Showing papers by "École Normale Supérieure published in 1981"

Photothermal deflection spectroscopy and detection

Abstract: The theory for a sensitive spectroscopy based on the photothermal deflection of a laser beam is developed. We consider cw and pulsed cases of both transverse and collinear photothermal deflection spectroscopy for solids, liquids, gases, and thin films. The predictions of the theory are experimentally verified, its implications for imaging and microscopy are given, and the sources of noise are analyzed. The sensitivity and versatility of photothermal deflection spectroscopy are compared with thermal lensing and photoacoustic spectroscopy.

Superlattice band structure in the envelope-function approximation

Abstract: The band structure of GaAs-GaAlAs and InAs-GaSb superlattices is calculated by matching propagating or evanescent envelope functions at the boundary of consecutive layers. For GaAs-GaAlAs materials, the envelope functions are the solutions of an effective Hamiltonian in which both band edges and effective masses are position dependent. The effective-mass jumps modify the boundary conditions which are imposed to the eigenstates of the effective-mass Hamiltonian. In InAs-GaSb superlattices, the dispersion relations, although quite similar to those obtained in GaAs-GaAlAs materials, reflect the genuine symmetry mismatch of InAs (electrons) and GaSb (light-holes) levels. The evolution of the InAs-GaSb band structure with increasing periodicity is calculated and found to be in excellent agreement with previous LCAO results. The dispersion relations of heavy-hole bands are obtained.

Hydrogenic impurity states in a quantum well: A simple model

Abstract: A variational calculation of hydrogenic impurity states in a quantum well has been performed. The binding energy of donor (acceptor) levels is calculated as a function of layer thickness and of the impurity position. It is found that the ground impurity state degeneracy with respect to the impurity position is lifted, leading to the formation of some sort of an "impurity band." The density of states of this impurity band exhibits one or two peaks energetically located at the "band" extrema. This one-dimensional feature can be evidenced in the optical absorption associated with valence subband\ensuremath{\rightarrow}donor transitions, whereas acceptor\ensuremath{\rightarrow}conduction processes are almost featureless. In the case of conduction\ensuremath{\rightarrow}acceptor luminescence a smooth curve is obtained for degenerate electronic distribution, whereas nondegenerate electron\ensuremath{\rightarrow}trapped hole recombination spectra should again exhibit a double peak.

Coexistence of Spin-Glass and Ferromagnetic Orderings

Abstract: The phase diagram of the infinite-range model of spin-glasses exhibits two mixed phases. In these mixed phases, ferromagnetism and spin-glass order coexist, due to freezing of the transverse degrees of freedom or replica symmetry breaking. This may help to interpret a number of recent experimental findings, e.g., in $\mathrm{Au}\mathrm{Fe}$.

Critical Behavior of Branched Polymers and the Lee-Yang Edge Singularity

Abstract: The exponents for large branched dilute polymers (which are also connected with the exponents of the lattice animals) are related in $D$ dimensions to the exponents of the Lee-Yang edge singularity of the Ising model in $D\ensuremath{-}2$ dimensions. From the exact solution of the Ising model in zero and one dimension, one gets the polymer exponents in two and three dimensions, $\ensuremath{\theta}(D=2)=1$ and $\ensuremath{\theta}(D=3)=\frac{3}{2}$.

Mass-Transfer Rates in Gas-Liquid Absorbers and Reactors

Abstract: Publisher Summary At the heart of the liquid-phase processes, gas scrubbing process, manufacturing of pure products, and biological systems, there exists the absorber or the reactor of a particular configuration best suited to the chemical absorption or reaction being carried out. Its selection, design, sizing, and performance depend on the hydrodynamics and axial dispersion, mass and heat transfer, and reaction kinetics. This chapter focuses on the subject of mass transfer with chemical reaction. It presents the techniques, results, and opinions on mass-transfer coefficients and interfacial areas in most types of absorbers and reactors. To study gas–liquid mass-transfer phenomena, it is convenient to consider steady-state situations in which the composition of the gas and the liquid are statistically constant when averaged over time in a specified region, such as a short, vertical slice of a tubular column or the entire volume of a single-compartment agitated vessel. Useful predictions have been developed for describing the behavior of complicated systems, using highly simplified models that simulate the situation for practical purposes without introducing a large number of parameters. The procedure differs depending on whether physical or chemical absorption is involved.

A Study of Barotropic Model Flows: Intermittency, Waves and Predictability

Abstract: The regime flows corresponding to the barotropic nondivergent equation with forcing, drag and subgrid-scale dissipation are studied using spectral model on the plane and on the sphere. The flow regimes obtained exhibit clear evidence of the existence of an enstrophy-cascading inertial range, together with a reverse energy cascade toward small wavenumbers. It is shown, however, that the enstrophy cascade is not associated with the k−3 spectral slope expected from the Kolmogorov-Kraichnan theory of two-dimensional turbulence; the slopes obtained are significantly steeper. This apparent paradox is tentatively resolved by a phenomenological theory of space-time intermittency in two dimensions; it is further shown that such intermittency associated with steeper spectra also restores locality of the nonlinear transfers in wavenumber space. In contrast to the well-known nonlocality typical of two-dimensional non-intermittent turbulent flows. The effect of differential rotation in connection with Rossby ...

Stress-Wave Probing of Electric Field Distributions in Dielectrics

Abstract: The spatial distribution of the electric field within a dielectric sample is shown to be obtainable unambiguously from the time dependence of the open-circuit voltage or short-circuit current during the propagation of a stress wave across the sample. Experiments in which the pressure wave is generated by the impact of a pulsed laser beam on a metal target bonded to the dielectric plate under investigation have led to the first straightforward visualization of electric field distributions in solid dielectrics.

Alcoholic fermentation: On the inhibitory effect of ethanol

Abstract: The inhibitory effect of ethanol was studied during alcoholic fermentation under strictly anaerobic conditions assured by stripping dissolved oxygen with pure nitrogen. It is shown that ehthanol produced during batch fermentation is more inhibitory than added ethanol in the range of 0 to 76 g/liter. Thus, the inhibition constant is 105.2 and 3.8 g/liter for added and produced ethanol, respectively. The measurement of the intracellular alcohol concentration can explain the difference in inhibitory effects of added and produced oxygen. 15 refs.

On the use of the geodesic metric in image analysis

Abstract: SUMMARY Let X be a phase in a specimen. Given two arbitrary points x and y of X, let us define the number dx(x, y) as follows: dx(x, y) is the greatest lower bound of the lengths of the arcs in X ending at points x and y, if such arcs exist, and + ∞ if not. The function dX is a distance function, called ‘geodesic distance’. Note that if x and y belong to two disjoint connected components of X, dx(x, y) = + ∞. In other words, dx seems to be an appropriate distance function to deal with connectivity problems. In the metric space (X, dx), all the classical morphological transformations (dilation, erosion, skeletonizations, etc.) can be defined. The geodesic distance dx also provides rigourous definitions of topological transformations, which can be performed by automatic image analysers with the help of iterative algorithms. All these notions are illustrated with several examples (definition of the length of a fibre; automatic detection of cells having at least one nucleus, or having exactly a single nucleus; definitions of the geodesic centre and of the ends of a particle without holes, etc.). As an application, a general problem of segmentation is treated (automatic separation of balls in a polished section).

Modulated phase of an ising system with competing interactions on a Cayley tree

Abstract: A system with competing nearest-neighbour and next-nearest-neighbour interactions is considered on a Cayley tree The phase diagram contains a modulated phase, as found for similar models on periodic lattices, but the multicritical Lifshitz point is at zero temperature The variation of the wavevector with temperature in the modulated phase is studied in detail, it shows narrow commensurate steps between incommensurate regions (“incomplete devil's staircase”) The behaviour of the coherence length near the different transitions is also analyzed

Lead isotopic composition of Hercynian granitic K-feldspars constrains continental genesis

Abstract: Lead isotope compositions have been determined in many K-feldspars from granitic rocks emplaced during the Variscan (Hercynian) orogeny sensu lato (around 300 Myr) in various parts of Europe and the eastern US. The ratios show a regular geographical pattern with μ contours arched around southern France and parallel to the Atlantic coast of the US. This distribution parallels predriftpalaeogeographical zones during the Palaeozoic. The lead isotope pattern of Variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an Hercynian mantle-crust material mixing. The continental crust from these areas must be older than 3,000 Myr. Large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes.

International cooperative comparison of instrument methods for cooked rice texture

Abstract: The texture of 10 milled rices cooked by a standardized method in excess water was measured in 11 laboratories using various instrument methods that had been developed specifically for national samples. The samples selected represented a wide range of rice textures. Instrument indexes for hardness and stickiness of cooked rice generally were more sensitive than the corresponding taste panel scores in discriminating among the 10 cooked rice samples. Instrument indexes for hardness correlated positively among each other as did those for stickiness. Most hardness indexes showed significant negative correlation with stickiness indexes. Most hardness indexes were positively correlated with amylose content whereas stickiness indexes were negatively correlated with amylose content. The continued use of amylose content as an index of eating quality in a rice breeding program is justified.

Corona and Insulation

Abstract: The aim of this paper is to review our present knowledge of the physics of electrical coronas and their interaction with surfaces, with some emphasis on phenomena which seem of importance for high voltage insulation. We will concentrate on fundamental and general subjects, in order to limit the scope of the paper and to make it serve as an introduction to the more specialized papers that follow.

Role of cyclic AMP in a serotonin-evoked slow inward current in snail neurones.

Abstract: One model of synaptic transmission suggests1 that transmitters modify postsynaptic permeability through the intermediary of cyclic AMP. Thus, serotonin (5-hydroxytryptamine) evokes in molluscan neurones a decrease in a voltage-dependent K+ conductance2,3 which in turn generates a slow inward current when studied in steady voltage-clamp conditions3. The serotonin-induced increase of the plateau phase of the spike of an Aplysia sensory neurone can be mimicked by both intracellularly injected cyclic AMP and extracellularly applied phosphodiesterase inhibitors4, suggesting that cyclic AMP mediates the effect. We have tested whether a similar mechanism could account for the serotonin slow inward current in identified snail neurones and have found that the intracellular injection of cyclic AMP, but not of cyclic GMP or 5′-AMP, evokes a slow inward current showing similar voltage dependence, inversion potential and ionic properties to the serotonin slow inward current. Phosphodiesterase inhibitors at low concentrations (1–20 µM) potentiate the serotonin slow inward current and at higher concentrations evoke by themselves an inward current, partially or totally occluding the serotonin and cyclic AMP currents. Finally, we have found that in homogenates of pooled identified snail neurones serotonin stimulates the adenylate cyclase, increasing its activity by 50–100%.

The role of syllables in speech processing: infant and adult data

Abstract: An empirical account is offered of some of the constants that infants and adults appear to use in processing speech-like stimuli. From investigations carried out in recent years, it seems that syllable-like sequences act as minimal accessing devices in speech processing. Ss are aware in real time of syllabic structure in words and respond differently to words with the same initial three phonemes if the segmental one is CV/... and the other CVC/.... Likewise, infants seem to be aware that a 'good' syllable must have at least one alternation if it is composed of more than one phoneme. When the segment is only one phoneme long, its status is necessarily somewhere between that of the phoneme and the syllable. An important problem that arises with the syllable is that it is an unlikely device for speech acquisition. Indeed, there are a few thousand syllables and the attribution of a given token to a type is far from obvious. Even if physical invariants for syllables in contexts were to be found, the task facing the child still remains one of sorting thousands of types from many more tokens. Issues concerning acquisition versus stable performance will be addressed to further constrain possible models. In addition, I try to show that even though information processing models are useful tools for describing synchronic sections of organisms, the elements that can account for development will have to be uncovered in neighbouring branches.