with in the paper: In which ways can regional development theory profit by linking the milieu and social capital approaches? Are there important complementarities in connecting both concepts, or do we rather find redundancies as they both address the same categories of local interaction and must be considered kind of synonymous? The paper tries to explore these issues by combining theoretical considerations with the application to a case study. It first highlights major elements of both the concept of innovative milieus and the notion of social capital (in the context of regional economic development). The emphasis is put on deriving complementary aspects of both approaches: while the first one sets the spotlight on the qualities of heterogeneous networks of agents that creatively combine widely differing competencies, the second one rather relates to the collaboration externalities of more homogeneous groups of actors. It shall be shown that the connection of both concepts helps to better explain the role of personal and informal relationships of agents as promoters of innovation-based regional restructuring. The theoretical conclusions are then examplified by looking at the features and factors of industrial transformation of the German region of Aachen. Formerly an 'old industrialised' area based on textile and mining industries, Aachen has managed to successfully restructure its economy through the growing emergence of innovative, knowledge-intensive firms. This process, which has gained momentum since the mid 1980s, has its main foundations in formally and informally constructed systems of personal collaboration and mutual support. A closer look at the patterns of interaction reveals that this success can much better be explained when distinctions are drawn between milieu and social capital externalities.

Quantum Chaos in Quantum Wells

We develop a quantitative semiclassical theory for the resosnant tunneling through a quantum well in a tilted magnetic field. It is shown, that in the leading semiclassical approximation the tunneling current depends only on periodic orbits within the quantum well. Further corrections (due to e.g. "ghost" effect) can be expressed in terms of closed, but non-periodic orbits, started at the "injection point". The results of the semiclassical theory are shown to be in good agreement with both the experimental data and numerical calculations.Comment: 25 pages, 15 figures, accepted for publication in Physica

Resonant control of cold-atom transport through two optical lattices with a constant relative speed

We show theoretically that the dynamics of cold atoms in the lowest energy band of a stationary optical lattice can be transformed and controlled by a second, weaker, periodic potential moving at a constant speed along the axis of the stationary lattice. The atom trajectories exhibit complex behavior, which depends sensitively on the amplitude and speed of the propagating lattice. When the speed and amplitude of the moving potential are low, the atoms are dragged through the static lattice and perform drifting orbits with frequencies an order of magnitude higher than that corresponding to the moving potential. Increasing either the speed or amplitude of the moving lattice induces Bloch-like oscillations within the energy band of the static lattice, which exhibit complex resonances at critical values of the system parameters. In some cases, a very small change in these parameters can reverse the atom's direction of motion. In order to understand these dynamics we present an analytical model, which describes the key features of the atom transport and also accurately predicts the positions of the resonant features in the atom's phase space. The abrupt controllable transitions between dynamical regimes, and the associated set of resonances, provide a mechanism for transporting atoms between precise locations in a lattice: as required for using cold atoms to simulate condensed matter or as a stepping stone to quantum information processing. The system also provides a direct quantum simulator of acoustic waves propagating through semiconductor nanostructures in sound analogs of the optical laser (SASER)

Innovative milieu and social capital - exploring conceptual complementarities (example of the Aachen region, Germany)

Since local systems of relationships (e.g. networks) have been recognized as crucial promoters of regional economic success and restructuring through industrial innovativeness, several concepts try to capture the essence of relevant developments. Much discussed notions such as industrial district, learning region, or innovative milieu each aim at highlighting certain facets of areas that prosper on the base of interacting agents. They represent specific, however complementary concepts, which are linked by the common central idea of the importance of intraregional collaboration. This paper picks up two approaches whose complementarities in explaining regional development and restructuring deserve to get further explored: innovative milieu and social capital. They both put into terms the benefits of trustful, information rich personal relationships in creating supportive externalities for innovative firms. In this regard the notion of social capital - originating from sociological discourse - has recently gained importance for explaining successful economic development as well. The following conceptual questions emerge and will be dealt with in the paper: In which ways can regional development theory profit by linking the milieu and social capital approaches? Are there important complementarities in connecting both concepts, or do we rather find redundancies as they both address the same categories of local interaction and must be considered kind of synonymous? The paper tries to explore these issues by combining theoretical considerations with the application to a case study. It first highlights major elements of both the concept of innovative milieus and the notion of social capital (in the context of regional economic development). The emphasis is put on deriving complementary aspects of both approaches: while the first one sets the spotlight on the qualities of heterogeneous networks of agents that creatively combine widely differing competencies, the second one rather relates to the collaboration externalities of more homogeneous groups of actors. It shall be shown that the connection of both concepts helps to better explain the role of personal and informal relationships of agents as promoters of innovation-based regional restructuring. The theoretical conclusions are then examplified by looking at the features and factors of industrial transformation of the German region of Aachen. Formerly an 'old industrialised' area based on textile and mining industries, Aachen has managed to successfully restructure its economy through the growing emergence of innovative, knowledge-intensive firms. This process, which has gained momentum since the mid 1980s, has its main foundations in formally and informally constructed systems of personal collaboration and mutual support. A closer look at the patterns of interaction reveals that this success can much better be explained when distinctions are drawn between milieu and social capital externalities

Atom chips with two-dimensional electron gases: theory of near surface trapping and ultracold-atom microscopy of quantum electronic systems

We show that current in a two-dimensional electron gas (2DEG) can trap ultracold atoms $<1 \mu$m away with orders of magnitude less spatial noise than a metal trapping wire. This enables the creation of hybrid systems, which integrate ultracold atoms with quantum electronic devices to give extreme sensitivity and control: for example, activating a single quantized conductance channel in the 2DEG can split a Bose-Einstein condensate (BEC) for atom interferometry. In turn, the BEC offers unique structural and functional imaging of quantum devices and transport in heterostructures and graphene.Comment: 5 pages, 4 figures, minor change