Microscopic theory of the proximity effect in superconductor-graphene nanostructures

We present a theoretical analysis of the proximity effect at a graphene-superconductor interface. We use a tight-binding model for the electronic states in this system which allows to describe the interface at the microscopic level. Two different interface models are proposed: one in which the superconductor induces a finite pairing in the graphene regions underneath, thus maintaining the honeycomb structure at the interface and one that assumes that the graphene layer is directly coupled to a bulk superconducting electrode. We show that properties like the Andreev reflection probability and its channel decomposition depend critically on the model used to describe the interface. We also study the proximity effect on the local density of states on the graphene. For finite layers we analyze the induced minigap and how it is reduced when the length of the layer increases. Results for the local density of states profiles for finite and semi-infinite layers are presented.Comment: 9 pages, 7 figures, submitted to Phys. Rev.

Effect of a weak ion collisionality on the dynamics of kinetic electrostatic shocks

In strictly collisionless electrostatic shocks, the ion distribution function can develop discontinuities along phase-space separatrices, due to partial reflection of the ion population. In this paper, we depart from the strictly collisionless regime and present a semi-analytical model for weakly collisional kinetic shocks. The model is used to study the effect of small but finite collisionalities on electrostatic shocks, and they are found to smooth out these discontinuities into growing boundary layers. More importantly, ions diffuse into and accumulate in the previously empty regions of phase space, and, by upsetting the charge balance, lead to growing downstream oscillations of the electrostatic potential. We find that the collisional age of the shock is the more relevant measure of the collisional effects than the collisionality, where the former can become significant during the lifetime of the shock, even for weak collisionalities.Comment: Published in J. Plasma Phy

An ultrasonic technique for the measurement of elastic properties of soft surface coatings

The properties of thin layers of materials can be different from those in the bulk form. The response of a coating to any given load and its ability to remain bonded to the substrate will depend on its elastic modulus and Poisson's ratio. In this study a measurement method based on ultrasonic bulk wave reflection was evaluated. As a model system, a thin layer of polyethylene was pressed between two solid steel bodies. The reflection spectra of longitudinal and shear ultrasonic waves were recorded from the coating. The frequencies at which the layer resonates were measured and from this the wave speeds deduced. The Poisson's ratio can be determined from these two wave speeds and if the layer thickness is known the modulus is also available. The tests yielded reasonable values for both. This approach is only suitable if the layer can be made to resonate by the available ultrasonic frequencies; typically this will be the case for thicker coatings (tens of microns). Further, good coupling between the layer material and the steel bodies is necessary so that the interfaces do not themselves act to reflect ultrasound. This is better achieved with a smooth soft coating

Design of a film surface roughness-minimizing molecular beam epitaxy

Molecular beam epitaxy of germanium was used along with kinetic Monte Carlo simulations to study time-varying processing parameters and their effect on surface morphology. Epitaxial Ge films were deposited on highly oriented Ge(001) substrates, with reflection high-energy electron diffraction as a real-time sensor. The Monte Carlo simulations were used to model the growth process, and physical parameters were determined during growth under time-varying flux. A reduced version of the simulations was generated, enabling the application on an optimization algorithm. Temperature profiles were then computed that minimize surface roughness subject to various experimental constraints. The final roughness after two layers of growth was reduced to 0.32, compared to 0.36 at the maximum growth temperature. The study presented here is an initial demonstration of a general approach that could also be used to optimize properties in other materials and deposition processes

Becoming an effective practitioner through guided reflection

The study aimed to develop, monitor and explore the process and outcomes of guided reflection and its impact on enabling practitioners to achieve desirable and effective caring practice. A secondary focus of the study was to monitor and explore the process and outcomes of guided reflection as a form of critical action research which may generate theoretical insights regarding its use in clinical supervisory practices. The process referred to as 'guided reflection' was developed and used to guide this study. Guided reflection represents a form of social action research which was framed within an ontology and process of critical and reflexive phenomenology of experience between practitioners and their supervisors over a period of four years. Whilst each guided reflection relationship was written as a critical narrative to illuminate the reflexive development of effective practice, these narratives became a secondary level of analysis to construct meta-narratives of the nature of effective work and dynamics of guided reflection. Various frameworks were developed and tested within a reflexive process that was appropriately informed and juxtaposed with extant theory to adequately interpret and present the process and ou.tc omes of the study. The method and process of guided reflection generated two major empirical and theoretical insights. • 'The 'Being available' framework to know effective caring practice, presented as one major exemplar of 'Pru'. • Meta-reflection of methods and process of guided reflection. Three frameworks in particular are significant: • 'Being available' as a parallel framework for effective supervision practice. This parallel framework supports the coherence between developmental and research processes. • The Model for Structured Reflection as an heuristic device for knowing reflection. • 'Framing perspectives' as a series of integrated lenses to focus on discrete layers of learning within reflection. The insights gained through the study have considerable significance for informing and guiding the future development of reflective practice within nursing curriculum, clinical supervision within practice, and the future development of nursing knowledge. The development of nursing knowledge is of particular significance in understanding the meaning and nuances of holistic nursing as a lived reality and have significantly contributed to the reflexive development of the Burford NDU Model: Caring in Practice. The study has become a springboard for research to gain further insight into the factors that facilitate or constrain the efficacy of guided reflection in enabling practitioners to know and realise desirable practice within everyday practic

Effects of electric charge on osmotic flow across periodically arranged circular cylinders

An electrostatic model is developed for osmotic flow across a layer consisting of identical circular cylinders with a fixed surface charge, aligned parallel to each other so as to form an ordered hexagonal arrangement. The expression of the osmotic reflection coefficient is derived for spherical solutes with a fixed surface charge suspended in an electrolyte, based on low-Reynolds-number hydrodynamics and a continuum, point-charge description of the electric double layers. The repulsive electrostatic interaction between the surface charges with the same sign on the solute and the cylinders is shown to increase the exclusion region of solute from the cylinder surface, which enhances the osmotic flow. Applying the present model to the study of osmotic flow across the endothelial surface glycocalyx of capillary walls has revealed that this electrostatic model could account well for the reflection coefficients measured for charged macromolecules, such as albumin, in the physiological range of charge density and ion concentration

Finite amplitude gravity waves in the Venus atmosphere generated by surface topography

A two-dimensional, fully nonlinear, nonhydrostatic, gravity wave model is used to study the evolution of gravity waves generated near the surface of Venus. The model extends from near the surface to well above the cloud layers. Waves are forced by applying a vertical wind at the bottom boundary. The boundary vertical wind is determined by the product of the horizontal wind and the gradient of the surface height. When wave amplitudes are small, the near-surface horizontal wind is the zonally averaged basic-state zonal wind, and the length scales of the forcing that results are characteristic of the surface height variation. When the forcing becomes larger and wave amplitudes affect the near-surface horizontal wind field, the forcing spectrum becomes more complicated, and a spectrum of waves is generated that is not a direct reflection of the spectrum of the surface height variation. Model spatial resolution required depends on the amplitude of forcing; for very nonlinear cases considered, vertical resolution was 250 m, and horizontal resolution was slightly greater than 1 km. For smaller forcing amplitudes, spatial resolution was much coarser, being 1 km in the vertical and about 10 km in the horizontal. Background static stability and mean wind are typical of those observed in the Venus atmosphere

Development of Water Management Modeling by using GIS in Chirchik River Basin, Uzbekistan

AbstractThe management of water resources has become a critical need for Chirchik river basin in Uzbekistan due to water deficit and salt damage. Hence, plans must be developed for water efficient use through better management at the river basin level. In this paper, fundamental concept of the proposed water management modeling of the Chirchik river basin and overview of hydrological model using Geomorphology Based Hydrological Model (GBHM). Furthermore, various spatial data such as land-use layers and hydrological layers are developed in this study by conducting latest GIS (Geographical Information System) technology. By extracting ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) Digital Elevation Model (DEM) and ALOS (Advanced Land Observing Satellite) data on autumn and spring, a series of land-use classification is created using the supervised classification method. For regional-scale hydrological modeling, GBHM as a powerful tool is used to analyze the river basin by utilizing the geomorphological properties data for each catchment and hillslope hydrological processes. As a result, development of spatial modeling is obtained and GIS-based analysis is an effective method to study water management in Chirchik river basin in Uzbekistan