Near-Perfect Correlation of the Resistance Components of Mesoscopic Samples at the Quantum Hall Regime

We study the four-terminal resistance fluctuations of mesoscopic samples near the transition between the $\nu=2$ and the $\nu=1$ quantum Hall states. We observe near-perfect correlations between the fluctuations of the longitudinal and Hall components of the resistance. These correlated fluctuations appear in a magnetic-field range for which the two-terminal resistance of the samples is quantized. We discuss these findings in light of edge-state transport models of the quantum Hall effect. We also show that our results lead to an ambiguity in the determination of the width of quantum Hall transitions.Comment: As publishe

Conacyt y el posgrado: políticas de evaluación y calidad

1 archivo PDF (12 páginas). magyefDesde la década de los setenta, ante el imperativo de organizar un subsistema de posgrado integrado y de calidad, el Conacyt, en su Plan Nacional Indicativo de Ciencia y Tecnología, manifestó su decisión de iniciar la estructuración de un sistema de posgrado nacional, cuyas principales características serían las de contribuir a la formación de recursos de alto nivel y así promover el desarrollo de la cultura científica y tecnológica en el país. El objetivo del trabajo es presentar un análisis de algunas tendencias del comportamiento del posgrado en las últimas décadas, y el papel desempeñado por el Consejo Nacional de Ciencia y Tecnología (Conacyt) y las políticas de evaluación en el cambio de la organización del posgrado. ABSTRACT: Since the seventies, in order to organize an integrated subsystem of quality postgraduate, the Conacyt, in its National Plan for Science and Technology, took the decision to start the organization of a national postgraduate system, whose main characteristics would be to contribute to the formation of high-level resources and thereby promote the development of scientific and technological culture in the country. The aim of this paper is to present an analysis of trends graduate behavior in recent decades, and the role played by the National Council of Science and Technology (Conacyt) and assessment policies in changing the organization of postgraduate. PALABRAS CLAVE: Sistema de educación, educación superior, sistemas de evaluación, competencia, Consejo Nacional de Ciencia y Tecnología, evaluación. KEYWORDS: System education, education higher, evaluation system, competition, Consejo Nacional de Ciencia y Tecnología, evaluation

Two-Dimensional Electron Gas in InGaAs/InAlAs Quantum Wells

We designed and performed low temperature DC transport characterization studies on two-dimensional electron gases confined in lattice-matched In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48}$As quantum wells grown by molecular beam epitaxy on InP substrates. The nearly constant mobility for samples with the setback distance larger than 50nm and the similarity between the quantum and transport life-time suggest that the main scattering mechanism is due to short range scattering, such as alloy scattering, with a scattering rate of 2.2 ps$^{-1}$. We also obtain the Fermi level at the In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48}$As surface to be 0.36eV above the conduction band, when fitting our experimental densities with a Poisson-Schr\"odinger model.Comment: Accepted in Applied Physics Letter

Predicting species‐specific responses of fungi to climatic variation using historical records

Although striking changes have been documented in plant and animal phenology over the past century, less is known about how the fungal kingdom's phenology has been changing. A few recent studies have documented changes in fungal fruiting in Europe in the last few decades, but the geographic and taxonomic extent of these changes, the mechanisms behind these changes, and their relationships to climate are not well understood. Here, we analyzed herbarium data of 274 species of fungi from Michigan to test the hypotheses that fruiting times of fungi depend on annual climate and that responses depend on taxonomic and functional groups. We show that the fungal community overall fruits later in warmer and drier years, which has led to a shift toward later fruiting dates for autumn‐fruiting species, consistent with existing evidence. However, we also show that these effects are highly variable among species and are partly explained by basic life‐history characteristics. Resulting differences in climate sensitivities are expected to affect community structure as climate changes. This study provides a unique picture of the climate dependence of fungal phenology in North America and an approach for quantifying how individual species and broader fungal communities will respond to ongoing climate change.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/100145/1/gcb12278.pd

Historia y noticias del culto a la Virgen en el antiguo reino de León

Copia digital. Valladolid : Junta de Castilla y León. Consejería de Cultura y Turismo, 2009-201

Measurement of Length and Position with Frequency Combs

We show two different absolute distance measurement methods with micrometer accuracy based on frequency combs, and we discuss possible applications. Using a mode- locked laser and MEMS-based tracking optics, we measure the 3D position of a retroreflector within 10 ms and with a 24 μm volumetric accuracy. We also investigate modulator-based combs and show that they enable highly sensitive surface topography measurements with microsecond acquisition times and micrometer precision. Moreover, the potential for photonic integration of frequency comb sources is explored

Localization properties of a one-dimensional tight-binding model with non-random long-range inter-site interactions

We perform both analytical and numerical studies of the one-dimensional tight-binding Hamiltonian with stochastic uncorrelated on-site energies and non-fluctuating long-range hopping integrals . It was argued recently [A. Rodriguez at al., J. Phys. A: Math. Gen. 33, L161 (2000)] that this model reveals a localization-delocalization transition with respect to the disorder magnitude provided . The transition occurs at one of the band edges (the upper one for and the lower one for). The states at the other band edge are always localized, which hints on the existence of a single mobility edge. We analyze the mobility edge and show that, although the number of delocalized states tends to infinity, they form a set of null measure in the thermodynamic limit, i.e. the mobility edge tends to the band edge. The critical magnitude of disorder for the band edge states is computed versus the interaction exponent by making use of the conjecture on the universality of the normalized participation number distribution at transition.Comment: 7 pages, 6 postscript figures, uses revtex