The Quantum-Mechanical Position Operator in Extended Systems

The position operator (defined within the Schroedinger representation in the standard way) becomes meaningless when periodic boundary conditions are adopted for the wavefunction, as usual in condensed matter physics. We show how to define the position expectation value by means of a simple many-body operator acting on the wavefunction of the extended system. The relationships of the present findings to the Berry-phase theory of polarization are discussed.Comment: Four pages in RevTe

Neuronal Density in Primary Visual Cortex (17 Visual Area), in Two Species of Octodon

Indexación: ScieloEstudios experimentales demuestran que modificaciones medioambientales pueden producir alteraciones en el desarrollo normal de la corteza cerebral visual y sus conexiones. Por otra parte, es posible que en condiciones naturales, las especies animales hayan desarrollado adaptaciones genéticas a las distintas condiciones de luminosidad en que realizan su actividad. Recientemente, se han observado variaciones significativas en la densidad neuronal cortical del área 17 (área visual primaria), en roedores silvestres con diferentes períodos diarios de actividad y relación filogenética distante (Abrothrix olivaceus y Phyllotis darwini), pero aún no se ha determinado la naturaleza genética o plástica de dichas diferencias. En este trabajo se compararon especies con una mayor cercanía filogenética, para disminuir al máximo la variable taxonómica. Se estudió la corteza visual primaria (área 17), de roedores silvestres nativos, de las especies Octodon degus (n=5) y Octodon bridgesi (n=3), pertenecientes a la Familia Octodontidae, con el propósito de evidenciar cambios a través de la medición de la densidad neuronal, mediante la técnica del disector óptico, en cortes de 40 µm, incluidos en celoidina y teñidos con Nissl. Complementariamente, se realizó una cuantificación de la densidad neuronal de la corteza motora de las especies en estudio. O. degus, que presenta un período de actividad diurna, evidenció una densidad neuronal menor en la corteza visual (34,32 ± 2,51 x 104 neuronas/mm3), que la observada en O. bridgesi (39,55 ± 0,64 x 104 neuronas/mm3), especie de período de actividad nocturna; lo cual fue estadísticamente significativo (t=3,44; p<0,05). Las diferencias encontradas se podrían relacionar con el tipo de condiciones de luminosidad en que se desenvuelven dichas especies, aunque no se puede descartar la influencia de otros factores. SUMMARY: Studies show that environmental modifications can produce profound alterations in the normal development of the visual cortex and its connectivity. For the other hand it is possible that in natural conditions, animal species have developed genetic adaptations to the different conditions of luminance in which they normally behave. Recently have observed significant changes in cortical neuronal density of area 17 (primary visual area), in two sympatric Chilean rodents with different daily activity (Phyllotis darwini and Abrothrix olivaceus), but have not yet determined the genetic nature or plastic such differences. In this paper we compared species with a closer phylogenetic relation so as to minimize the taxonomic variable. We studied the primary visual cortex (area 17) of wild rodents native of the species Octodon degus (n=5) and Octodon bridgesi (n=3), belonging to the Octodontidae family, in order to show changes in the neuronal density, using celloidin-embedded, 40µm-thickness Nissl sections, with the aid of an optical dissector. In addition, we performed a quantification of the neuronal density of the motor cortex of the species under study. O. degus, bearing a crepuscular-diurnal activity pattern, showed a lower neuronal density in the visual cortex (34.32 ± 2.51 x104 neuron/mm3) than that observed in O. bridgesi (39.55 ± 0.64 x104 neuron/mm3), a species that exhibits a nocturnal phase preference, which was statistically significant (t=3.44; p<0.05). These differences might be related to differences in daily activity in two species, but we cannot discount the influence of other factors

Quality assurance systems in the production and marketing of fresh peaches.

Due to the fast rate of peach post-harvest ripening, damage due to mechanical handling, externally appreciated as bruises and soft areas, is a real problem that leads to an early harvesting and poor quality of the fruits, as perceived by the consumers. More and more, the European consumer asks for good taste and freshness of fruits and vegetables, and these quality factors are not included in standards, nor in most of the producers' practices. Fruit processing and marketing centres (co-operatives) are increasingly interested in adopting quality controls in their processes. ISO 9000 procedures are being applied in some food areas, primarily milk and meat processors, but no generalised procedures have been developed until the present time to be applied to fresh product processes. All different peach and nectarine varieties that are harvested and handled in Murcia cooperatives and sold in a large supermarket in Madrid were analysed during the whole 1997 season (early May to late August). A total number of 78 samples of 25 fruits (co-operative) or 10 fruits (market), were tested in the laboratory for mechanical, optical, chemical and tasting quality. The variability and relationships between all these quality parameters are presented and discussed, and sampling unit sizes which would be advisable for quality control are calculated

Aplicación del análisis de imagen a la caracterización de la marmatita l.t. (Marmato, Colombia)

El presente trabajo investiga la diferencia existente entre el contenido de Fe teórico definido en marmatita (esfalerita con 10% a 26% Fe) y el contenido real (9% Fe) determinado por Microsonda Electrónica (MS) en muestras de este mineral procedentes del locus typicus. Se trata de determinar si el mineral estaba correctamente definido en origen, combinando los estudios de MS y el Análisis Digital de Imagen (ADI) sobre probetas pulidas de la Mina de Marmato, Colombia. Se demuestra que las inclusiones microscópicas de otras fases portadoras de Fe (pirrotita, calcopirita) en esfalerita aportan una cantidad de Fe que puede ser significativa (~1.5 % en las muestras analizadas). Dichas fases no podían separarse cuando se definió la marmatita (1929) y, por tanto, el Fe de las mismas se habría atribuido a la esfalerita

Algunas soluciones exactas para una ecuación de Klein Gordon

In solving practical problems in science and engineering arises as a direct consequence differential equations that explains the dynamics of the phenomena. Finding exact solutions to this equations provides importan information about the behavior of physical systems. The Lie symmetry method allows tofind invariant solutions under certain groups of transformations for differential equations.This method not very well known and used is of great importance in the scientific community. By this approach it was possible to find several exactinvariant solutions for the Klein Gordon Equation uxx − utt = k(u). A particularcase, The Kolmogorov equation uxx − utt = k1u + k2un was considered.These equations appear in the study of relativistic and quantum physics. The general solutions found, could be used for future explorations on the study for other specific K(u) functions.Al resolver problemas prácticos en ciencia e ingeniería surge como consecuencia directa las ecuaciones diferenciales que explican la dinámica de los fenómenos. Encontrar soluciones exactas a estas ecuaciones proporciona información importante sobre el comportamiento de los sistemas físicos. El método de simetría de Lie permite encontrar soluciones invariantes bajo ciertos grupos de transformaciones para ecuaciones diferenciales. Este método, poco conocido y utilizado, es de gran importancia en la comunidad científica. Mediante este enfoque, fue posible encontrar varias soluciones exactas invariables para la ecuación de Klein Gordon uxx - utt = k (u). Un caso particular, se consideró la ecuación de Kolmogorov uxx - utt = k1u + k2un. Estas ecuaciones aparecen en el estudio de la física relativista y cuántica. Las soluciones generales encontradas podrían utilizarse para futuras exploraciones en el estudio para otras funciones específicas de K (u)

Depositional models of lacustrine evaporites in the SE margin of the Ebro Basin (Paleogene, NE Spain)

An important evaporitic sedimentation occurred during the Paleogene (Eocene to lower Oligocene) in the Barberà sector of the southeastern margin of the Tertiary Ebro Basin. This sedimentation took place in shallow lacustrine environments and was controlled by a number of factors: 1) the tectonic structuration of the margin; 2) the high calcium sulphate content in the meteoric waters coming from the marginal reliefs; 3) the semiarid climate; and 4) the development of large alluvial fans along the basin margin, which also conditioned the location of the saline lakes. The evaporites are currently composed of secondary gypsum in surface and anhydrite at depth. There are, however, vestiges of the local presence of sodium sulphates. The evaporite units, with individual thicknesses ranging between 50 and 100 m, are intercalated within various lithostratigraphic formations and exhibit a paleogeographical pattern. The units located closer to the basin margin are characterized by a massive gypsum lithofacies (originally, bioturbated gypsum) bearing chert, and also by meganodular gypsum locally (originally, meganodules of anhydrite) in association with red lutites and clastic intercalations (gypsarenites, sandstones and conglomerates). Chert, which is only linked to the thickest gypsum layers, seems to be an early diagenetic, lacustrine product. Cyclicity in these proximal units indicates the progressive development of low-salinity, lacustrine bodies on red mud flats. At the top of some cycles, exposure episodes commonly resulted in dissolution, erosion, and the formation of edaphic features. In contrast, the units located in a more distal position with regard to the basin margin are formed by an alternation of banded-nodular gypsum and laminated gypsum layers in association with grey lutites and few clastic intercalations. These distal units formed in saline lakes with a higher ionic concentration. Exposure episodes in these lakes resulted in the formation of synsedimentary anhydrite and sabkha cycles. In some of these units, however, outer rims characterized by a lithofacies association similar to that of the proximal units occur (nodular gypsum, massive gypsum and chert nodules)