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Search Results: 1 - 10 of 717662 matches for " C. R. A. Catlow "
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Controlling bulk conductivity in topological insulators: Key role of anti-site defects
D. O. Scanlon,P. D. C. King,R. P. Singh,A. de la Torre,S. McKeown Walker,G. Balakrishnan,F. Baumberger,C. R. A. Catlow
Physics , 2012, DOI: 10.1002/adma.201200187
Abstract: The binary Bi-chalchogenides, Bi2Ch3, are widely regarded as model examples of a recently discovered new form of quantum matter, the three-dimensional topological insulator (TI) [1-4]. These compounds host a single spin-helical surface state which is guaranteed to be metallic due to time reversal symmetry, and should be ideal materials with which to realize spintronic and quantum computing applications of TIs [5]. However, the vast majority of such compounds synthesized to date are not insulators at all, but rather have detrimental metallic bulk conductivity [2, 3]. This is generally accepted to result from unintentional doping by defects, although the nature of the defects responsible across different compounds, as well as strategies to minimize their detrimental role, are surprisingly poorly understood. Here, we present a comprehensive survey of the defect landscape of Bi-chalchogenide TIs from first-principles calculations. We find that fundamental differences in the energetics of native defect formation in Te- and Se-containing TIs enables precise control of the conductivity across the ternary Bi-Te-Se alloy system. From a systematic angle-resolved photoemission (ARPES) investigation of such ternary alloys, combined with bulk transport measurements, we demonstrate that this method can be utilized to achieve true topological insulators, with only a single Dirac cone surface state intersecting the chemical potential. Our microscopic calculations reveal the key role of anti-site defects for achieving this, and predict optimal growth conditions to realize maximally-resistive ternary TIs.
Determination of the nitrogen vacancy as a shallow compensating center in GaN doped with divalent metals
J. Buckeridge,C. R. A. Catlow,D. O. Scanlon,T. W. Keal,P. Sherwood,M. Miskufova,A. Walsh,S. M. Woodley,A. A. Sokol
Physics , 2014, DOI: 10.1103/PhysRevLett.114.016405
Abstract: We report accurate energetics of defects introduced in GaN on doping with divalent metals, focussing on the technologically important case of Mg doping, using a model which takes into consideration both the effect of hole localisation and dipolar polarisation of the host material, and includes a well-defined reference level. Defect formation and ionisation energies show that divalent dopants are counterbalanced in GaN by nitrogen vacancies and not by holes, which explains both the difficulty in achieving p-type conductivity in GaN and the associated major spectroscopic features, including the ubiquitous 3.46 eV photoluminescence line, a characteristic of all lightly divalent metal-doped GaN materials that has also been shown to occur in pure GaN samples. Our results give a comprehensive explanation for the observed behaviour of GaN doped with low concentrations of divalent metals in good agreement with relevant experiment.
Embedded-Cluster Calculations in a Numeric Atomic Orbital Density-Functional Theory Framework
Daniel Berger,Andrew J. Logsdail,Harald Oberhofer,Matthew R. Farrow,C. Richard A. Catlow,Paul Sherwood,Alexey A. Sokol,Volker Blum,Karsten Reuter
Physics , 2014, DOI: 10.1063/1.4885816
Abstract: We integrate the all-electron electronic structure code FHI-aims into the general ChemShell package for solid-state embedding (QM/MM) calculations. A major undertaking in this integration is the implementation of pseudopotential functionality into FHI-aims to describe cations at the QM/MM boundary through effective core potentials and therewith prevent spurious overpolarization of the electronic density. Based on numeric atomic orbital basis sets, FHI-aims offers particularly efficient access to exact exchange and second order perturbation theory, rendering the established QM/MM setup an ideal tool for hybrid and double-hybrid level DFT calculations of solid systems. We illustrate this capability by calculating the reduction potential of Fe in the Fe-substituted ZSM-5 zeolitic framework and the reaction energy profile for (photo-)catalytic water oxidation at TiO2(110).
Microscopic Origins of Electron and Hole Stability in ZnO
C. Richard A. Catlow,Alexey A. Sokol,Aron Walsh
Physics , 2011, DOI: 10.1039/C1CC10314H
Abstract: A novel and direct method is proposed to assess the doping limits of semiconducting materials. Applied to the case of ZnO, our first-principles calculations demonstrate that p-type ZnO is thermodynamically unstable.
Study of the Oceanic Heat Budget Components over the Arabian Sea during the Formation and Evolution of Super Cyclone, Gonu  [PDF]
P. R. Jayakrishnan, C. A. Babu
Atmospheric and Climate Sciences (ACS) , 2013, DOI: 10.4236/acs.2013.33030
Abstract:

Oceans play a vital role in the global climate system. They absorb the incoming solar energy and redistribute the energy through horizontal and vertical transports. In this context it is important to investigate the variation of heat budget components during the formation of a low-pressure system. In 2007, the monsoon onset was on 28th May. A well-marked low-pressure area was formed in the eastern Arabian Sea after the onset and it further developed into a cyclone. We have analysed the heat budget components during different stages of the cyclone. The data used for the computation of heat budget components is Objectively Analyzed air-sea flux data obtained from WHOI (Woods Hole Oceanographic Institution) project. Its horizontal resolution is 1° × 1°. Over the low-pressure area, the latent heat flux was 180 Wm2. It increased to a maximum value of 210 Wm2 on 1st June 2007, on which the system was intensified into a cyclone (Gonu) with latent heat flux values ranging from 200 to 250 Wm2. It sharply decreased after the passage of cyclone. The high value of latent heat flux is attributed to the latent heat release due to the cyclone by the formation of clouds. Long wave radiation flux is decreased sharply from 100 Wm2 to 30 Wm2 when the low-pressure system intensified into a cyclone. The decrease in long wave radiation flux is due to the presence of clouds. Net heat flux also decreases sharply to 200 Wm2 on 1st June 2007. After the passage, the flux value increased to normal value (150 Wm2) within one day. A sharp increase in the sensible heat flux value (20 Wm2) is observed on 1st June 2007 and it decreased thereafter. Short wave radiation flux decreased from 300 Wm2 to 90 Wm2

Assessment of Convective Activity Using Stability Indices as Inferred from Radiosonde and MODIS Data  [PDF]
P. R. Jayakrishnan, C. A. Babu
Atmospheric and Climate Sciences (ACS) , 2014, DOI: 10.4236/acs.2014.41014
Abstract:

The combined use of both radiosonde data and three-dimensional satellite derived data over ocean and land is useful for a better understanding of atmospheric thermodynamics. Here, an attempt is made to study the thermodynamic structure of convective atmosphere during pre-monsoon season over southwest peninsular India utilizing satellite derived data and radiosonde data. The stability indices were computed for the selected stations over southwest peninsular India viz: Thiruvananthapuram and Cochin, using the radiosonde data for five premonsoon seasons. The stability indices studied for the region are Showalter Index (SI), K Index (KI), Lifted Index (LI), Total Totals Index (TTI), Humidity Index (HI), Deep Convective Index (DCI) and thermodynamic parameters such as Convective Available Potential Energy (CAPE) and Convective Inhibition Energy (CINE). The traditional Showalter Index has been modified to incorporate the thermodynamics over tropical region. MODIS data over South Peninsular India are also used for the study. When there is a convective system over south peninsular India, the value of LI over the region is less than ?4. On the other hand, the region where LI is more than 2 is comparatively stable without any convection. Similarly, when KI values are in the range 35 to 40, there is a possibility for convection. The threshold value for TTI is found to be between 50 and 55. Further, we found that prior to convection, dry bulb temperature at 1000, 850, 700 and 500 hPa is minimum and the dew point temperature is a maximum, which leads to increase in relative humidity. The total column water vapor is maximum in the convective region and minimum in the stable region. The threshold values for the different stability indices are found to agree with that reported in literature.

Thermodynamics of the Reduction Roasting of Nickeliferous Laterite Ores  [PDF]
R. Elliott, C. A. Pickles, J. Forster
Journal of Minerals and Materials Characterization and Engineering (JMMCE) , 2016, DOI: 10.4236/jmmce.2016.46028
Abstract: The global nickel sulphide resources are becoming more difficult to mine and, as a result, there is increasing interest in the current and future development of the oxidic nickel laterite deposits. In comparison to the sulphide ores, the nickel laterites cannot be readily upgraded by conventional means and growing attention is being focused on the development of new methods for processing these ores. In this paper, firstly, brief overviews of laterite ore mineralogy and the conventional techniques used to extract the nickel from both the limonitic and the saprolitic nickeliferous laterites are provided. Secondly, previous research on the thermodynamic modelling of the reduction of the laterites is discussed. Thirdly, an improved thermodynamic model is used to predict the equilibrium products arising from the solid state reduction of both the limonitic and the saprolitic ores. Based on these thermodynamic predictions, the reduction behaviors of the two ore types are compared in terms of nickel recovery and grade in the ferronickel product. The effects of reduction temperature, ore composition and carbon additions were studied. Finally, the results from the simulations are compared to the experimental data available in the literature.
Imperfect Trapping in a Random Walk with Both Species Mobile  [PDF]
Natalia C. Bustos, Miguel A. Ré
Advances in Pure Mathematics (APM) , 2018, DOI: 10.4236/apm.2018.81007
Abstract: It is presented here a continuous time random walk model for diffusion mediated reactions with both species mobile. The random walk is carried out over an infinite homogeneouos lattice. They are calculated the probability density for the time of reaction of a pair, the reaction rate and the time evolution of the concentration of the majority species. Analytical results are obtained in the Fourier-Laplace transform representation. Known results for a fixed trap are reobtained with appropriate marginal probabilities. It is thus justified Smoluchowski’s original approximation considering the trap at a fixed position and the majority species diffusing with a coefficient sum of the individual coefficients. The results obtained are illustrated by a one dimensional model with bias.
Analysis, Modeling and Simulation of a Poly-Bag Manufacturing System  [PDF]
R. A. R. C. Gopura, T. S. S. Jayawardane
Engineering (ENG) , 2012, DOI: 10.4236/eng.2012.45034
Abstract: The cost of raw material of poly-bags increases and fluctuates with an unpredictable trend. Further, legal restrictions imposed on some types of polythene products adversely affects for the demand. In this context, entrepreneurs engaging in poly-bag manufacturing face major challenges. With the purview of optimizing the poly-bag manufacturing process, authors attempted to analyze, model and simulate the poly-bag manufacturing process in the light of posed challenges. This paper presents preliminary analysis, modeling and simulation strategies of a poly-bag manufacturing system. In addition, a risk prioritization method is proposed in the preliminary analysis and also a simulation tool is developed.
A Hand Prosthesis with an Under-Actuated and Self-Adaptive Finger Mechanism  [PDF]
R. A. R. C. Gopura, D. S. V. Bandara
Engineering (ENG) , 2018, DOI: 10.4236/eng.2018.107031
Abstract: One of the major problems faced by hand amputees is the unavailability of a lightweight and powered multi-functional hand prosthesis. Under-actuated finger designs play a key role to make the hand prosthesis lightweight. In this paper, a hand prosthesis with an under-actuated and self-adaptive finger mechanism is proposed. The proposed finger is capable to generate passively different flexion/extension angles for a proximal interphalangeal (PIP) joint and a distal interphalangeal (DIP) joint for each flexion angle of metacarpophalangeal (MCP) joint. In addition, DIP joint is capable to generate different angles for the same angle of PIP joint. Hand prosthesis is built on the proposed finger mechanism. The hand prosthesis enables user to grasp objects with various geometries by performing five grasping patterns. Thumb of the hand prosthesis includes opposition/apposition in addition to flexion/extension of MCP and interphalangeal (IP) joint. Kinematic analysis of the proposed finger has been carried out to verify the movable range of the joints. Simulations and experiments are carried out to verify the effectiveness of the proposed finger mechanism and the hand prosthesis.
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