Microclimate patterns in an agroforestry intercropped vineyard: first results

info:eu-repo/semantics/publishedVersio

Anisotropic random resistor networks: a model for piezoresistive response of thick-film resistors

A number of evidences suggests that thick-film resistors are close to a metal-insulator transition and that tunneling processes between metallic grains are the main source of resistance. We consider as a minimal model for description of transport properties in thick-film resistors a percolative resistor network, with conducting elements governed by tunneling. For both oriented and randomly oriented networks, we show that the piezoresistive response to an applied strain is model dependent when the system is far away from the percolation thresold, while in the critical region it acquires universal properties. In particular close to the metal-insulator transition, the piezoresistive anisotropy show a power law behavior. Within this region, there exists a simple and universal relation between the conductance and the piezoresistive anisotropy, which could be experimentally tested by common cantilever bar measurements of thick-film resistors.Comment: 7 pages, 2 eps figure

Nonadiabatic Superconductivity and Vertex Corrections in Uncorrelated Systems

We investigate the issue of the nonadiabatic superconductivity in uncorrelated systems. A local approximation is employed coherently with the weak dependence on the involved momenta. Our results show that nonadiabatic vertex corrections are never negligible, but lead to a strong suppression of $T_c$ with respect to the conventional theory. This feature is understood in terms of the momentum-frequency dependence of the vertex function. In contrast to strongly correlated systems, where the small ${\bf q}$-selection probes the positive part of vertex function, vertex corrections in uncorrelated systems are essentially negative resulting in an effective reduction of the superconducting pairing. Our analysis shows that vertex corrections in nonadiabatic regime can be never disregarded independently of the degree of electronic correlation in the system.Comment: 4 pages, 3 eps fig

Synthesis and characterization of hydrazine derivatives of coumarin for bioorthogonal chemistry inside cells

Protein carbonylation is one manifestation of oxidative stress, which is characteristic of many diseases. Our lab has developed several coumarin-hydrazine based fluorescent probes capable of detecting carbonylation in live cells via the formation of hydrazone conjugates. As an effort to expand and improve on these achievements, this work presents new data regarding the synthesis and characterization of three such probes. Two of the probes—julolidine coumarin hydrazide (JCH) and trifluoromethyl coumarin hydrazine (TFCH)—were previously synthesized by our lab. Here, JCH is studied in terms of its reaction speed and fluorescence enhancement in acylhydrazone formation with aldehydes under various conditions and catalytic schemes. For TFCH, the previously developed synthetic scheme is replicated, and the molecule is subjected to analysis of its reaction with various aldehydes, as well as stability studies. The synthesis of a third probe—coumarin acid hydrazine (CAH)—is developed, adding a novel analog to our library. CAH is substituted with a carboxylic acid at the 4-position, which may confer the benefit of enhanced water solubility and situates it as the precursor to an ester substituted derivative for future work

Enhanced tunneling conductivity induced by gelation of attractive colloids

We show that the formation of a gel by conducting colloidal particles leads to a dramatic enhancement in bulk conductivity, due to inter-particle electron tunneling, combining predictions from molecular dynamics simulations with structural measurements in an experimental colloid system. Our results show how colloidal gelation can be used as a general route to huge enhancements of conductivity, and suggest a feasible way for developing cheap materials with novel properties and low metal content.Comment: 8 pages, 8 figures, 2 table

Nonadiabatic Pauli susceptibility in fullerene compounds

Pauli paramagnetic susceptibility $\chi$ is unaffected by the electron-phonon interaction in the Migdal-Eliashberg context. Fullerene compounds however do not fulfill the adiabatic assumption of Migdal's theorem and nonadiabatic effects are expected to be relevant in these materials. In this paper we investigate the Pauli spin susceptibility in nonadiabatic regime by following a conserving approach based on Ward's identity. We find that a sizable renormalization of $\chi$ due to electron-phonon coupling appears when nonadiabatic effects are taken into account. The intrinsic dependence of $\chi$ on the electron-phonon interaction gives rise to a finite and negative isotope effect which could be experimentally detected in fullerides. In addition, we find an enhancement of the spin susceptibility with temperature increasing, in agreement with the temperature dependence of $\chi$ observed in fullerene compounds. The role of electronic correlation is also discussed.Comment: Revtex, 10 pages, 8 figures include

Influence de la déforestation sur le fonctionnement hydrologique de petits bassins versants tropicaux

Les régions tropicales subissent une déforestation importante. En Amérique du Sud,la forêt est généralement remplacée par une prairie, C'est pourquoi nous avons étudié le comportement hydrologique de 2 petits (1,5 ha) bassins versants. Un bassin (bassin B) est recouvert par une forêt primaire, tandis que le second (bassin A) a été défriché et transformé en prairie (Digitaria swazilandensis, programme ÉCÉREX, ORSTOM/CTFT). Ces bassins, situés en Guyane Française, sont proches (500 m), escarpés et principalement constitués par des sols à drainage vertical ralenti. Le climat est de type tmpical humide avec une température moyenne (26 °C) et des précipitations moyennes annuelles (3500 à 3900 mm/an) élevées. L'évapotranspiration réelle et potentielle de la forêt primaire sont respectivement égales à 1470 mm/an et 1565 mm/an, En période d'étiage, nous avons observé un écoulement permanent à l'exutoire du bassin A, alors que le bassin B en est dépourvu. Deux crues (24 mai 1992 et 15 mai 1993) ont été étudiées, simultanément sur les 2 bassins. Pendant les crues, nous avons prélevé des échantillons d'eau des précipitations (pluie et pluviolessivat), des ruisseaux et du sol. Sur ces sites, l'eau circulant dans les couches peu profondes du sol présente une concentration élevée en K+ et faible en Cl-. Une signature opposée caractérise l'eau des couches pmfondes du sol. L'analyse des relations existant entre les traceurs chimiques (K+, Cl-) et isotopique l80) ainsi l'étude des propriétés hydrodynamiques du sol permet de décomposer qualitativement l'hydrogramme de crue en 3 réservoirs: sol superficiel (écoulement hypodermique), sol intermédiaire (de 0 à - 0,4 m), sol profond (bassin B) ou nappe (bassin A). Une décomposition quantitative a été effectuée en utilisant des traceurs chimique (Cl-) et isotopique l80). Nous avons ainsi montré que les crues sur les 2 bassins sont dominées par l'écoulement issu des couches intermédiaires du sol qui représente environ la moitié de l'écoulement total de crue. Cependant,les mécanismes de génération des crues diffèrent sur les 2 bassins. Sur le bassin A, les couches profondes du sol sont saturées avant la crue et participent donc à la totalité de la crue. Au contraire, sur le bassin B, les couches profondes de sol atteignent la saturation peu de temps avant le pic de crue et participent donc essentiellement aux écoulement pendant la décrue. Ces résultats confirment les études hydrologiques réalisées précédemment (FRITSCH, 199Ù) et permettent d'identifier les mécanismes de genèse des crues et ainsi de mettre en évidence l'effet de la déforestation.The tropical regions are subjected to fast deforestation. In South America, the tropical rain forest is being replaced by grassland. Thus, we have studied the hydrological behaviour of two small (1.5 ha) watersheds. One basin (hereafter named "B" basin) is still covered by primary forest while the second one (hereafter named "A" basin) was cleared and transformed to grassland (Digitaria swazilandensis, ÉCÉREX program, supported by ORSTOM/CTFT). These basins, located in French Guyana, are close to one another (500 m), steep, and are principally constituted of soils showing lateral drainage. The tropical humid climate is characterized by a high mean interannual temperature (26¡C), which varies slightly from month to month, and by a high mean annual precipitation (3500 to 3900 mm yr-1). Precipitation mainly occurs during the main wet season from May to June and during a secondary wet season from December to January. Real evapotranspiration of the natural forest is 1470 mm yr-1 and potential evapotranspiration is 1565 mm yr-1. During the low-water level period, we have observed perennial runoff at the outlet at the "A" basin while the "B" basin is without permanent flow. We have studied two runoff events (24 May 1992 and 15 May 1993) in both basins. On 24 May 1992, the runoff event was caused by a rainfall lasting for about 10 hours. Total precipitation was 53.8 mm. The main event amounted to 32 mm. The main peak of the hydrograph corresponded to the heaviest rainfalls. On 15 May 1993, the runoff event was caused by a rain lasting for about 13 hours. Total precipitation was 64.0 mm. The main peak of the hydrograph (86.2 L s-1) corresponded again to the heaviest rainfalls. Spatial variability of the precipitation amount was high, especially for the most intense events that have the largest standard deviations. Interception by the canopy amounted to 5.3% of the rainfall in 1992 and 4.3% in 1993. High rapid runoff coefficients were observed, i.e., 0.28 for 24 May 1992 and 0.43 for 15 May 1993. No overland flow was observed in the watershed.Samples of rainwater, throughfall, stream water, and soil water were regularly collected in both watersheds during the runoff events. Temporal variations in the isotopic composition of the stream water at the outlet of the watershed paralleled variations in rainwater but with a distinct shift. The difference between the two signatures could be due to a mixture between:- Rainwater and water present in the watershed before the event and whose isotopic composition is different and variable over space. - Rainwater and water originating from various reservoirs whose contribution to the stream varies with time. The analysis of runoff events using the isotope tracer method revealed the existence in the stream of a mixture of water originating from rain and from one or several other reservoirs in the watershed. Isotope tracers alone were not sufficient to estimate the depth of the soil water contributing to the runoff event. On one hand, temporal variability in the isotopic composition of rainwater was very similar to the vertical spatial variability in the isotopic composition of soil water. On the other hand, surface evaporation in the watershed was negligible: the isotopic signature of water originating from soil during runoff events was the consequence of successive infiltrated rain events. Oxygen-18 content in rain water strongly varied with time but only slightly with space because of the small area of the watershed. Because of this temporal variability, an average isotope content of rainwater could not be used when calculating the contribution of "new water" at the outlet of the watershed.Using chemical and isotope tracers is a way to identify and quantify the contribution of the various water reservoirs to runoff. We were thus able to separate runoff hydrographs into simple components (water from superficial layer, intermediate layer and deep layer). In these watersheds, shallow water was characterized by relatively high concentration in potassium and very low concentration in chloride. An opposite signature characterized deep waterA "deep water" chemical tracer (chloride) - isotope tracer (18O) diagram shows the evidence of a hysteresis relationship:1. The decreasing limb of this relationship (rising segment of the hydrograph) is due to a decrease in heavy isotope content resulting from the decrease of oxygen-18 content in the precipitation and from the arrival of water from upper soil layers with low concentrations of chloride. 2. The increasing limb (falling segment of the hydrograph and recession) is associated with the arrival at the outlet of deep waters containing relatively high concentrations of chloride and heavy isotopes. Using chemical (Cl-) and isotope (18O) tracers, quantitative hydrograph separation was achieved with a simple 2- or 3- component conservative-mixing model. This information allowed qualitative hydrograph separation into 3 reservoirs: superficial soil layers, intermediate soil layers (0 to -0.4 m), deep soil layers ("B" watershed) or ground water ("A" watershed).Thus, the runoff event of both basins was dominated by the intermediate soil layers reservoir, which represents half of the total flow for both basins. However, the processes of runoff generation differ: in the "A" watershed, the deep soil layers were saturated before the rain: the contribution is significant throughout the runoff . In the "B" watershed, the deep soil layers become saturated a few times before the peak flow: their contribution dominates during the recession. These results confirm previous hydrological studies (Fritsch, 1990), which showed the high reactivity of the watershed, and give a better insight into the mechanisms involved.Some of these observations can also be used at a larger scale: 1. Identification of the reservoirs contributing to the runoff event by analyzing the relationships between oxygen-18 content and the flow rate, and between isotope and chemical tracers. 2. Simultaneous samplings along the stream in order to detect a possible zonation of the watershed. These samples must be taken during a runoff event as well as during a low-water level period to check whether the tracer concentrations in the continuous or discontinuous water table supplying the stream are heterogeneous. If the signature of the water table is heterogeneous or if the stream is supplied by several water tables with different chemical concentrations, the watershed must be divided into several homogeneous sub-watersheds

Longitudinal and transversal piezoresistive response of granular metals

In this paper, we study the piezoresistive response and its anisotropy for a bond percolation model of granular metals. Both effective medium results and numerical Monte Carlo calculations of finite simple cubic networks show that the piezoresistive anisotropy is a strongly dependent function of bond probability p and of bond conductance distribution width \Delta g. We find that piezoresistive anisotropy is strongly suppressed as p is reduced and/or \Delta g is enhanced and that it vanishes at the percolation thresold p=p_c. We argue that a measurement of the piezoresistive anisotropy could be a sensitive tool to estimate critical metallic concentrations in real granular metals.Comment: 14 pages, 7 eps figure