531 research outputs found
Analytic behavior of the QED polarizability function at finite temperature
We revisit the analytical properties of the static quasi-photon
polarizability function for an electron gas at finite temperature, in
connection with the existence of Friedel oscillations in the potential created
by an impurity. In contrast with the zero temperature case, where the
polarizability is an analytical function, except for the two branch cuts which
are responsible for Friedel oscillations, at finite temperature the
corresponding function is not analytical, in spite of becoming continuous
everywhere on the complex plane. This effect produces, as a result, the
survival of the oscillatory behavior of the potential. We calculate the
potential at large distances, and relate the calculation to the non-analytical
properties of the polarizability.Comment: 8 pages, 2 figure
A symmetry adapted approach to vibrational excitations in atomic clusters
An algebraic method especially suited to describe strongly anharmonic
vibrational spectra in molecules may be an appropriate framework to study
vibrational spectra of Na clusters, where nearly flat potential energy
surfaces and the appearance of close lying isomers have been reported. As an
illustration we describe the model and apply it to the Be, H, Be
and Na clusters.Comment: 8 pages with 2 tables, invited talk at `Atomic Nuclei & Metallic
Clusters: Finite Many-Fermion Systems', Prague, Czech Republic, September
1-5, 199
Comment on ``Boson-realization model for the vibrational spectra of tetrahedral molecules''
An algebraic model in terms of a local harmonic boson realization was
recently proposed to study molecular vibrational spectra [Zhong-Qi Ma et al.,
Phys. Rev. A 53, 2173 (1996)]. Because of the local nature of the bosons the
model has to deal with spurious degrees of freedom. An approach to eliminate
the latter from both the Hamiltonian and the basis was suggested. We show that
this procedure does not remove all spurious components from the Hamiltonian and
leads to a restricted set of interactions. We then propose a scheme in which
the physical Hamiltonian can be systematically constructed up to any order
without the need of imposing conditions on its matrix elements. In addition, we
show that this scheme corresponds to the harmonic limit of a symmetry adapted
algebraic approach based on U(2) algebras.Comment: 9 pages Revtex, submitted February 199
A general algebraic model for molecular vibrational spectroscopy
We introduce the Anharmonic Oscillator Symmetry Model to describe vibrational
excitations in molecular systems exhibiting high degree of symmetry. A
systematic procedure is proposed to establish the relation between the
algebraic and configuration space formulations, leading to new interactions in
the algebraic model. This approach incorporates the full power of group
theoretical techniques and provides reliable spectroscopic predictions. We
illustrate the method for the case of -triatomic molecules.Comment: 35 pages TEX, submitted to Annals of Physics (N.Y.
A symmetry-adapted algebraic approach to molecular spectroscopy
We apply a symmetry-adapted algebraic model to the vibrational excitations in
D_3h and T_d molecules. A systematic procedure is used to establish the
relation between the algebraic and configuration space formulations. In this
way we have identified interaction terms that were absent in previous
formulations of the vibron model. The inclusion of these new interactions leads
to reliable spectroscopic predictions. We illustrate the method for the D_3h
triatomic molecules, H_3^+, Be_3 and Na_3, and the T_d molecules, Be_4 and
CH_4.Comment: 16 pages with 4 tables, invited talk at `Symmetries in Science IX',
August 6-10, 199
Wind Resource Assessment on Puna Island
Puna Island, located in the Pacific Ocean off the southern coast of Ecuador, has a population of approximately 3344 inhabitants. However, not all inhabitants have access to electricity, which is largely supplied by diesel generators. Therefore, to identify a renewable, sustainable, environmentally friendly and low-cost alternative, a 40-m-high anemometer tower was installed for wind resource assessment and to determine the possibility of generating electricity from wind energy. Based on mathematical models for electricity generation from wind energy, data were analyzed using the software Windographer and WAsP, to determine a long-term wind speed of 4.8 m/s and a mean wind power density of 272 W/m(2). By simulating the use of a 3.3-MW wind turbine, we demonstrated that as much as 800 kWh could be generated during the hours when the wind reaches its highest speed. In addition to demonstrating the technical feasibility of meeting the electricity demands of Puna Island through wind power, this study exemplifies a method that can be used for wind resource assessment in any location
Analysis of Li-ion battery degradation using self-organizing maps
This paper proposes a new methodology to identify the different degradation processes of Li-Ion battery cells. The goal of this study is to determine if different degradation factors can be separated by waveform analysis from aged cells with similar remaining capacity. In contrast to other works, the proposed method identifies the past operating conditions in the cell, regardless of the actual State of Health. The methodology is based on a data-driven approach by using a SOM (Self-organizing map), an unsupervised neural network. To verify the hypothesis a SOM has been trained with laboratory data from whole data cycles, to classify cells concerning their degradation path and according to their discharge voltage patterns. Additionally, this new methodology based on the SOM allows discriminating groups of cells with different cycling conditions (based on depth of discharge, ambient temperature and discharge current). This research line is very promising for classification of used cells, not only depending on their current static parameters (capacity, impedance), but also the battery use in their past life. This will allow making predictions of the Remaining Useful Life (RUL) of a battery with greater precision
- …