474 research outputs found
Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride
We demonstrate that the valence energy-loss function of hexagonal boron
nitride (hBN) displays a strong anisotropy in shape, excitation energy and
dispersion for momentum transfer q parallel or perpendicular to the hBN layers.
This is manifested by e.g. an energy shift of 0.7 eV that cannot be captured by
single-particle approaches and is a demonstration of a strong anisotropy in the
two-body electron-hole interaction. Furthermore, for in-plane directions of q
we observe a splitting of the -plasmon in the M direction that is absent in the
K direction and this can be traced back to band-structure effects.Comment: 10 pages, 4 figure
Phonon surface mapping of graphite: disentangling quasi--degenerate phonon dispersions
The two-dimensional mapping of the phonon dispersions around the point of
graphite by inelastic x-ray scattering is provided. The present work resolves
the longstanding issue related to the correct assignment of transverse and
longitudinal phonon branches at . We observe an almost degeneracy of the
three TO, LA and LO derived phonon branches and a strong phonon trigonal
warping. Correlation effects renormalize the Kohn anomaly of the TO mode, which
exhibits a trigonal warping effect opposite to that of the electronic band
structure. We determined the electron--phonon coupling constant to be
166 in excellent agreement to calculations. These results
are fundamental for understanding angle-resolved photoemission,
double--resonance Raman and transport measurements of graphene based systems
Evaluación de riesgos laborales en Almazaras: Limitaciones y nuevas perspectivas
Risk assessment is an essential tool to support risk decisions. However, this process may not always be applied effectively, and this can limit the quality of the preventative action. This is particularly critical in sectors that have a lot of micro and small companies, such as Olive Oil Mills (OOMs). To better understand how to improve the quality of the risk assessment at OOMs, this study aims to analyze the views of Occupational Safety and Health (OSH) practitioners on the key difficulties/limitations in this process and identify some improvements to current practices. This analysis was based on a questionnaire that was developed and given to 13 OSH practitioners working for OOMs. The results showed that the time available to perform the risk assessment is the major limitation. The need for a specific tool for risk assessment in the OOM sector was identified, and the use of accident reports from the entire sector was indicated as an alternative to the absence of data at company level.La evaluación de riesgos es una herramienta esencial para apoyar las decisiones acerca del riesgo. No obstante, este proceso puede no siempre aplicarse con eficacia, y esto puede limitar la calidad de la acción preventiva. Esto es particularmente crÃtico en sectores que incluyen una gran cantidad de micro y pequeñas empresas, como las Almazaras. Para comprender cómo mejorar la calidad de la evaluación de riesgos en las Almazaras, este estudio tiene como objetivo analizar los puntos de vista de los profesionales de la Prevención de Riesgos Laborales (PRL) acerca de las principales dificultades/limitaciones en este proceso e identificar algunas mejoras en las prácticas actuales. Para este análisis fue desarrollado un cuestionario que fue aplicado a 13 profesionales de la PRL con experiencia en las Almazaras. Los resultados mostraron que el tiempo disponible para el desarrollo de la evaluación del riesgos es la principal limitación. Esta investigación identificó la necesidad de una herramienta especÃfica para la evaluación de riesgos en las Almazaras, siendo identificado el uso de informes de accidentes de todo el sector como una alternativa a la falta de datos a nivel de la empresa.(undefined
Instrumentation system for location of partial discharges using acoustic detection with piezoelectric transducers and optical fiber sensors
In this paper, a multichannel instrumentation system for the location of partial discharges (PDs) in power transformers is presented. It is based on the detection of the acoustic emissions from PDs in oil with several piezoelectric (PZT) and fiber optic sensors. An acoustic detection and location approach is proposed based on a time reference given by one fiber optic sensor installed inside the tank and the times of arrival to several PZT sensors installed outside the tank (two in front of each phase windings of a three phase transformer for a typical application). The signal processing includes digital denoising techniques and time-difference lookup table-based 3-D location algorithms; these algorithms have been implemented with virtual instrumentation. The system is tested in an acoustic experimental setup and the location accuracy is evaluated. Finally, the error propagation from the times of arrival and the influence of the number of sensors and their sites in the 3-D location algorithm were analyzed.This work was supported in part by the Spanish National Ministry of Science and Innovation under the Project DPI 2009-14628-C03-01 and in part by the scholarship FPI under Grant BES-2010-04208
Practical Characterization of Cell-Electrode Electrical Models in Bio-Impedance Assays
This paper presents the fitting process followed to adjust the parameters of the electrical model associated to a cell-electrode system in Electrical Cell-substrate Impedance Spectroscopy (ECIS) technique, to the experimental results from cell-culture assays. A new parameter matching procedure is proposed, under the basis of both, mismatching between electrodes and time-evolution observed in the system response, as consequence of electrode fabrication processes and electrochemical performance of electrode-solution interface, respectively. The obtained results agree with experimental performance, and enable the evaluation of the cell number in a culture, by using the electrical measurements observed at the oscillation parameters in the test circuits employed.Ministerio de EconomÃa y Competitividad TEC2013-46242-C3-1-
Ab initio determination of the ionization potentials of DNA and RNA nucleobases
Quantum chemical high level ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute vertical and adiabatic ionization potentials of the five canonical DNA and RNA nucleobases: uracil, thymine, cytosine, adenine, and guanine. Several states of their cations have been also calculated. The present results represent a systematic compendium of these magnitudes, establishing theoretical reference values at a level not reported before, calibrating computational strategies, and guiding the assignment of the features in the experimental photoelectron [email protected] [email protected]
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Vibrational properties of hexagonal boron nitride by Inelastic X-ray scattering and ab initio calculations
The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations show an outstanding agreement and reconcile the controversies raised by recent experimental data obtained by electron-energy loss spectroscopy and second-order Raman scattering
Simultaneous Surface Plasmon Resonance and X-ray Absorption Spectroscopy
We present here an experimental set-up to perform simultaneously measurements
of surface plasmon resonance (SPR) and X-ray absorption spectroscopy (XAS) in a
synchrotron beamline. The system allows measuring in situ and in real time the
effect of X-ray irradiation on the SPR curves to explore the interaction of
X-rays with matter. It is also possible to record XAS spectra while exciting
SPR in order to detect the changes in the electronic configuration of thin
films induced by the excitation of surface plasmons. Combined experiments
recording simultaneously SPR and XAS curves while scanning different parameters
can be carried out. The relative variations in the SPR and XAS spectra that can
be detected with this set-up ranges from 10-3 to 10-5, depending on the
particular experiment
Trajectory definition with high relative accuracy (HRA) by parametric representation of curves in nano-positioning systems
Nanotechnology applications demand high accuracy positioning systems. Therefore, in order to achieve sub-micrometer accuracy, positioning uncertainty contributions must be minimized by implementing precision positioning control strategies. The positioning control system accuracy must be analyzed and optimized, especially when the system is required to follow a predefined trajectory. In this line of research, this work studies the contribution of the trajectory definition errors to the final positioning uncertainty of a large-range 2D nanopositioning stage. The curve trajectory is defined by curve fitting using two methods: traditional CAD/CAM systems and novel algorithms for accurate curve fitting. This novel method has an interest in computer-aided geometric design and approximation theory, and allows high relative accuracy (HRA) in the computation of the representations of parametric curves while minimizing the numerical errors. It is verified that the HRA method offers better positioning accuracy than commonly used CAD/CAM methods when defining a trajectory by curve fitting: When fitting a curve by interpolation with the HRA method, fewer data points are required to achieve the precision requirements. Similarly, when fitting a curve by a least-squares approximation, for the same set of given data points, the HRA method is capable of obtaining an accurate approximation curve with fewer control points
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