Showing papers by "École Normale Supérieure published in 1984"
TL;DR: The voltage dependence of the NMDA receptor-linked conductance appears to be a consequence of the voltage dependenceof the Mg2+ block and its interpretation does not require the implication of an intramembrane voltage-dependent ‘gate’.
Abstract: The responses of vertebrate neurones to glutamate involve at least three receptor types. One of these, the NMDA receptor (so called because of its specific activation by N-methyl-D-aspartate), induces responses presenting a peculiar voltage sensitivity. Above resting potential, the current induced by a given dose of glutamate (or NMDA) increases when the cell is depolarized. This is contrary to what is observed at classical excitatory synapses, and recalls the properties of 'regenerative' systems like the Na+ conductance of the action potential. Indeed, recent studies of L-glutamate, L-aspartate and NMDA-induced currents have indicated that the current-voltage (I-V) relationship can show a region of 'negative conductance' and that the application of these agonists can lead to a regenerative depolarization. Furthermore, the NMDA response is greatly potentiated by reducing the extracellular Mg2+ concentration [( Mg2+]o) below the physiological level (approximately 1 mM). By analysing the responses of mouse central neurones to glutamate using the patch-clamp technique, we have now found a link between voltage sensitivity and Mg2+ sensitivity. In Mg2+-free solutions, L-glutamate, L-aspartate and NMDA open cation channels, the properties of which are voltage independent. In the presence of Mg2+, the single-channel currents measured at resting potential are chopped in bursts and the probability of opening of the channels is reduced. Both effects increase steeply with hyperpolarization, thereby accounting for the negative slope of the I-V relationship of the glutamate response. Thus, the voltage dependence of the NMDA receptor-linked conductance appears to be a consequence of the voltage dependence of the Mg2+ block and its interpretation does not require the implication of an intramembrane voltage-dependent 'gate'.
3,977 citations
TL;DR: In this article, the Na-exchanged clinoptilolite in presence of ammonium ions has been analyzed and the selectivity of the Na exchange is calculated by plotting the exchange isotherms relative to the various cations.
1,364 citations
TL;DR: A pyrope-quartzite originally described by Vialon (1966) from the Dora Maira massif was resampled and reinvestigated in this article, showing that the whole matrix has once been coesite.
Abstract: A pyrope-quartzite originally described by Vialon (1966) from the Dora Maira massif was resampled and reinvestigated. Garnet (up to 25 cm in size), phengite, kyanite, talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz. The garnet is a pyrope-almandine solid solution with 90 to 98 mol % Mg end-member. It contains inclusions of coesite which has partially inverted to quartz, resulting in a typical radial cracking of the host garnet around the inclusions. Several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite. The formidable pressures of formation implied (≧28 kbar) are independently indicated by i) the coexistence of nearly pure pyrope with free silica and talc, ii) the coexistence of jadeite with kyanite, iii) the high Si content of phengite. Water activity must have been low. The stability of talc-phengite and the presence of rare glaucophane inclusions in pyrope point to low formation temperatures (about 700 °C) and to a probable Alpine age for the assemblage. This is evidence that low temperature gradients, how essentially transient they are, may nevertheless persist to considerable depths. Moreover, the upper crustal (evaporite-related?) origin of the quartzite and its interbedding within a continental unit implies that continental crust may also be subducted to depths of 90 km or more. The return back to the surface is problematic; the retrograde assemblages observed show that it must be tectonic. If the rocks remain at depth, new perspectives open for the genesis of intermediate to acidic magmas. Eventually, the role of continental crust in geodynamics may have to be reconsidered.
1,197 citations
TL;DR: In this paper, the authors considered a class of principal-agent problems with the following features: there is adverse selection because the principal ignores the value of one parameter of the agent's true characteristics, and the optimization is limited to the class of non-stochastic mechanisms.
723 citations
TL;DR: In this paper, the general couplings of vector multiplets to N = 2 supergravity were investigated and the possibility of generalized duality invariance of the bosonic field equations was investigated.
620 citations
TL;DR: In this article, a probability distribution has been proposed as an order parameter for spin-glasses, and this probability depends on the particular realization of the couplings even in the thermodynamic limit.
Abstract: A probability distribution has been proposed recently by one of us as an order parameter for spin-glasses We show that this probability depends on the particular realization of the couplings even in the thermodynamic limit, and we study its distribution We also show that the space of states has an ultrametric topology
383 citations
TL;DR: In this paper, the quantum mechanical field operator of the two-dimensional Liouville theory in a finite box was constructed, and a new type of triangle relation was discovered, which does not reduce to the already known ones.
357 citations
TL;DR: In this paper, the authors considered a set of trips where each trip is specified a priori by a place of origin, a destination, a duration, a cost, and a time interval within which the trip must begin.
Abstract: Consider a set of trips where each trip is specified a priori by a place of origin, a destination, a duration, a cost, and a time interval within which the trip must begin. The trips may include visits to one or more specific points. Our problem is to determine the number of vehicles required, together with their routes and schedules, so that each trip begins within its given time interval, while the fixed costs related to the number of vehicles, and the travel costs between trips, are minimized. The problem is a generalization of the m-traveling salesman problem. We use column generation on a set partitioning problem solved by simplex and branch-and-bound; columns are generated by a shortest path algorithm with time windows on the nodes. Numerical results for several school bus transportation problems with up to 151 trips are discussed.
342 citations
TL;DR: In this article, it was shown that in renormalizable models in four dimensions, the only infinity that can arise is at the one-loop order in the Yang-Mills coupling constant.
336 citations
TL;DR: In this article, a probability distribution for spin glasses is proposed, which depends on the particular realization of the couplings even in the thermodynamic limit, and its distribution is studied.
Abstract: A probability distribution has been proposed recently by one of us as an order parameter for spin glasses. We show that this probability depends on the particular realization of the couplings even in the thermodynamic limit, and we study its distribution. We also show that the space of states has an ultrametric topology Etude d'une distribution de probabilite, proposee recemment, comme parametre d'onde pour les verres de spin: la probabilite depend de la realisation particuliere des couplages, meme a la limite thermodynamique. Etude de la distribution de cette probabilite. Mise en evidence de la topologie ultrametrique de l'espace des etats
332 citations
09 Jul 1984
TL;DR: Closures are presently employed to prevent external contaminants such as dust and liquids from entering bearing housings at the region of clearance between the shaft and the housing through the use of mated, washer-like surfaces.
Abstract: Closures are presently employed to prevent external contaminants such as dust and liquids from entering bearing housings at the region of clearance between the shaft and the housing. One such closure relies on the use of mated, washer-like surfaces wherein such surfaces are perpendicular to the axis of the shaft. Exclusion of contaminants is enhanced, by inclining the surfaces toward the direction of ingress of the contaminating substances, at an angle of about 10-40 DEG .
TL;DR: The findings are indicative of two types of adaptation of the central program of posture regulation to weightlessness: fast, short-term adaptation, characterized by a quasi-instantaneous redistribution of motor commands between ankle flexors and extensors (an “operative process”) and slow, long- term adaptation, exemplified by the loss of anticipatory activation of certain muscles by the end of the flight (a “conservative process’).
Abstract: Adaptation of motor control to weightlessness was studied during a 7-day spaceflight. The maintenance of control of upright posture was examined during a voluntary raising movement of the arm and during the voluntary raising on tiptoe. In order to evaluate the contribution of visual cues, three types of visual situations were examined: normal vision, central vision, and without vision. On the basis of cinematographic and mechanographic data, the postural perturbations consecutive to the movement of a body part in conditions of weightlessness were found to be similar to those observed on earth. However, in weightlessness, in contrast to the ground-based situation, erectness of posture was maintained primarily due to the predominant contraction of the ankle flexor muscles. The sequences of postural leg muscle activity associated with the arm or foot movement were well structured and varied slightly in the course of the flight. In addition, the initial posture, that is the erect posture before the movement was executed, changed throughout the flight from an exaggerated oblique position to a terrestrial standing position. Visual information was preponderant at the beginning of the space mission for the recalibration of other sensory cues affected by weightlessness. The findings are indicative of two types of adaptation of the central program of posture regulation to weightlessness: fast, short-term adaptation, characterized by a quasi-instantaneous redistribution of motor commands between ankle flexors and extensors (an "operative process") and slow, long-term adaptation, exemplified by the loss of anticipatory activation of certain muscles by the end of the flight (a "conservative process").
TL;DR: A review of the application of liquid crystals to chirality studies can be found in this article, where a survey of the methods currently employed for the determination of the pitch and handedness of cholesteric liquid crystals is presented.
Abstract: Nematic liquid crystals are extremely sensitive to optically active molecules, which, at very low concentration, convert them into cholesteric liquid crystals. The purpose of this report is to review recent applications of liquid crystals to chirality studies. A brief discussion of the structure and properties of liquid crystals is followed by a survey of the methods currently being employed for the determination of the pitch and handedness of cholesteric liquid crystals. Of special interest in the formation of induced-cholestric phases are the twisting powers of optically active compounds and the structural relationships between dopants and nematics. Liquid crystals can be used, inter alia, for the detection of small optical activities and for the determination of thermal racemization barriers and absolute configurations.
TL;DR: The seven participating laboratories received films of two different thicknesses of Sc2O3 and Rh, and the various methods used for determination of the optical constants of these coating materials were described.
Abstract: The seven participating laboratories received films of two different thicknesses of Sc2O3 and Rh. All samples of each material were prepared in a single deposition run. Brief descriptions are given of the various methods used for determination of the optical constants of these coating materials. The measurement data are presented, and the results are compared. The mean of the variances of the Sc2O3 refractive-index determinations in the 0.40–0.75-nm spectral region was 0.03. The corresponding variances for the refractive index and absorption coefficient of Rh were 0.35 and 0.26, respectively.
TL;DR: In this article, the density of states and the nature of the eigenmodes of the vibrating d-dimensional Sierpinski gasket are investigated, and the spectral measure is shown to be a superposition of two distinct pure point measures of relative weights d/(d + 1) and 1/(d+ 1).
Abstract: The density of states and the nature of the eigenmodes of the vibrating d-dimensional Sierpinski gasket are investigated. The results hold for the tight-binding or any general quadratic Hamiltonian. For d>1, the spectral measure is show to be a superposition of two distinct pure point measures of relative weights d/(d+1) and 1/(d+1). The eigenmodes associated with each part are explicitly calculated. The first part of the spectrum is associated with localized modes, with non zero amplitudes only on a finite number of sites (molecular modes), Whereas the second part is associated with a new kind of states: the hierarchical modes. The influence of the boundary conditions is also elucidated as well as the importance of this kind of spectrum in quantum percolation and incommensurate potentials Etude de la densite d'etats et de la nature des etats propres d'un reseau de Sierpinski a n dimensions, les resultats etant valables aussi pour le modele de liaisons fortes et pour tout hamiltonien quadratique. Pour d>1, la mesure spectrale est la superposition de deux mesures purement ponctuelles, de poids relatifs d/(d+1) et 1/(d+1). Calcul explicite des modes propres associes. Association de la 1ere partie du spectre a des modes localises d'amplitudes non nulles seulement sur un nombre fini de sites (modes moleculaires), et de la 2eme partie a un nouveau type d'etats: des modes hierarchiques. Interpretation de l'influence des conditions aux limites et de l'importance de ce type de spectre en percolation quantique et pour les potentiels incommensurables
TL;DR: Dopamine and HVA levels in the striatum of subjects with supranuclear palsy indicate that the nigrostriatal system was lesioned to the same degree in this disease as in idiopathic Parkinsonism, but spiperone binding was reduced by half in all the structures studies.
TL;DR: In this article, the influence of quantum phase fluctuations which affect single frequency semiconductor lasers in various coherent detection systems is discussed in terms of photocurrent autocorrelation and spectral density functions.
Abstract: The influence of quantum phase fluctuations which affect single frequency semiconductor lasers in various coherent detection systems is discussed in terms of photocurrent autocorrelation and spectral density functions. The general treatment given in this paper can be applied in diverse practical cases and points out the problems of phase correlation and phase matching between the two mixed optical beams. In the more general case the photocurrent spectrum is found to be composed of discrete and quasi-Lorentzian parts whose energies and spectral spreads are discussed as a function of the laser line width, the phase matching and the phase correlation between the two coherently combined fields.
TL;DR: In this article, the results of low-temperature photoluminescence experiments performed on GaAs single quantum wells grown by metal organic chemical-vapor deposition are presented, showing that the luminescence line is down shifted by a few milli-electron-volts below the heavy-hole exciton absorption peak.
Abstract: We present the results of low-temperature photoluminescence experiments performed on GaAs single quantum wells grown by metal organic chemical-vapor deposition. The luminescence line is down shifted by a few milli-electron-volts below the $n=1$ heavy-hole exciton absorption peak. This behavior is interpreted in terms of exciton trapping on interface defects. A simple model provides reasonable values for the exciton binding energy on these defects as well as insights on the lack of thermalization which characterizes the trapped exciton photoluminescence.
18 Jun 1984
TL;DR: In this article, the authors investigated the dynamics of exciton screening of a MQWS with sub-picosecond resolution, at T = 15 K and T = 220 K.
Abstract: Multiple-quantum-well structures (MQWS) are attracting much interest for their potential use as efficient active devices in optoelectronics and nonlinear optical signal processing. Based on their strong excitonic optical nonlinearity, operation of MQWS as optical bistable elements and logical gates has been demonstrated both at low and room temperatures. To obtain information about the intrinsic response time of such devices, we have investigated the dynamics of exciton screening of a MQWS with subpicosecond resolution, at T = 15 K and T = 220 K. We have observed an unexpected shift of the exciton resonance to higher energy as a result of exciton-exciton interactions.
TL;DR: In the Gironde (France) as mentioned in this paper, the mixing of polluted particles with old deposits probably has a minor role in the decrease in paniculate lead. But it is not known whether this is the case in the lower Gironde estuary. And it is possible that in this lower estuary, lead-depleted particles are recycled to the tidal estuary by landward currents, where they become mixed with fresh river-borne particles.
Abstract: The cycling of lead in estuaries involves a complex exchange between dissolved and paniculate phases. In macrotidal estuaries such as the Gironde (France), dissolved lead is adsorbed onto particles due to an increase in turbidity and in the specific surface area of particles in the tidal estuary. In the lower estuary a mobilization of exchangeable and carbonate lead occurs simultaneously with a coagulation of organic dissolved lead associated with iron–manganese hydrous oxides. The lead-depleted particles are subsequently recycled to the tidal estuary by landward currents, where they become mixed with fresh river-borne particles. Lead isotopic ratios and paniculate lead speciation studies indicate that in this estuary the mixing of polluted particles with old deposits probably has a minor role in the decrease in paniculate lead.
TL;DR: In this paper, a tracer (tritium) method was used to establish the chemisorption isotherm of stearic acid on a CaCO3 filler and the induced changes in surface properties have been studied.
TL;DR: The Nicalon SiC fiber offers the possibility of the development of composite materials for use above 1000° C as mentioned in this paper. But, the properties of the SiC fibres have not yet been investigated.
Abstract: The Nicalon SiC fibre offers the possibility of the development of composite materials for use above 1000° C. This study has compared two types of Nicalon fibre, one which was shown to be amorphous and the other microcrystalline with a SiC grain radius of 1.7 nm. Both fibres behaved in a brittle manner when tested in air and in argon up to 1300° C and their strengths fell above 1000° C. There were indications that new defects were created in the amorphous fibre when tested in air above 1000° C. The microstructural analysis showed that as well as SiC, both SiO2 and free carbon were present in both fibres. The segregations of free carbon had a mean radius of 2.0 nm in the amorphous fibre and were grouped into two populations of 1.5 and 2.2 nm in the microcrystalline fibre.
TL;DR: In this paper, a simple elastic model for a coesite inclusion in pyrope explains why coesites has been exclusively preserved in the inclusions and not in the matrix; this finding has an important bearing on the significance of P-T conditions estimated from minerals occurring as inclusions, and on actual behaviour of continental crust in collision zones.
TL;DR: The first transfer-matrix calculation of the superconductivity exponent of a random mixture of normal and superconducting elements is presented in this article, where the exponent is defined through the divergence of the conductivity as the critical fraction of superconducted elements.
Abstract: The first transfer-matrix calculation of the superconductivity exponent $s$ of a random mixture of normal and superconducting elements is presented: The exponent $s$ is defined through the divergence of the conductivity $\ensuremath{\Sigma}$ as the critical fraction ${p}_{c}$ of superconducting elements is approached: $\ensuremath{\Sigma}\ensuremath{\sim}{(p\ensuremath{-}{p}_{c})}^{\ensuremath{-}s}$. We obtain very accurate values for the exponents which disagree with the Alexander-Orbach conjecture as well as other conjectures. Our results are $\frac{s}{\ensuremath{
u}}=0.977\ifmmode\pm\else\textpm\fi{}0.010$ in two dimensions and $\frac{s}{\ensuremath{
u}}=0.85\ifmmode\pm\else\textpm\fi{}0.04$ in three dimensions.
TL;DR: In this article, the authors studied the evolution of the albedo of the Sahel region since 1972 and found that dry season albedodensity in the region declined from a maximum close to 0.30 in 1973 to values close to 1.20 in 1979.
Abstract: The persistence of the Sahel drought, which reached a peak in 1973, appears to be typical for such dry episodes over past decades and centuries1–3. Such strong persistence can be understood if a strong positive feedback mechanism is operating, partly driven by changes in surface properties4,5. The key factors in the mechanisms thus far studied are the surface albedo and the soil moisture, both of which affect the radiation balance at the surface, the first directly, the second indirectly through its influence on the latent heat flux. We have now studied the evolution of the albedo of this region since 1972. We find that dry season albedo in the Sahel (notably Ferlo and Gondo regions, see Fig. 1b) declined from a maximum close to 0.30 in 1973 to values close to 0.20 in 1979. This decline is consistent with changes in plant cover determined by analysis of spectral changes in the Landsat multispectral scanner (MSS) data and field studies6,7.
TL;DR: A theoretical evaluation for Peek's law which yields the corona onset gradient is made in air for positive wire/cylinder and positive point/plane geometries as discussed by the authors, where the physical and geometrical parameters used in Meek's criterion take into account the variation of apparent ionization coefficient?1 of air with humidity, pressure, and temperature and the curvature radius R of the rod tip (or the wire diameter D) A single analytic integrable expression for?1/P which may be used for any value of the reduced field E/p.
Abstract: A theoretical evaluation for Peek's law which yields the corona onset gradient is made in air for positive wire/cylinder and positive point/plane geometries The physical and geometrical parameters used in Meek's criterion take into account the variation of apparent ionization coefficient ?1 of air with humidity, pressure, and temperature and the curvature radius R of the rod tip (or the wire diameter D) A single analytic integrable expression is introduced for ?1/P which may be used for any value of the reduced field E/p The results covering a wide range of parameters 10-7 m < R or D < 107 m (up to a plane/plane geometry) are compared with a variety of experimental results
TL;DR: In this paper, a phengite-talc-chloritoid-chlorite-kyanite-quartz assemblage is reported from a nearly undeformed quartz-rich metapelite found in the Monte Rosa massif (Western Alps).
Abstract: A phengite-talc-chloritoid-chlorite-kyanite-quartz assemblage is reported from a nearly undeformed quartz-rich metapelite found in the Monte Rosa massif (Western Alps). Chloritoid contains up to 74 mol % of the Mg end member and is the most magnesian ever reported. Textural relationships and mineral compositions suggest equilibrium and therefore a low-variance assemblage which represents the high-pressure stability limit of chlorite+quartz according to the terminal reaction
$${\text{chlorite + quartz }} \rightleftarrows {\text{ talc + chloritoid + kyanite + H}}_{\text{2}} {\text{O}}{\text{.}}$$
Mineral compositions combined with new experimental data on the stability of the Mg-chloritoid end member lead, for a temperature close to 500° C, to a pressure estimate of 16 kbar and a water activity of 0.6 which is supported by fluid inclusions study. Chloritoid composition is in fact a fine metamorphic indicator which opens new ways for barometry in high-grade blueschists. It demonstrates here the existence of a high-pressure metamorphism in the Monte Rosa massif. The assemblage remained mineralogically unaffected during the subsequent lower-pressure evolution. Two size fractions of the single phengite generation were analysed by the 39Ar-40Ar incremental release method. Both spectra are identical with a plateau at 110±3 Ma representing over 96% of the 39Ar degassed. The ages of the first heating steps are discordant and increase with increasing temperature from values near 70 Ma to the plateau age. Isotope correlation diagrams show two 36Ar components, one released at high temperature and correlated with 40Ar and 39Ar, the other released at low temperature in a mixture of atmospheric argon and of a loosely held argon of 70 Ma apparent age. The 110 Ma plateau age may reflect the presence of homogeneously incorporated excess argon, the 70 Ma value might then be a true age. However we favour the alternative hypothesis that the 110 Ma plateau age is a true age, implying that the internal crystalline massifs of the Western Alps have endured high-pressure metamorphism as early as mid-Cretaceous. Whatever the interpretation chosen, the preserved high-pressure mineral assemblage remained isotopically unaffected during the low-pressure mid-Tertiary event which is recorded by the 37 Ma plateau age of phengite from a foliated, recrystallised quartzite collected in the same, westernmost part of the massif. The contrasting behaviour of the two samples shows that even at temperatures as high as 400–450° C deformation and recrystallisation are also major controlling factors of isotope mobility.
TL;DR: In this paper, dual lagrangian representations at the quantum level were discussed and a dual representation which was shown to be asymptotically free was proposed. But the dual representation was not shown to hold for the two-dimensional nonlinear σ-model.
TL;DR: In this paper, two types of Nicalon SiC fibres having different structures have been examined up to 1300° C. The fibres were found to creep at temperatures above 1000° C when loads greater than a threshold level were applied.
Abstract: Two types of Nicalon SiC fibres having different structures have been examined. Their mechanical properties and their microstructures have been studied up to 1300° C. The fall in strength above 1000° C has been shown to be due to the microcrystallization of the fibre structure. Under low loads this change in structure led to a shrinkage of the fibre. The fibres were found to creep at temperatures above 1000° C when loads greater than a threshold level were applied. The creep of the fibres has been shown to be controlled by the changes which occur to the fibre structure. Degradation of the fibres on heating in air or argon has been shown to depend on SiO2 and free carbon, which have been shown to exist in the fibre.
TL;DR: This paper considers a version of the vehicle routing problem in which all vehicles are identical and where the distance travelled by any vehicle may not exceed A prespecified upper bound.
Abstract: This paper considers a version of the vehicle routing problem in which all vehicles are identical and where the distance travelled by any vehicle may not exceed A prespecified upper bound. The problem is first formulated as an integer program which is solved by means of a constraint relaxation procedure. Two exact algorithms are developed: one based on Gomory cutting planes and one on branch and bound. Numerical results are reported. (Author/TRRL)