Vol. 54 No. 3 (2008): Revista Mexicana de Física.

Published: 2008-01-01

Articles

  • Noise-assisted synchronization of the transition times of a set of uncoupled bistable elements

    E. Cortés, J. Fujioka, R.F. Rodríguez
    176-0
    Abstract:
    We study the noise-induced synchronization of the transition times of a set of non-interacting bistable systems undergoing an activation process in the presence of an external periodic field. Starting from a collection of time series corresponding to these transition times, we first explore graphically the extent of the synchronization for different values of the noise intensity and the frequency of the periodic signal. Then we quantify this phenomenon by means of the fractional fluctuation of the transition times. We show that this quantity has an optimal behavior when we plot it as a function of the noise intensity and the frequency of the periodic external force. Three-dimensional plots of the fractional fluctuation versus these two parameters clearly exhibit the parameter region of optimal synchronization.
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  • Optical spectroscopy and multivariate analysis of biomedical optics

    J.L. Pichardo-Molina, C. Araujo-Andrade, G. Gutiérrez-Juárez
    180-0
    Abstract:
    Although some optical spectroscopy methods were introduced more than ten decades ago, they are still finding new applications in many areas of science. Specifically, medicine and biology are two areas of research where optical methods may facilitate and improve the study and characterization of tissue and biological molecules in order to improve medical diagnosis. Optical spectroscopy can aid in the study and detection of some diseases faster than standard laboratory techniques. This work demonstrates applications of Micro-Raman spectroscopy and multivariate analysis to biomedical problems such as: breast cancer detection, toxoplasmosis study through indirect antibody detection, and discrimination between antibody isotypes (IgG and IgM).
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  • Linearized five dimensional Kaluza-Klein theory as a gauge theory

    G. Atondo-Rubio, J.A. Nieto, L. Ruiz, J. Silvas
    188-0
    Abstract:
    We develop a linearized five-dimensional Kaluza-Klein theory as a gauge theory. By perturbing the metric around flat and de Sitter backgrounds, we first discuss linearized gravity as a gauge theory in any dimension. In the particular case of five dimensions, we show that in using the Kaluza-Klein mechanism, the field equations of our approach imply both linearized gauge gravity and Maxwell theory in flat and de Sitter scenarios. As a possible further development of our formalism, we also discuss an application in the context of gravitational polarization scattering by means of the analogue of the Mueller matrix in optical polarization algebra. We argue that this application can be of particular interest in gravitational wave experiments.
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  • Measurements of local radii of curvature by the retrocollimated interferometric method

    J. D, F.S. Granados-Agustín, A.A. Cornejo-Rodríguez
    194-0
    Abstract:
    With the measurements of the local radii of curvature of an aspherical optical, convex or concave surface, the shape of the surface can be found. In this paper a method is proposed for taking radius measurements, for off-axis sections of an optical surface, using a retrocollimated interference method (rim) that was previously developed by several authors[1-3]; but in particular, in this paper, the one described by Xiang[4] for measuring long radii of curvature[5,6] is used.
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  • A low-temperature and seedless method for producing hydrogen-free Si$_{3}$N$_{4}$

    A.L. Leal-Cruz, M.I. Pech-Canul, J.L. de la Peña
    200-0
    Abstract:
    A simple, seedless method for the synthesis of Si$_{3}$N$_{4}$ from a hydrogen-free precursor system (Na$_{2}$SiF$_{6(s)}$-N$_{2(g)})$ was developed. From thermodynamic calculations and experimental results it is concluded that the gaseous chemical species SiF$_{x}$ (SiF$_{4}$,$_{ }$SiF$_{3}$, SiF$_{2}$, SiF and Si) formed during the low-temperature dissociation of Na$_{2}$SiF$_{6}$ in a conventional CVD system react in-situ with nitrogen to produce Si$_{3}$N$_{4}$. Whiskers, fibers, coatings and powders were obtained via the Na$_{2}$SiF$_{6}$-N$_{2}$ system at pressures slightly above atmospheric pressure. Not only does the feasibility of the reactions for Na$_{2}$SiF$_{6}$ dissociation and Si$_{3}$N$_{4}$ formation increase with the temperature but also, once the SiF$_{x}$ chemical species are formed by the former, the latter reaction is even more viable. Amorphous Si$_{3}$N$_{4}$ is obtained at temperatures of up to 1173 K while crystalline $\alpha $- and $\beta $-Si$_{3}$N$_{4}$ are formed in the range 1273-1573 K and with processing times as short as 120 minutes. Optimal conditions for maximizing Si$_{3}$N$_{4 }$formation were determined.
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  • Nuclear structure for the isotopes $^{3}$He and $^{4}$He in k$^{+}$N scattering

    R. Arceo
    208-0
    Abstract:
    The study of the nuclear structure of $^{3}$He and $^{4}$He was performed by using K$^{+}$N phase shifts at incident energies up to 900 MeV. The analysis and comparison with previous experiments and theoretical results were made, obtaining better results with high precision.
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  • Transceiver 4-leg birdcage for high field MRI: knee imaging

    S.E. Solis, G. Cuellar, R.L. Wang, D. Tomasi, A.O. Rodriguez
    215-0
    Abstract:
    The radiofrequency coil is a crucial component of the magnetic resonance imaging scanners, so that a solid knowledge on the design and physical characteristics is important for those interested in its development. A birdcage coil with a 10 cm radius and 4 legs (length = 12cm), and a separation between the copper strips of 4 cm, was developed for magnetic resonance imaging (MRI) of the human knee and tuned at the resonance frequency of protons at 4 Tesla (170.3 MHz). MR images were acquired with this coil in phantoms and in the knee of a healthy volunteer using a standard spin echo sequence, The phantom images demonstrated the high uniformity of the radiofrequency field with high signal-to-noise ratio, a characteristic of all birdcage RF coils. The in vivo knee images demonstrated that this birdcage geometry is ideal for knee imaging, promising MR images of the knee with higher spatial resolution at 4 Tesla. This work also demonstrates that volume coils are a good choice for high-field MRI applications.
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  • Laser frequency stabilization using fm optical pumping spectroscopy

    E. de Carlos López, J.M. López Romero
    222-0
    Abstract:
    We show a novel method for laser stabilization using FM optical pumping spectroscopy with counter-propagating beams. This method uses a variation of FM saturation spectroscopy, where the saturation and probe beam are replaced by two laser beams with different frequencies. In this scheme, the probe beam is produced by a laser stabilized with FM saturation spectroscopy, whereas the saturation beam (pumping beam) is generated by a second laser, without any modulation. The spectra thus obtained lack the crossover lines and Doppler well that are characteristic of conventional saturation spectroscopy. We show the results in frequency stabilization of a laser diode using this technique and $^{133}$Cs, finding relative stability values of the same order of magnitude as other traditional stabilization techniques.
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  • Optical properties and aggregation of 1-N-methylamino-4'-nitroazobenzene in various environments

    M. del P, M. Gutiérrez-Nava, O.G. Morales-Saavedra, J.M. Re, na-González., E. Rivera
    229-0
    Abstract:
    Aggregation of 1-N-methylamino-4'-nitroazobenzene (RED-H) was studied by UV-vis spectroscopy in solution, using different methanol: water mixtures as solvents, at different concentrations. In 100 % methanol, methanol: water 80:20 and 60:40, RED-H exhibited an maximum absorption wavelength at $\lambda $ = 477 nm and no aggregation was observed. By contrast, in mixtures of methanol: water 40:60 and 20:80 the appearance of an additional blue shifted band around $\lambda $ =430 nm and a red shift of the absorption band to $\lambda $ = 500 nm followed by a long tail confirmed the presence of both H-and J-aggregates in these solvents. Theoretical estimations of aggregate stability carried out at the LMP2/aug-cc-PVTZ(-f)//MP2/6-31G level of theory showed that the H-aggregate is the more stable one (-14.9 kcal/mol) stabilized mostly by electronic correlation while the J-aggregate is much less stable, being stabilized by electrostatic interactions (-3.4 kcal/mol). Theoretical estimation of the absorption spectra of RED-H, H- and J-aggregates carried out using the TD-B3LYP method reproduces the experimentally observed spectra. All electronic transitions show a strong charge-transfer component.
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  • Convección natural de fluidos incompresibles y viscosos en cavidades rectangulares

    B. Bermúdez, A. Nicolás
    236-0
    Abstract:
    Natural convection numerical results for incompressible viscous flows are presented in rectangular cavities with different aspect ratios. This kind of flows may be governed by the time-dependent Boussinesq approximation in the stream function-vorticity formulation. The results are obtained with a simple numerical scheme previously reported for isothermal/thermal (mixed convection) flows. The numerical scheme is based mainly on a fixed point iterative process applied to the non-linear elliptic system that results after a second order time discretization is made. The iterative process leads to the solution of uncoupled, well-conditioned, symmetric linear elliptic problems. The evolution of the thermal flow depends on the parameters given by the Rayleigh number $Ra$, in the range $10^4\leq Ra \leq 10^6$, and the aspect ratio of the cavity $G$, in the range $1/3 \leq G \leq 3$. There are also shown a result related with cat's eyes instability, $G=16$, and other as an example of time-dependent thermal flows, $G=1/16$. To the best of our knowledge, some results with %$A \neq 1$ $G$ different of the unity are being reported for the first time.
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  • Alturas de equilibrio de líquidos en celdas de Hele-Shaw corrugadas bajo penetración capilar espontánea

    M. Francisco, A. Medina, F.A. Sánchez, F.J. Higuera
    247-0
    Abstract:
    A simple and general method to calculate the equilibrium heights of a liquid that penetrates spontaneously, due to capillarity, into a corrugated Hele-Shaw cell is presented. A cosine-like corrugation was imposed on each internal face of the plates that made the cell and periodic channels were generated. It were found complex equilibrium heights which allow to evaluate the volumes of absorbed liquid or expelled gas. Equilibrium heights were also evaluated in corrugated tilted cells.
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  • Optical sensing technique for Young's modulus measurements in piezoelectric materials

    S.J. Pérez Ruíz, J.A. Montero Díaz, S. Alcántara Iniesta, P.R. Hernández, R. Castañeda G.
    253-0
    Abstract:
    In the design of microsensors using microelectromechanical system (MEMS) technology, it is necessary to know the elastic properties of the materials employed in their fabrication. Reliable mechanical properties of the materials are critical to the safety and correct functioning of these microdevices. Mechanical testing of microstructures that are only a few microns thick requires novel techniques and specialized procedures for preparation and handling. In this paper a simplified optic sensing is used to measure the Young's modulus in piezoelectric cantilever. This optical technique was chosen because it is the most appropriate when working with small devices, besides being easily implemented and low cost.
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  • Absolute values of transport mean free path of light in non-absorbing media using transmission and reflectance measurements

    J. Galvan-Mi, oshi., R. Castillo
    257-0
    Abstract:
    We derived a relation between the transport mean free path of light, and transmittance and the reflectance in non-absorbing turbid media. This allowed us to develop an experimental procedure to obtain absolute values for the transport mean free path of light just by measuring in an integrating sphere both the transmittance and the reflectance in this kind of system. We determined how accurate our method was by comparing our transport mean free path measurements with calculations made for colloidal suspensions of particles using Mie scattering theory and with measurements made in colloidal suspensions of polystyrene microspheres using diffusive wave spectroscopy. The agreement is excellent.
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