Finite-difference numerical methods for solving the energy-momentum transport equations in two-valley semiconductors

Two finite-difference methods for solving the energy-momentum transport equations for electrons in two-valley semiconductors are analyzed. For each method, stability analyses are carried out including the electric field terms and relaxation terms in the equations. Results of large-signal simulations of GaAs IMPATTs using these numerical methods are presented and compared.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25635/1/0000185.pd

Ultrathin cobalt films on ruthenium (0001): growth, structure, and magnetism

Tesis doctoral inédita. Universidad Autónoma de Madrid. Facultad de Ciencias, Departamento de Física Teórica de la Materia Condensada. Fecha de lectura: 24-11-2006Texto en inglés. Conclusiones en españo

The Search for a New Theory of Sustainable Architectural Design: Breathing Architecture

To maintain the reduction in fuel consumption and the need to modify the techniques concerning climate change new ideas for example “Parisian worker gardens”, New York’s “community gardens”, and “vegetable squares” of Muscovite, come into the picture. The city aims to produce more clean energy than it consumes within fifteen years for a positive energetic assessment. The city plans to shift more towards renewable energies for example photovoltaic cells, biomass, green architecture such as walls and roofs, etc. The new architecture should also follow these green technologies and methods. Methodology—The methodology of this paper is based on a qualitative research format for findings and results, depending on: Data collection, Data Analysis, Theoretical analysis and analytical cases studies integrating the interdisciplinary fields of green technologies and sustainability. Thus producing a new language of architecture based on the dynamic relation among materials, structures and finally a new spatial idea. Empirical application and study. Besides rational comparisons and conclusions

The Search for a New Theory of Sustainable Architectural Design: Breathing Architecture

To maintain the reduction in fuel consumption and the need to modify the techniques concerning climate change new ideas for example “Parisian worker gardens”, New York’s “community gardens”, and “vegetable squares” of Muscovite, come into the picture. The city aims to produce more clean energy than it consumes within fifteen years for a positive energetic assessment. The city plans to shift more towards renewable energies for example photovoltaic cells, biomass, green architecture such as walls and roofs, etc. The new architecture should also follow these green technologies and methods. Methodology—The methodology of this paper is based on a qualitative research format for findings and results, depending on: Data collection, Data Analysis, Theoretical analysis and analytical cases studies integrating the interdisciplinary fields of green technologies and sustainability. Thus producing a new language of architecture based on the dynamic relation among materials, structures and finally a new spatial idea. Empirical application and study. Besides rational comparisons and conclusions

In-plane orientation effects on the electronic structure, stability and Raman scattering of monolayer graphene on Ir(111)

We employ angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic structures of two rotational variants of epitaxial, single-layer graphene on Ir(111). As grown, the more-abundant R0 variant is nearly charge-neutral, with strong hybridization between graphene and Ir bands near the Fermi level. The graphene Fermi surface and its replicas exactly coincide with Van Hove singularities in the Ir Fermi surface. Sublattice symmetry breaking introduces a small gap-inducing potential at the Dirac crossing, which is revealed by n-doping the graphene using K atoms. The energy gaps between main and replica bands (originating from the moir\'e interference pattern between graphene and Ir lattices) is shown to be non-uniform along the mini- zone boundary due to hybridization with Ir bands. An electronically mediated interaction is proposed to account for the stability of the R0 variant. The variant rotated 30{\deg} in-plane, R30, is p-doped as grown and K doping reveals no band gap at the Dirac crossing. No replica bands are found in ARPES measurements. Raman spectra from the R30 variant exhibit the characteristic phonon modes of graphene, while R0 spectra are featureless. These results show that the film/substrate interaction changes from chemisorption (R0) to physisorption (R30) with in-plane orientation. Finally, graphene-covered Ir has a work function lower than the clean substrate but higher than graphite.Comment: Manuscript plus 7 figure

Determining the structure of Ru(0001) from low-energy electron diffraction of a single terrace

While a perfect hcp (0001) surface has three-fold symmetry, the diffraction patterns commonly obtained are six-fold symmetric. This apparent change in symmetry occurs because on a stepped surface, the atomic layers on adjacent terraces are rotated by 180 degrees. Here we use a Low-Energy Electron Microscope to acquire the three-fold diffraction pattern from a single hcp Ru terrace and measure the intensity-vs-energy curves for several diffracted beams. By means of multiple scattering calculations fitted to the experimental data with a Pendry R-factor of 0.077, we find that the surface is contracted by 3.5(+-0.9) at 456 K.Comment: 10 pages, 4 figures. Corrected some typos, added more details. Accepted for publication in Surface Science (Letters

Three-Fold Diffraction Symmetry in Epitaxial Graphene and the SiC Substrate

The crystallographic symmetries and spatial distribution of stacking domains in graphene films on SiC have been studied by low energy electron diffraction (LEED) and dark field imaging in a low energy electron microscope (LEEM). We find that the graphene diffraction spots from 2 and 3 atomic layers of graphene have 3-fold symmetry consistent with AB (Bernal) stacking of the layers. On the contrary, graphene diffraction spots from the buffer layer and monolayer graphene have apparent 6-fold symmetry, although the 3-fold nature of the satellite spots indicates a more complex periodicity in the graphene sheets.Comment: An addendum has been added for the arXiv version only, including one figure with five panels. Published paper can be found at http://link.aps.org/doi/10.1103/PhysRevB.80.24140

Labyrinthine Island Growth during Pd/Ru(0001) Heteroepitaxy

Using low energy electron microscopy we observe that Pd deposited on Ru only attaches to small sections of the atomic step edges surrounding Pd islands. This causes a novel epitaxial growth mode in which islands advance in a snakelike motion, giving rise to labyrinthine patterns. Based on density functional theory together with scanning tunneling microscopy and low energy electron microscopy we propose that this growth mode is caused by a surface alloy forming around growing islands. This alloy gradually reduces step attachment rates, resulting in an instability that favors adatom attachment at fast advancing step sections

Highly Enhanced Concentration and Stability of Reactive Ce^3+ on Doped CeO_2 Surface Revealed In Operando

Trivalent cerium ions in CeO_2 are the key active species in a wide range of catalytic and electro-catalytic reactions. We employed ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy to quantify simultaneously the concentration of the reactive Ce^3+ species on the surface and in the bulk of Sm-doped CeO_2(100) in hundreds of millitorr of H2–H2O gas mixtures. Under relatively oxidizing conditions, when the bulk cerium is almost entirely in the 4+ oxidation state, the surface concentration of the reduced Ce^3+ species can be over 180 times the bulk concentration. Furthermore, in stark contrast to the bulk, the surface’s 3+ oxidation state is also highly stable, with concentration almost independent of temperature and oxygen partial pressure. Our thermodynamic measurements reveal that the difference between the bulk and surface partial molar entropies plays a key role in this stabilization. The high concentration and stability of reactive surface Ce^3+ over wide ranges of temperature and oxygen partial pressure may be responsible for the high activity of doped ceria in many pollution-control and energy-conversion reactions, under conditions at which Ce^3+ is not abundant in the bulk