Scanning Tunneling Microscopy and Tunneling Luminescence of the Surface of GaN Films Grown by Vapor Phase Epitaxy

We report scanning tunneling microscopy (STM) images of surfaces of GaN films and the observation of luminescence from those films induced by highly spatially localized injection of electrons or holes using STM. This combination of scanning tunneling luminescence (STL) with STM for GaN surfaces and the ability to observe both morphology and luminescence in GaN is the first step to investigate possible correlations between surface morphology and optical properties.Comment: 12 pages, Revtex 3.0, submitted to Appl. Phys. Lett., three figures available from Jian Ma at [email protected]

Multiple configurations and fluctuating trophic control in the Barents Sea food-web

The Barents Sea is a subarctic shelf sea which has experienced major changes during the past decades. From ecological time-series, three different food-web configurations, reflecting successive shifts of dominance of pelagic fish, demersal fish, and zooplankton, as well as varying trophic control have been identified in the last decades. This covers a relatively short time-period as available ecological time-series are often relatively short. As we lack information for prior time-periods, we use a chance and necessity model to investigate if there are other possible configurations of the Barents Sea food-web than those observed in the ecological time-series, and if this food-web is characterized by a persistent trophic control. We perform food-web simulations using the Non-Deterministic Network Dynamic model (NDND) for the Barents Sea, identify food-web configurations and compare those to historical reconstructions of food-web dynamics. Biomass configurations fall into four major types and three trophic pathways. Reconstructed data match one of the major biomass configurations but is characterized by a different trophic pathway than most of the simulated configurations. The simulated biomass displays fluctuations between bottom-up and top-down trophic control over time rather than persistent trophic control. Our results show that the configurations we have reconstructed are strongly overlapping with our simulated configurations, though they represent only a subset of the possible configurations of the Barents Sea food-web.publishedVersio

Combined effects of temperature and fishing mortality on the Barents Sea ecosystem stability

Temporal variability in abundance and composition of species in marine ecosystems results from a combination of internal processes, external drivers, and stochasticity. One way to explore the temporal variability in an ecosystem is through temporal stability, measured using the inverse of the coefficient of variation for biomass of single species. The effect of temperature and fisheries on the variability of the Barents Sea food web is still poorly understood. To address this question, we simulate the possible dynamics of Barents Sea food web under different temperature and fishery scenarios using a simple food-web model (Non-Deterministic Network Dynamic [NDND]). The NDND model, which is based on chance and necessity (CaN), defines the state space of the ecosystem using its structural constraints (necessity) and explores it stochastically (chance). The effects of temperature and fisheries on stability are explored both separately and combined. The simulation results suggest that increasing temperature has a negative effect on species biomass and increasing fisheries triggers compensatory dynamics of fish species. There is a major intra-scenario variability in temporal stability, while individual scenarios of temperature and fisheries display a weak negative impact and no effect on stability, respectively. However, combined scenarios indicate that fisheries amplify the effects of temperature on stability, while increasing temperature leads to a shift from synergistic to antagonistic effects between these two drivers

Low Mach Preconditioning For Harmonic Balance Solvers

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Low Mach Preconditioning for Harmonic Balance Solvers

Density-based solvers suffer from a decrease in solution quality and convergence rate as the Mach number decreases. Previous studies have demonstrated that low Mach preconditioning improves the performance of density-based steady-state solvers at low Mach numbers. Applying a similar procedure, this study presents a low Mach preconditioning approach for harmonic balance solvers, which enhances the solution quality and optimizes convergence rates of frequency-domain computations in the low Mach regime. The convergence rate is improved by multiplying the time derivative with a preconditioning matrix, equalizing the acoustic and convective propagation velocities. However, for unsteady computations, the preconditioner introduces a frequency dependence to the acoustic propagation velocity. Thus, frequency-dependent preconditionining parameters are required for harmonic balance computations. Excessive numerical dissipation in the Roe solver causes inaccuracies at low Mach numbers. To address this, a preconditioned Roe solver for unsteady computations is employed, ensuring correct Mach number scaling in the artificial diffusion. The preconditioned harmonic balance solver is applied to a low Mach cylinder and the results are compared with non-preconditioned computations and experimental data. This comparison highlights shortcomings of the non-preconditioned Roe scheme in predicting unsteady flows and demonstrates that preconditioned computations are in excellent agreement with the experiments across all Mach numbers. Furthermore, the use of a frequency-dependent preconditioner is shown to achieve the fastest convergence

Low Mach Preconditioning for Harmonic Balance Solvers

Density-based solvers suffer from a decrease in solution quality and convergence rate as the Mach number decreases. Previous studies have demonstrated that low Mach precon�ditioning improves the performance of density-based steady-state solvers at low Mach numbers. Applying a similar procedure, this study presents a low Mach preconditioning approach for har�monic balance solvers, which enhances the solution quality and optimizes convergence rates of frequency-domain computations in the low Mach regime. The convergence rate is improved by multiplying the time derivative with a preconditioning matrix, equalizing the acoustic and convective propagation velocities. However, for unsteady computations, the preconditioner introduces a frequency dependence to the acoustic propagation velocity. Thus, frequency-dependent preconditionining parameters are required for harmonic balance computations. Excessive numerical dissipation in the Roe solver causes inaccuracies at low Mach numbers. To address this, a preconditioned Roe solver for unsteady computations is employed, ensuring correct Mach number scaling in the artificial diffusion. The preconditioned harmonic balance solver is applied to a low Mach cylinder and the results are compared with non-preconditioned computations and experimental data. This comparison highlights shortcomings of the non-preconditioned Roe scheme in predicting unsteady flows and demonstrates that preconditioned computations are in excellent agreement with the experiments across all Mach numbers. Furthermore, the use of a frequency-dependent preconditioner is shown to achieve the fastest convergence

Low Mach preconditioned non-reflecting boundary conditions for the harmonic balance solver

In CFD, unsteady computations are cost-intensive. The HB method represents a cost-efficient alternative. Here, for time-periodic flows, the governing equations are recasted in the Fourier domain. In the low Mach regime, the compressible governing equations are stiff. Therefore, density-based solvers converge slowly. Low Mach preconditioning equalizes the eigenvalues of the system of equations, to improve the condition number and remove the stiffness of the system. In this paper, low Mach preconditioning is applied to the HB method, with emphasis on the NRBCs. These boundary conditions have a crucial impact on the flow inside the truncated computational domains used in CFD. Improper boundary conditions reflect waves exiting the computational domain and deteriorate the quality of the solution. However,NRBCs avoid spurious reflections. We explain that to precondition the NRBC its formulation in terms of characteristics has to be adapted. An academic wave propagation test case is computed for different wave configurations to validate the preconditioned boundary conditions. The use of non-preconditioned NRBCs in a preconditioned computation leads to instabilities and reflections at the boundaries of the domain. A consistent setup with preconditioned NRBCs improves the stability and leads to good non-reflecting properties for all presented wave configurations

Light emission from a scanning tunneling microscope: Fully retarded calculation

The light emission rate from a scanning tunneling microscope (STM) scanning a noble metal surface is calculated taking retardation effects into account. As in our previous, non-retarded theory [Johansson, Monreal, and Apell, Phys. Rev. B 42, 9210 (1990)], the STM tip is modeled by a sphere, and the dielectric properties of tip and sample are described by experimentally measured dielectric functions. The calculations are based on exact diffraction theory through the vector equivalent of the Kirchoff integral. The present results are qualitatively similar to those of the non-retarded calculations. The light emission spectra have pronounced resonance peaks due to the formation of a tip-induced plasmon mode localized to the cavity between the tip and the sample. At a quantitative level, the effects of retardation are rather small as long as the sample material is Au or Cu, and the tip consists of W or Ir. However, for Ag samples, in which the resistive losses are smaller, the inclusion of retardation effects in the calculation leads to larger changes: the resonance energy decreases by 0.2-0.3 eV, and the resonance broadens. These changes improve the agreement with experiment. For a Ag sample and an Ir tip, the quantum efficiency is $\approx$ 10$^{-4}$ emitted photons in the visible frequency range per tunneling electron. A study of the energy dissipation into the tip and sample shows that in total about 1 % of the electrons undergo inelastic processes while tunneling.Comment: 16 pages, 10 figures (1 ps, 9 tex, automatically included); To appear in Phys. Rev. B (15 October 1998

Recursos humanos y su influencia en la gestión administrativa del Hotel Qalasaya de la ciudad de Puno periodos 2009 – 2010

El trabajo de investigación titulado “RECURSOS HUMANOS Y SU INFLUENCIA EN LA GESTION ADMINISTRATIVA DEL HOTEL QALASAYA DE LA CIUDAD DE PUNO PERIODOS 2009 – 2010”. El objetivo trazado es el de analizar la influencia de recursos humanos en la gestión administrativa en la prestación de servicios. Luego de haber alcanzado los resultados se procedió a su análisis y discusión, arribándose entre las más importantes a las siguientes conclusiones: Se ha determinado que no existe una adecuada capacitación del personal tanto administradores como demás trabajadores por lo que no se lleva una debida administración ni una adecuada atención al cliente En el transcurso de la investigación se pudo detectar que no existe una evaluación y control interno en estos establecimientos de hospedaje, como también la falta de asociaciones con otras servicios afines a este negocio como restaurantes, bares, artesanías, agencias de viajes, etc. Por lo que se considera es de vital importancia para brindar un servicio de calidad y seguridad al cliente en su estadía del lugar donde se hospedan. Dentro de las principales recomendaciones tenemos, capacitar alos propietarios como al personal que labora en los establecimientos de hospedaje para mejorar la atención y servicio al cliente a la vez asociarse con empresas que generan servicios afines para de esa manera contar con servicios adicionales o recomendar a clientes que lo requieran durante su estadía en el lugar donde se hospedan aplicar un control y evaluación constante al personal que labora en la empresa, realizar nuevas estrategias para un mejor funcionamiento de la empresa.Tesi

Low Mach preconditioning for turbomachinery flow simulations with cavities and variable gas compositions

The optimization of turbomachines increasingly relies on highly accurate numerical performance predictions. Loss predictions require the cavities of the machine to be included in numerical simulations. Commonly, in cavities, the velocity of the simulated fluid is small. For density-based solvers, this results in slow convergence and inaccurate computations. Further, the fluid in cavities is often composed of several gases. This paper presents the low Mach preconditioning method for multi-component thermally perfect gas of DLR’s inhouse solver TRACE. Two low Mach academic test cases, a lid driven cavity and an air and exhaust gas mixing layer, are computed to validate the preconditioner. Both test cases show an accelarated convergence and an improved accuracy, when preconditioning is used. A 1.5 stage low-pressure turbine rig with a labyrinth seal is computed with thermally perfect air. The result shows a good agreement with the experimental reference. The fluid is then changed to exhaust gas, and two air inflows are added in the labyrinth seal, to analyze the effect of low Mach preconditioning on the mixing of the two gases. The preconditioned computation shows an improved convergence in the cavity. Moreover, the wall temperature and the gas distribution in the cavity differ, when preconditioning is applied