Showing papers by "National University of Cuyo published in 1998"

High-temperature spin dynamics in CMR manganites: ESR and magnetization

Abstract: We have performed electron spin resonance (ESR) and dc susceptibility measurements in Mn perovskites up to 1000 K. Assuming an effective Heisenberg-like interaction for ${\mathrm{Mn}}^{3+}\mathrm{\ensuremath{-}}{\mathrm{Mn}}^{4+}$ spin pairs, the dc susceptibility, ${\ensuremath{\chi}}_{\mathrm{dc}}(T),$ is well described in the paramagnetic regime by the constant coupling approximation. Absolute determination of the ESR intensity indicates that all Mn spins contribute to the ESR line in the temperature range studied. The ESR linewidth can be described by $\ensuremath{\Delta}{H}_{\mathrm{pp}}(T)=\ensuremath{\Delta}{H}_{\mathrm{pp}}(\ensuremath{\infty})[C/T{\ensuremath{\chi}}_{\mathrm{dc}}(T)],$ thus presenting a universal behavior in a temperature scale normalized to ${T}_{c}.$ A single relaxation mechanism, related to spin-only interactions, explains the $T$ dependence of $\ensuremath{\Delta}{H}_{\mathrm{pp}}(T)$ for all the compounds studied: ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3},$ ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3},$ ${\mathrm{Pr}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3},$ and ${\mathrm{La}}_{0.67}{\mathrm{Pb}}_{0.33}{\mathrm{MnO}}_{3}.$ The dc susceptibility and the ESR linewidth and intensity all reflect the progressive importance of magnetic clustering below $\ensuremath{\approx}{2T}_{c}.$

Fractional Dynamics in Random Velocity Fields

Abstract: We develop an iterative approach, from which an exact systematic calculation of the moments of the probability density of test particles, under a generalized Fickian evolution and in the presence of a stratified random velocity field, can be calculated. The type of fractional diffusion-advection equation studied is useful for transport in anisotropic and heterogeneous media. Our method stresses the relevance of the coupling between anomalous diffusion and convection by stratified random velocity fields. [S0031-9007(98)07342-6]

Stages of the cycle of the seminiferous epithelium of the viscacha (Lagostomus maximus maximus).

Abstract: The adult male viscacha (Lagostomus maximus maximus) is a seasonal rodent It exhibits a short period of testicular regression with partial arrest of spermatogenesis during winter (July-August) The present study provides the first description of the viscacha spermatogenic cycle during the period of maximum gonadal activity (summer-autumn) The testes were processed by using conventional techniques of light and electron microscopy One-micrometer-thick sections stained with toluidine blue were used to clearly define the different cell associations Spermatogenesis in this rodent is well organized and synchronized and has been divided into nine stages The present classification is based on the morphogenesis of the acrosomal system (stages I–V) and the degree of elongation and condensation of the heads of the differentiating spermatids (stages VI–IX) Stage I is principally defined by round spermatids with a well-developed, juxtanuclear Golgi complex and without acrosomal components that are visible under the light microscope Sperm release from the viscacha seminiferous epithelium occurs either in late stage III or in early stage IV Stage IX is characterized by diplotene spermatocytes, figures of meiosis I or II, and secondary spermatocytes Anat Rec 252:8–16, 1998 © 1998 Wiley-Liss, Inc

Electronic structure of CaMnO x with 2.66 x 3.00 studied with photoemission and x-ray-absorption spectroscopy

Abstract: We studied the electronic structure of ${\mathrm{CaMnO}}_{x}$ $(2.66l~xl~3.00)$ using x-ray photoemission and O $1s$ x-ray-absorption spectroscopy. Analyzing the spectra of the two end members with the configuration-interaction cluster model, we have determined all the main parameters of the electronic structure. We have found that the ground states of ${\mathrm{CaMnO}}_{2.5}$ and ${\mathrm{CaMnO}}_{3}$ are highly covalent, with approximately the same number of ligand holes per oxygen atom: 0.22--0.23. The main separations between the Mn $3d$ bands closest to the Fermi level are of the order of 3 eV and the band gaps are of the charge-transfer type $(Ug\ensuremath{\Delta})$. The main effects of the oxygenation are the disappearance of the occupied ${e}_{g\ensuremath{\uparrow}}$ band and an approximately rigid shift of about 1 eV to lower energies of all the other Mn $3d$ bands. Finally, we discuss the most probable causes for the absence of a metallic phase in ${\mathrm{CaMnO}}_{x}.$

Gastroduodenal mucosal protection induced by dehydroleucodine: mucus secretion and role of monoamines.

Abstract: In previous work we have demonstrated that dehydroleucodine (DhL) prevents gastric damage induced by necrosis-inducing agents such as absolute ethanol (EtOH). In this study we examine the effects of DhL on gastroduodenal morphology and monoamine levels by histological and biochemical methods, respectively, as an approach to elucidating the cytoprotective mechanism of the drug. Histological evidence shows that DhL prevents formation of gastroduodenal mucosal lesions induced by EtOH and that this protective effect is related to the ability of the drug to stimulate mucus production. DhL itself does not affect the tissue concentration of NE, DA and 5-HT. However, it prevents the depletion of DA and 5-HT provoked by EtOH. We propose that the abundant mucoid blanket secreted after treatment with DhL acts as a diffusion barrier against EtOH. It is also possible that DhL could act as a "cell stabilizer," by inhibiting the degranulation of cells containing monoamines.

Glassylike low-frequency dynamics of globular proteins

Abstract: It is shown that globular proteins display a universal average structure factor accounting for their common aminoacid base. This property, linked to the construction of a random elastic network to study the low-frequency dynamics, leads to striking similarities between the physics of globular proteins and that of glassy or amorphous nanocrystals. A few consequences of this interpretation are inquired into, particularly, in connection with some of the universal properties of the low-frequency dynamics that were recently discovered for globular proteins.

Numerical study of hierarchical quantum Hall edge states in the disk geometry

Abstract: We present a detailed analysis of the exact numerical spectrum of up to ten interacting electrons in the first Landau level on the disk geometry We study the edge excitations of the hierarchical plateaus and check the predictions of two relevant conformal field theories: the multicomponent Abelian theory and the ${W}_{1+\ensuremath{\infty}}$ minimal theory of the incompressible fluids We introduce two criteria for identifying the edge excitations within the low-lying states: the plot of their density profiles and the study of their overlaps with the Jain wave functions in a meaningful basis We find that the exact bulk and edge excitations are very well reproduced by the Jain states; these, in turn, can be described by the multicomponent Abelian conformal theory Most notably, we observe that the edge excitations form subfamilies of the low-lying states with a definite pattern, which is explained by the ${W}_{1+\ensuremath{\infty}}$ minimal conformal theory Actually, the two conformal theories are related by a projection mechanism whose effects are observed in the spectrum Therefore, the edge excitations of the hierarchical Hall states are consistently described by the ${W}_{1+\ensuremath{\infty}}$ minimal theory, within the finite-size limitations

The bcc-to-fcc phase transformation during dezincification of β-phase Cu-Zn

Abstract: β-phase Cu-48at% Zn single-crystal slices with (100)β and (110)p surface orientations have been dezincified in vacuum at 450 and 600°C for various times, and the resulting surfaces have been studied by optical microscopy, and also by scanning and transmission electron microscopy. On dezincification, the β phase becomes unstable and two different competing nucleation processes occur, one being a coherent homogeneous nucleation within the β-phase matrix of a non-equilibrium 4H type of hexagonal structure, and the other a heterogeneous formation of long straight needles of the fcc α phase, with an interface orientation given by (211)β parallel to (121)α and a parallelism of the close packed directions [011]β and [111]α within the planes.

Characterization of the electronic density of metals in terms of the bulk modulus

Abstract: In the present work a semiempirical method for characterizing the electronic density of pure metals (or, equivalently, the mean radius, rs, per electron) from the bulk modulus is proposed. The general features of this method are discussed and shown to be simple and useful for many situations. Knowledge of rs is important because it determines several important properties of the metal. For instance, the interaction with a hydrogen impurity is dominated by the interstitial electron density, and we check the applicability of the obtained values of rs by evaluating the volume of solution of hydrogen in pure metals. Our results are in excellent agreement with the available experimental data.

Luminescence and resonant Raman scattering of color centers in irradiated crystalline L-alanine

Abstract: The color centers in electron irradiated L-alanine crystals are studied by means of resonant Raman scattering and photoluminescence. We find a selective resonant enhancement of some of the Raman-active vibrations. The Raman resonant modes are either localized at the methyl group or belong to lattice modes comprising displacements of the ammonium group in the damaged amino acid molecules. The luminescence intensity wanes with increasing temperature above an onset of ;150 K, indicating a strong interaction of the photoexcited electrons in the defects with the lattice modes. These results are compared with the electronparamagnetic-resonance absorption brought forth by these defects. A simple model calculation that explains the experimental results qualitatively is presented. It is shown that optical spectroscopy provides a valuable technique to study color centers in organic crystals on account of the spatial information of the wave function that can be obtained by resonant Raman scattering. @S0163-1829~98!03221-4#

Stochastic resonance in an activator–inhibitor system through adiabatic and quasi-variational approaches

Abstract: We study the phenomenon of stochastic resonance in a spatially extended system. An activator–inhibitor reaction–diffusion model is analyzed through two different approximations: an adiabatic one leading to a known form of the Graham’s nonequilibrium potential, and a quasi-variational approach that allows to obtain an approximated form of Graham’s potential for a different parameter region. Those potentials have been exploited to obtain, firstly the probability for the decay of the metastable extended states, and secondly expressions for the correlation function and for the signal-to-noise ratio, within the framework of a two state description. The analytical results show how this ratio depends on both local and nonlocal coupling parameters.

Sertoli cell nuclear pore number changes in some stages of the spermatogenic cycle of the rat seminiferous epithelium

Abstract: In an earlier study we described changes in the number and distribution of nuclear pores during maturation of germ cells at given stages of the spermatogenic cycle; these changes were related to the activity of nucleus-cytoplasm transport. Similarly, the present work was performed by combining freeze-fracture techniques with Sertoli nuclei identification criteria, using fragments of tubules isolated by transillumination under stereo-microscopy. We studied the density of nuclear pores in freeze-fracture replicas of the Sertoli nuclear envelope at stages XIII-XIV-I compared with stages IX-XII. Pore counts were carried out on photographs of the platinum replicas using a digitalized morphometric board. The results were statistically analyzed using Student's t test. The difference in density (pore number/μm 2 ± SEM) was significant between stages IX-XII (8.25 ± 0.63) and XIII-XIV-I (10.80 ± 0.60). We postulate that this density appears to be increased at the time of increased metabolic requirements of the Sertoli cell.

Dynamical Cosmological Constant and Relations Among Pseudo Goldstone Bosons

Abstract: The present apparent cosmological constant is interpreted as the potential of the lightest pseudo-Goldstone boson. Some numerical relations among cosmological parameters and particle masses are shown to arise from the mixture of this very light particle, whose interaction is of the gravitational order, with other pseudo-Goldstone bosons like the pion and the axion which feel the strong interaction.

Baryogenesis through gradual collapse of vortons

Abstract: We evaluate the matter-antimatter asymmetry produced by emission of fermionic carriers from vortons which are assumed to be destabilized at the electroweak phase transition.The velocity of contraction of the vorton, calculated through the decrease of its magnetic energy, originates a chemical potential which allows a baryogenesis of the order of the observed value. This asymmetry is not diluted by reheating if the collapse of vortons is distributed along an interval of ~ 10^-9 sec.

Infant botulism during a one year period in San Luis, Argentina

Abstract: Five cases of infant botulism which occurred during the period from May 1995 to May 1996 in San Luis, Argentina, are reported. Infant botulism was confirmed in all patients by isolation of Clostridium botulinum type A from stool culture and by the toxin assay. Toxin was found in the serum of one of them. All patients required hospitalization with treatment consisting of supportive especially respiratory and nutritional care. At the time of discharge from the hospital, three patients had a good recovery, although two of them had mild difficulties in sucking or constipation. C. botulinum was not detected in samples of honey which had been given to two of the patients.

Empresarios-políticos y el control del estado: Renovación en la élite y construcción de una economía regional en el marco nacional. Mendoza, Argentina. 1850-1890

Abstract: The present paper deals with the formation of an entrepreneurial group, whose origins are essentially mercantile and which, having gained access to the political power, comes to constitute an oligarchy. Belonging as it is to the local hegemonic sector, this group stands out because of its modernizaing inclinations, and, when facting national and international circumstances that were adverse to the mercantile economy, it has been able to design and carry out a model of capitalistic development based on the establishment of a viticulturist-viniculturist agro-industrial society. In Argentina the province of Mendoza, together with that of San Juan, constituted one of the two regional economies (the other one being Tucuman, with sugar) that specialized in productions destined for the internal market and functioned as a complement to the region of La Pampa, which was the agro- exporting realm whereby the country became integrated to the international division of labor. In their analysis, the authors provide original appraisals of the way the hegemonic group's economic practices and network of relationships fostered a phase of accumulation - a stage that proved to be decisive for the further arrival of modernization. They also study the changes that ocurred inside the elite: once gropus of businessmen substitute for sectors tied to the tradition of farming, they will obtain political power, renew the prevalent ideas, give the state a modern character, and subordinate themselves to the developing central power.

Effect of strong magnetic field on the first-order electroweak phase transition

Abstract: The broken-symmetry electroweak vacuum is destabilized in the presence of a magnetic field stronger than a critical value. Such magnetic field may be generated in the phase transition and restore the symmetry inside the bubbles. A numerical calculation indicates that the first-order phase transition is delayed but may be completed for a sufficient low value of the Higgs mass unless the magnetic field is extremely high.

[Accumulation of actin and adhesion to HEp-2 cells of strains of Escherichia coli isolated from children with diarrhea in Mendoza, Argentina].

Abstract: E. coli strains are the major bacterial cause of diarrhea among children under 2 years of age residing in Mendoza, Argentina. Detection of diarrheogenic E. coli is made after coproculture, by agglutination tests using O-group antisera including most enteropathogenic E. coli (EPEC) serogroups and others often isolated in diarrhea. Although there are many O serogroups and H serotypes of E. coli strongly associated with infantile diarrhea, a number of studies have shown differences in the rate of isolation of EPEC between cases and controls using DNA probes. We compared the diagnosis of EPEC infections by traditional serogrouping tests with other detection methods using cell culture, involving the screening of isolates for adherence patterns to HEp-2 cells. A total of 140 isolates from children less than 24 months old with acute, persistent and chronic diarrhea and 40 isolates from controls were recovered. Three distinct patterns of adherence, termed localised (LA), diffuse (DA) and aggregative (EAgg) adherence were found. The fluorescence actin staining assay was used as indicative of the ability of some EPEC strains that produce attaching-effacing (A/E) lesions. Positive serogrouping strains were strongly associated with adherence (P = 0.0001). LA adherence pattern occurred in 11% of cases with acute diarrhea associated with these serogroups (P = 0.001) and children under 12 months (P = 0.0001). The FAS test was positive in 80% of them. EAgg adherence was found only in patients (20% P = 0.0001) and DA occurred both in cases (29%) and controls (2.5% P = 0.0001). Diagnosis of EPEC infections has traditionally been performed by identifying organisms belonging to a number of serogroups or serotypes epidemiologically linked to diarrhea. Evidence is presented in this paper to show that pathogenicity is not restricted to serogroups. Isolation of many adherent strains not belonging to traditional EPEC O serogroups, shows the need for alternative methods to be used to detect and identify E. coli.

Evidence that a novel type of progestational phase control occurs in the corn mouse, a South American murid rodent.

Abstract: Most Muridae display a short luteal phase that becomes functional as a consequence of the prolactin release induced by the stimulation of copulation and/or lactation. The corn mouse also shows a short luteal phase, and we wanted to know whether copulation and/or lactation would release prolactin and maintain progesterone secretion in this species. Females in postpartum estrus were either allowed to copulate with an intact male or not, and either to lactate their young or not. Afterward, plasma progesterone was elevated over the baseline level only in females that had copulated and were bearing growing embryos (whether or not they were lactating), while prolactin was elevated only in lactating females. In another experiment, endometrial scratching induced decidualization both in females that had copulated with a vasectomized male and in those that had not copulated; sham operations had no effect in either case. Progesterone levels were elevated in decidualized animals as compared with their sham-operated controls. Results indicate that the initiation of the progestational phase in the corn mouse is not dependent on prolactin release. A short luteal phase during which nidation may occur has not yet been described in any other mammal.

Dynamic screening of fast heavy ions in solids

Abstract: The dynamic screening on a fast ion moving through a solid medium is described in a linear response model, with contributions of bound target electrons approximated by that of an inhomogeneous electron gas. Projectile ions with one and two bound electrons are considered in order to study the mixing and shifting of their energy levels by the induced wake potential. The detailed space dependence of this potential is shown to be responsible for some observed features of the excitation spectrum of the projectile ion.