Showing papers by "École Normale Supérieure published in 1972"

Outline of a theory of practice

Abstract: Outline of a Theory of Practice is recognized as a major theoretical text on the foundations of anthropology and sociology. Pierre Bourdieu, a distinguished French anthropologist, develops a theory of practice which is simultaneously a critique of the methods and postures of social science and a general account of how human action should be understood. With his central concept of the habitus, the principle which negotiates between objective structures and practices, Bourdieu is able to transcend the dichotomies which have shaped theoretical thinking about the social world. The author draws on his fieldwork in Kabylia (Algeria) to illustrate his theoretical propositions. With detailed study of matrimonial strategies and the role of rite and myth, he analyses the dialectical process of the 'incorporation of structures' and the objectification of habitus, whereby social formations tend to reproduce themselves. A rigorous consistent materialist approach lays the foundations for a theory of symbolic capital and, through analysis of the different modes of domination, a theory of symbolic power.

A neural effect of partial visual deprivation in humans.

Abstract: Certain human subjects have considerable sensitivity differences in the visual resolution of vertical gratings as compared to horizontal gratings. Although only subjects with pronounced ocular astigmatism exhibit this effect, the differences are of neural, rather than optical, origin. It is argued that the resolution anisotropies result from early abnormal visual input caused by astigmatism. This abnormal input permanently modifies the brain.

Expansion of Cylindrical Probes in Cohesive Soils

Abstract: The present study shows that the cylindrical expansion tests can not only solve foundation problems by empirical method but are also a means to study the undrained behavior of cohesive soils. On theoretical grounds, it has been shown that the undrained stress-strain curve of the soil can be derived from the pressure meter curve. New testing techniques have been developed, much more sophisticated than the classical ones. The following information has been obtained on a few cohesive soils: horizontal at rest pressure; undrained stress-strain curve of the undisturbed soil for strains up to 5% and in situ undrained cohesion, which appears higher than those determined by classical methods.

Stereology and structuring elements

Abstract: SUMMARY An object's quantitative analysis is not purely objective. On the contrary, it results from the interaction of a structuring element chosen by the observer, with the object to be analysed. After defining this relation in a quantitative way (hit-or-miss transformation), and recalling its probabilistic interpretation (Matheron, 1971) some examples of structuring elements are given: The couple of points which measures the covariances matrix. It allows harmonic analysis and the solving of sample problems. The segment which measures the chord distribution, and leads to the stereo-logical parameter ‘the star’. The parallel segments doublet, which measures curvature radius distribution and the number of particles in the two-dimensional space. The hexagon, which measures bi-dimensional granulometries, etc. These measurements have been made on the texture analyser, built by us especially for that purpose. As a conclusion, we give an application of structuring elements to clustering measurements.

Recherches sur les textures des états mésomorphes - 2. — Les champs polygonaux dans les cholestériques

Abstract: Des textures polygonales ont ete examinees en microscopie a contraste de phase et entre polaroides croises, avec des melanges de MBBA (nematique a la temperature ordinaire) et de petites quantites de benzoate de cholesterol ou de baume de Canada. La mesophase cholesterique ainsi obtenue est arrangee selon un ensemble de domaines qui ne sont pas exactement focaux. Leurs regles d'association different fondamentalement de celles connues classiquement dans les smectiques. Les courbes focales appartiennent generalement a plusieurs domaines imparfaits et peuvent etre des coniques (segments d'hyperbole), mais ne sont pas associees de maniere biunivoque en position focale. Les domaines rentrent en contact selon des surfaces coniques qui ne sont pas de revolution, en general. Des dislocations de rotation (+ π et - π) et des dislocations-coins interviennent dans certains polygones ainsi que dans les regions qui limitent les champs polygonaux. Dans certains cas, les dislocations de rotation sont en meme temps des segments focaux et introduisent de fortes variations du pas cholesterique. Le pas helicoidal presente egalement un maximum relatif au niveau des courbes focales classiques. On peut demontrer dans quelques cas assez exceptionnels l'existence d'une dislocation-vis superposee a un segment focal. Cependant, les preparations ne permettent pas de mettre en evidence un grand nombre de dislocations-vis affectant la periodicite cholesterique, bien que leur presence puisse paraitre necessaire. La distribution des axes cholesteriques selon des segments rectilignes, tendus entre des courbes focales qui ne sont pas des coniques focales, est solenoidale et conduit a des dislocations-vis. En fait, les enveloppes des axes cholesteriques ne sont pas exactement rectilignes et sont meme fortement recourbees au voisinage des lignes focales et des divers types de dislocations. L'orientation des doubles spirales tres visibles dans les polygones est liee a la chiralite de la mesophase cholesterique. De nombreux details morphologiques ont ete elucides. Des ramifications des cotes des polygones sont frequentes et ne sont pas compatibles avec leur forme rigoureusement hyperbolique. Ce fait implique egalement des variations locales du pas cholesterique. Les cotes des polygones peuvent etre egalement interrompus par des petits quadrilateres montrant une dissymetrie liee au caractere helicoidal de la mesophase. Plusieurs sortes de fissures ou de champs polygonaux dissocies ont ete analyses (fissures avec ou sans chevrons, avec ou sans dislocations de rotation). Dans de nombreux cas, l'interpretation en termes de domaines n'est pas evidente.

Photon Generation by Joule Heating in Metal Films

Abstract: We calculate the evolution of the coupled electron-phonon system in a metal film subjected to an electric field. The detailed form of the phonon distribution is presented, and is shown to deviate significantly from a Bose distribution.

Rythme et Modalités des Plissements Superposés dans les Chaînes Subalpines Méridionales des Alpes Occidentales Françaises

Abstract: Les chaines subalpines des Alpes francaises offrent un bon exemple de plis de couverture decolles au niveau du Keuper a evaporites. Leur partie meridionale, au S du Vercors et au N et NE des Chaines Provencales, montre en outre l'entrecroisement de deux systemes plisses presque orthogonaux: un systeme E-W, dit pyreneoprovencal, dont la genese s'etale du Cretace superieur a l'Eocene moyen, et un systeme N-S a NW-SE, dit alpin, qui date de l'Oligocene et du Neogene. Il y a donc eu deux etapes majeures de plissements qui se sont succedees dans le temps; mais on doit se demander si chacune de ces etapes etait faite d'une succession de phases tectoniques courtes separees par des intervalles relativement plus longs de calme tectonique, ou bien si elles correspondent a de longues periodes de deformations continues; ces deux postulats paraissent ici egalement vraisemblables. La premiere periode, celle des plis pyreneens, semble avoir fait naitre des plis droits assez simples, parfois de style ejectif, dont les anticlinaux ont souvent, des l'Eocene, ete erodes jusqu'au Jurassique superieur. A l'Oligocene et au Miocene, des plis N-S a NW-SE sont nes, dans les domaines qui n'etaient pas deja plisses, tandis que les plis E-W deja existants ont ete accentues et rompus, degenerant parfois en chevauchements caracterises par des troncatures des flancs inverses des plis. A l'echelle des Alpes Occidentales comme a celle des Chaines Subalpines Meridionales et des « arcs » elementaires qui les composent, les structures plissees disposees en arc ne sont pas simples, mais sont l'aboutissement d'une evolution tectonique longue et complexe.

Piezoemission of GaSb: Impurities and Bound Excitons

Abstract: The effect of an uniaxial stress on the impurity and exciton lines is studied. The deformation potentials are determined. The analysis of the uniaxial-stress data shows that the residual acceptor in GaSb is a single acceptor and that the two bound excitons $\ensuremath{\beta}$ and $\ensuremath{\gamma}$ are excitons bound to neutral acceptors. We emphasize the importance of the $j\ensuremath{-}j$ splitting in the $\ensuremath{\gamma}$ complex. The temperature dependence of the photoconductivity signal reveals the thermal ionization of the complexes. The variation of the intensities of the lines with the temperature and the excitation rate gives evidence concerning the processes by which the injected carriers are bound at low temperature.

A generalization of the concept of size

Abstract: SUMMARY The size of a set is defined without ambiguity as a ratio of homothety with an elementary set having the same shape. In order to generalize this, we compare the object studied with the elements of the family {λB} of the homothetics of B. The size λ of a particular element B is going to act as a measure of the object. If the latter is made up of individualized elements, we can calculate the size of each one and construct the size histogram, assuming for example that the size of a connex component C according to B is the size of the largest B included in C. (Examples: inscribed circle radius, maximum intercept.) For any object (alveolar lung space, pore system), the size at each point x is defined as the largest λB included in the object and containing x. The set of points x of a given size λ is related to the opening with respect to λB. This geometrical transformation of opening has mathematical properties similar to those of the sieving process, and is the basis of the size distribution concept as generalized by mathematical morphology. In the plane, the texture analyser allows the above measurements.

Tin as a Vacancy Trap in Silicon at Room Temperature

Abstract: Silicon samples which contain a high concentration of dispersed tin have been irradiated at 80°, 140° or 300°K with 3 MeV electrons. The interaction of defects (vacancies and interstitials) with tin was studied by infrared absorption measurements. The tin-vacancy pair was found by monitoring the release of vacancies when the pair anneals near 460°K. Vacancies released from tin are then trapped on oxygen to form the oxygen-vacancy defect which is measured by the strength of the 835 cm-1 line. Because vacancies are trapped at room temperature by tin atoms, tin aids in the separation of vacancy and interstitial associated defects in silicon. An absorption band at 935 cm-1 produced independent of the tin concentration is attributed to silicon interstitials trapped by oxygen in Si. This interstitial defect is stable to 330°K. Applications to devices are suggested.

Diffusion dans les aluminiures de niobium

Abstract: Resume La cinetique de croissance des composes intermetalliques du systeme Aluminium-Niobium a ete etudiee dans le domaine de temperature 1000–1500 °C en elaborant des revetements d'aluminiures sur le Niobium par trempage ou cementation activee et en recuisant sous argon des revetements deja formes. La croissance des phases NbAl 3 , Nb 2 Al et Nb 3 Al a pu ainsi etre observee a partir de 1000 °C pour la premiere et de 1200 °C pour les deux autres. De ces cinetiques on a pu deduire la variation des coefficients d'interdiffusion dans les differentes phases en fonction de la temperature: ils suivent une loi d'Arrhenius. Les energies d'activation sont de 26000, 57000 et 86000 calories respectivement pour le coefficient d'interdiffusion dans NbAl 3 , Nb 2 Al et Nb 3 Al. En utilisant le profil de concentration determine a la microsonde dans le compose Nb 2 Al, il a ete possible de tracer la courbe de variation du coefficient d'interdiffusion dans ce compose en fonction de la concentration. Le mecanisme de diffusion a pu etre elucide pour le compose NbAl 3 : ce sont les atomes d'aluminium qui diffusent.

Crystallite Features of Valonia cellulose by Electron Diffraction and Dark-Field Electron Microscopy

Abstract: Individual cellulose crystallites from the cell wall of Valonia ventricosa have been studied by electron diffraction and observed by dark-field electron microscopy. These two techniques reveal that the crystalline zones which run along the fibrils are above 1000 A in length without any longitudinal periodicity. The width of the crystallites covers the width of the microfibrils and ranges from 140 to 180A without persistent 35 A subunits. In several instances, the crystalline zones terminate in the manner of a fork with two arms of 30 to 40 A in width.

Low-Frequency de Haas-van Alphen Effect in Cobalt

Abstract: The Fermi surface (FS) of ferromagnetic cobalt has been investigated using the de Haas-van Alphen effect. Measurements were made at temperatures down to 1.1 \ifmmode^\circ\else\textdegree\fi{}K and magnetic fields up to 78 kOe using a low-frequency field-modulation technique. Since cobalt is ferromagnetic, the dHvA oscillations are periodic in ${B}^{\ensuremath{-}1}$ rather than ${H}^{\ensuremath{-}1}$, as in nonmagnetic metals. Orientation studies of the dHvA frequencies in the ($10\overline{1}0$) plane show two sets of frequencies. The lower set [(1-2) \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ G] has two branches and may be tentatively assigned to pockets of $U$ symmetry of the spin-down FS. The higher frequencies [(3-5) \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ G], which can be observed up to 39\ifmmode^\circ\else\textdegree\fi{} away from the hexad axis, were attributed to the existence of a neck at the $\ensuremath{\Gamma}$ point of the spin-up FS. Agreement with band calculations is found if some modifications are made to the published FS models, mainly by reducing the exchange energy to fit the value found by photoemission studies. dHvA frequencies of (11-14) \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ G were also observed, but are still unexplained.

Elektronenüberführungs-, IR-, RAMAN- und Photoelektronenspektren sowie röntgenographische Untersuchungen von Tl2MO2S2 Tl2MOS3und TB2MS4 (MMo, W) sowie Photoelektronenspektren zahlreicher Übergangsmetallverbindungen zum Vergleich

Abstract: Die Elektronenuberfuhrungs-, IR-, RAMAN- und Photoelektronenspektren sowie die Pulverdiagramme von Tl2MoO2S2, Tl2WO2S2, Tl2MoOS3, TL2WOS3, Tl2MoS4 und Tl2WS4 werden angegeben und diskutiert. Die verschiedenen Mesmethoden erlauben eine Diskussion des stark polarisierenden Einflusses des Tl(I)-Kations auf die Anionen. Zum Vergleich wurden weiterhin die Photoelektronenspektren der Salze Tl3MX4(M = V, Nb, Ta; X = S, Se); T12MO4 (M = M, W); Cs2MX4 (M = Mo, W; X = S, Se) sowie weiterer ahnlicher Salze vermessen. The electron transfer, i. r., RAMAN and photoelectron spectra and the powder diagrams of Tl2MO2S2, TI2MOS3 Tl2MS4 (M = Mo, W) and, in comparison, the photoelectron spectra of some other transition metal compounds The electron transfer, i. r., RAMAN and photoelectron spectra and the powder diagrams of Tl2MoO2S2, Tl2WO2S2, Tl2MoOS3, Tl2WOS3, Tl2MoS4 and Tl2WS4 are reported and discussed. The different methods allow a discussion of the strong polarising effect of the Tl(I) cation on the anions. For the purpose of comparison the photoelectron spectra of the salts Tl3MX4 (M = V, Nb, Ta; X = S, Se); Tl2MO4 (M = Mo, W); Cs2MX4, (M = Mo, W; X = S, Se) and other similar salts were also measured.