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Search Results: 1 - 10 of 264574 matches for " Garcia David L. "
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Longitudinal Examination of Learning and Memory in Rats Following Adolescent Exposure to 3,4-Methylenedioxymethamphetamine or 5-Methoxy-N,N-Diisopropyltryptamine  [PDF]
David M. Compton, Kerri L. Dietrich, Peniel Esquivel, Corina Garcia
Journal of Behavioral and Brain Science (JBBS) , 2017, DOI: 10.4236/jbbs.2017.79028
Abstract: A drug of abuse, Foxy or Methoxy Foxy gained popularity among recreational users as an alternative to MDMA (Ecstasy). Considerable research into the consequences of MDMA use is available, yet much remains unknown about the neurobiological consequences of Foxy use. In addition, research into the long-term neuropsychological repercussions associated with these two compounds remains incomplete. The goal of the present research was to explore the effects of MDMA or Foxy on cognitive processes associated with adolescent exposure considered over much of the lifespan. Here we investigated whether the reported effects following adolescent exposure resolved in early adulthood or continued throughout life. The protocol involved repeated doses of either MDMA or Foxy during the period defined as mid-adolescence (postnatal days 34 - 46) in rats, followed by the use of four series of learning and memory tasks repeated at different points in the rodent lifespan. At four time points in adulthood, the animals were trained and tested on a on a series of spatial and non-spatial memory tasks designed to assess the impact and severity of Foxy and MDMA. Oddly, MDMA-treated rats were impaired on a step down passive avoidance task. The performance of the drug-treated rats was markedly inferior to that of the control animals on more demanding water maze tasks, with some results suggesting a lack of flexibility in adapting to changing task demands. MDMA rats were the most impaired. While some persistent cognitive deficits were found, no significant group differences in serotonin or dopamine levels were found in any of the measured regions of the brain changes, cortical or subcortical. These results provide evidence for compromised neurocognition that continues long after drug exposure in the absence of any discernable changes in neurotransmitter levels. Several possible physiological and neurochemical mechanisms associated with these compounds requiring further study are also outlined.
Finite-temperature properties of PZT alloys from first principles
L. Bellaiche,A. Garcia,David Vanderbilt
Physics , 2000, DOI: 10.1103/PhysRevLett.84.5427
Abstract: A first-principles-derived approach is developed to study finite-temperature properties of PbZr{1-x}Ti{x}O3 (PZT) solid solutions near the morphotropic phase boundary (MPB). Structural and piezoelectric predictions are in excellent agreement with experimental data and direct first-principles results. A low-temperature monoclinic phase is confirmed to exist, and is demonstrated to act as a bridge between the well-known tetragonal and rhombohedral phases delimiting the MPB. A successful explanation for the large piezoelectricity found in PZT ceramics is also provided.
Low-temperature properties of PZT solid solutions near the morphotropic phase boundary
L. Bellaiche,A. Garcia,David Vanderbilt
Physics , 2001,
Abstract: A first-principles-derived approach is used to study structural, piezoelectric and dielectric properties of Pb(Zr_{1-x}Ti_{x})O_{3} (PZT) solid solutions near the morphotropic phase boundary at low temperature. Three ferroelectric phases are found to exist: a tetragonal phase for larger x compositions, a rhombohedral phase for smaller x compositions, and the recently discovered monoclinic phase in between. In this monoclinic phase, the polarization associated with the Zr atoms behaves differently from the polarization associated with the Ti atoms. As the composition x decreases, the former rotates more quickly towards the pseudo-cubic [111] direction and grows in magnitude, while the latter lags in its rotation and its magnitude shrinks. The local microscopic structure is found to deviate significantly from the average structure in these PZT alloy phases as a result of fluctuations in the directions and magnitudes of the local polarizations. The monoclinic phase is characterized by a very large piezoelectric and dielectric response.
Electric-field induced polarization paths in PZT alloys
L. Bellaiche,A. Garcia,David Vanderbilt
Physics , 2001, DOI: 10.1103/PhysRevB.64.060103
Abstract: Properties of Pb(Zr_{1-x}Ti_x)O_3 (PZT) for compositions x near the morphotropic phase boundary and under an electric field are simulated using an ab-initio based approach. Applying an electric field of [111] orientation to tetragonal PZT (e.g., x=0.50) leads to the expected sequence of tetragonal, A-type monoclinic, and rhombohedral structures. However, the application of a field of orientation [001] to rhombohedral PZT (e.g., x=0.47) does not simply reverse this sequence. Instead, the system follows a complicated path involving also triclinic and C-type monoclinic structures. These latter phases are found to exhibit huge shear piezoelectric coefficients.
Integrated Off-Line Ballast for High Brightness LEDs with Dimming Capability  [PDF]
Jorge Garcia, Antonio J. Calleja, Emilio L. Corominas, David Gacio, Lidia Campa, Ramón E. Díaz
Circuits and Systems (CS) , 2011, DOI: 10.4236/cs.2011.24047
Abstract: This paper presents an off-line integrated full ballast to supply a 35W assembly of Power LEDs. The proposed solution integrates an input PFC stage (a flyback converter operating in DCM) and a DC-DC output converter (a buck converter) into a single switch power stage, operating with peak current control. As it will be shown, this control scheme maintains the current through the load constant, regardless of the instantaneous value of the DC link voltage. This issue allows the use of a small capacitor for the DC link, which enhances the overall system reliability. The complete ballast has full dimming capability, and all the analysis and design steps are presented, thus ensuring the fulfilling of the existing regulations. The novelty of the final solution comes from the simplicity and robustness of the control scheme in an integrated compact single-switch power stage. A final prototype of the ballast has been built and tested, and experimental results are shown in the last part of the paper. Finally, conclusions and future developments are shown.
Tratamiento físico y químico del suelo para el control Del marchitamiento vascular del clavel causado por el Hongo fusariumoxysporumf. Sp. Dianthi
Garcia David L.,Arbeláez Mario G.,Arbeláez German
Agronomía Colombiana , 1995,
Abstract: Los problemas fitosanitarios constituyen uno de los aspectos más importantes en el cultivo del clavel en Colombia, destacándose, entre ellos, el marchitamiento vascular del clavel, causado por el hongo F. oxysporum f.sp. dianthi. Con el propósito de avanzar en el manejo de dicha enfermedad. Se realizaron dos ensayos en dos fincas productoras de flores en la Sabana de Bogotá, comparando tratamientos físicos y químicos al suelo antes de la siembra. En el primer experimento efectuado con diferentes variedades de clavel estándar, se comprar el fumigante 1.3-0 Cloropicrina frente a la aplicación de vapor de agua. Los dos tratamientos fueren igualmente eficientes en la disminución de la incidencia de la enfermedad en todas las variedades evaluadas; de igual manera no se encontraron diferencias en cuanto a la reducción de la población del hongo en el suelo. Altura de las plantas y la producción y la calidad de las flores. En el segundo experimento, donde se evaluaron diferentes variedades de clavel miniatura, se compara la efectividad del fumigante 1.3-0 Cloropicrina frente al fumigante Dazomet. EI tratamiento al suelo con el fumigante 1,3-0 Cloropicrina fue más eficiente que el fumigante Dazomet en la reducción de la incidencia de la enfermedad y de la población del hongo en el suelo. En el desarrollo de las plantas y en la producción y la calidad de las flores.
Embryonic blood-cerebrospinal fluid barrier formation and function
David Bueno,Isma?l Hermelo,Jordi Garcia-Fernàndez
Frontiers in Neuroscience , 2014, DOI: 10.3389/fnins.2014.00343
Abstract: During embryonic development and adult life, brain cavities and ventricles are filled with cerebrospinal fluid (CSF). CSF has attracted interest as an active signaling medium that regulates brain development, homeostasis and disease. CSF is a complex protein-rich fluid containing growth factors and signaling molecules that regulate multiple cell functions in the central nervous system (CNS). The composition and substance concentrations of CSF are tightly controlled. In recent years, it has been demonstrated that embryonic CSF (eCSF) has a key function as a fluid pathway for delivering diffusible signals to the developing brain, thus contributing to the proliferation, differentiation and survival of neural progenitor cells, and to the expansion and patterning of the brain. From fetal stages through to adult life, CSF is primarily produced by the choroid plexus. The development and functional activities of the choroid plexus and other blood–brain barrier (BBB) systems in adults and fetuses have been extensively analyzed. However, eCSF production and control of its homeostasis in embryos, from the closure of the anterior neuropore when the brain cavities become physiologically sealed, to the formation of the functional fetal choroid plexus, has not been studied in as much depth and remains open to debate. This review brings together the existing literature, some of which is based on experiments conducted by our research group, concerning the formation and function of a temporary embryonic blood–CSF barrier in the context of the crucial roles played by the molecules in eCSF.
Global p-mode oscillations throughout the complete solar cycle 23 and the beginning of cycle 24
David Salabert,Rafael A Garcia,Pere L Palle,Antonio Jimenez
Physics , 2010, DOI: 10.1088/1742-6596/271/1/012030
Abstract: The parameters of the p-mode oscillations vary with solar activity. Such temporal variations provide insights for the study of the structural and dynamical changes occurring in the Sun's interior throughout the solar cycle. We present here a complete picture of the temporal variations of the global p-mode parameters (excitation, damping, frequency, peak asymmetry, and rotational splitting) over the entire solar cycle 23 and the beginning of cycle 24 as observed by the space-based, Sun-as-a-star helioseismic GOLF and VIRGO instruments onboard SoHO.
Effects of site dilution on the magnetic properties of geometrically frustrated antiferromagnets
Angel J. Garcia-Adeva,David L. Huber
Physics , 2001, DOI: 10.1103/PhysRevB.64.172403
Abstract: The effect of site dilution by non magnetic impurities on the susceptibility of geometrically frustrated antiferromagnets (kagome and pyrochlore lattices) is discussed in the framework of the Generalized Constant Coupling model, for both classical and quantum Heisenberg spins. For the classical diluted pyrochlore lattice, excellent agreement is found when compared with Monte Carlo data. Results for the quantum case are also presented and discussed.
Quantum corrections for the MSSM Higgs couplings to SM fermions
David Garcia
Physics , 2001,
Abstract: Higgs Yukawa couplings to down-type fermions receive, in the MSSM, supersymmetric quantum corrections that can be of order 1 for large values of tan\beta, provided \mu \sim MSUSY. Therefore, a sensitive prediction for observables driven by any of these couplings can only be obtained after an all-order resummation of the large corrections. We perform this necessary step and show, as an example, the effect of the resummation on the computation of the p\bar{p},pp \to t\bar{b}H^{-} + X cross-section and on the branching ratio BR(b\to s\gamma) at the next-to-leading order.
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