We study the dependence of the magnetic properties of NiPd and CoPd segregated nanoclusters and slabs as a function of the 3d magnetic component concentration (x), the chemical order, and size. In the case of clusters we consider 3d cores coated by Pd atoms (3d/Pd) and also onion-like (Pd/3d/Pd) configurations. For the slabs we consider different possible segregated structures of the 3d and 4d atoms and study the evolution of the magnetic properties on these different configurations. We show that Pd coating gives rise to an enhancement of the average magnetic moment of the Ni or Co core atoms and of the Pd atoms as well. Larger values of the average magnetic moments are obtained in the onion-like structures (Pd/3d/Pd) than in the core/shell (3d/Pd) configuration. We compare with theoretical and experimental results available in the literature.

Se estudia la dependencia de las propiedades magnéticas de nanocúmulos segregados y películas de NiPd y CoPd como función de la concentración de la componente magnética 3d (x), del orden químico y del tamaño. En el caso de los cúmulos, se consideran núcleos 3d cubiertos por átomos Pd (3d/Pd) y también configuraciones por capas tipo cebolla (Pd/3d/Pd). Para las películas se consideran diferentes posibles estructuras segregadas de los átomos 3d y 4d y se estudia la evolución de las propiedades magnéticas de las diferentes configuraciones. Se muestra que la cubierta de Pd da lugar a un incremento del momento magnético promedio de los átomos del núcleo de Ni o Co así como de los átomos de Pd también. Se obtienen valores más grandes de los momentos magnéticos promedio en las estructuras por capas (Pd/3d/Pd) que en las configuraciones núcleo/cubierta (3d/Pt). Se compara con resultados experimentales disponibles en la literatura.

A systematic study of the colineal and no-colineal magnetic properties and the metallic behavior in free standing clusters of PdN (2<N<21) is carried out. We also study the effect of the twinning in clusters on the magnetism in clusters up to N = 116 atoms. To this aim we use two techniques: in one hand the ab initio based on the density functional theory (DFT) and on another hand semi-empirical based on the tight binding (TB) Hamiltonian. These are two of the most common used techniques to study the electronic properties of clusters. We found that the colineal magnetism has a no-monotonic decreasing behavior as a function of the cluster size. We observe that the twinning of clusters in some cases can increase the magnetic moment of relatively large size clusters. We found that the no-colineal magnetism always corresponds to a low energy excited state, and that the ground state always presents a colineal magnetic order. Respect to the methods, in general we found that the semi-empirical ones describe very well the electronic properties, as the magnetism; that are obtained from the difference between electronic populations. Although the metallicity requires of the exact determination of the position of the electronic states, it is razonably described by the semi-empirical method if the clusters size is big enough. We compare our results with previous calculations as well as with some experiments reported in the literature.

Realizamos un estudio sistemático de las propiedades magnéticas colineales y no-colineales, así como de la metalicidad en cúmulos libres de PdN (2<N<21). Estudiamos también los efectos que la gemelación de cúmulos tiene sobre el magnetismo en cúmulos de hasta TV = 116 átomos. Para realizar estos estudios usamos dos técnicas: una de primeros principios basada en la teoría del funcional de la densidad (DFT) y otra semi-empírica basada en un Hamiltoniano de amarre fuerte (TB), que son dos de las técnicas más utilizadas en el campo cuando se trata de estudiar propiedades electrónicas de cúmulos. Encontramos que el magnetismo colineal presenta un comportamiento decreciente no-monotono como función del tamaño del cúmulo. Observamos también que la gemelación de los cúmulos puede, en algunos casos, aumentar el momento magnético en cúmulos de tamaño relativamente grande. En el caso de magnetismo no-colineal, encontramos que corresponde siempre a un estado excitado de baja energía, presentando el estado base siempre un orden magnético colineal. En cuanto a los métodos usados encontramos, en general, que las propiedades electrónicas, como el magnetismo, que se obtienen de la diferencia entre poblaciones electrónicas, están en general bien descritas por modelos semi-empíricos, aunque otras propiedades como el carácter metálico, cuya descripción requiere de la determinación exacta de la posición de los estados electrónicos, son también razonablemente descritas si el tamaño de los cúmulos es suficientemente grande. Comparamos nuestros resultados con cálculos previos reportados en la literatura así como con los resultados experimentales disponibles.

The magnetic properties of iron clusters (FeN, N = 9, 15, 27, 51, and 59 atoms) at T = 0 K with bcc-like structure and bulk parameters are studied using ab initio methods. In these studies we consider the spin-orbit coupling and applied external magnetic fields. The basis set includes wave functions of the s, p, d, and f valence electrons. An analysis of the spin and orbital magnetic moments for every shell of the different cluster sizes is performed. The results obtained in the present work agree with other results reported in the literature and call for additional experiments.

Se estudian las propiedades magnéticas de cúmulos de hierro (FeN, N = 9, 15, 27, 51, y 59 átomos) a T = 0 K y con estructura bcc, parámetros de bulk y usando métodos ab initio. En este estudio se considera el acoplamiento espín-orbita y se aplica un campo magnético externo. La base incluye funciones de onda de los electrones de valencia s, p, d, y f. Se lleva a cabo un análisis de los momentos magnéticos de espín y orbital para cada capa de los diferentes cúmulos considerados. Los resultados obtenidos aquí están están en acuerdo con otros resultados reportados en la literatura y provoca la realización de experimentos adicionales.

The magnetism of free vanadium clusters (V N, 2 ≤ N ≤ 169) with a bcc-like structure is studied using a d electron tight binding Hamil-tonian. The spin polarized density of states is calculated self-consistently in the unrestricted Hartree-Fock approximation. We have found that the free V clusters shown different magnetic behavior depending on their size. Results for the local magnetic moments, µi, average magnetic moment, ū, and the magnetic order at T = 0 are obtained as a function of N. For the V N clusters ū varies as a function of N due to the interplay between the changes in coordination number and local environment; for N ≤ 4 the ground state is ferromagnetic whereas for N ≥ 9 the ground state is mainly antiferromagnetic in agreement with ab initio and tight binding calculations, respectively. In the case of large size clusters the local magnetic moments of most of the internal atomic shells vanish to avoid local frustrations leading the system in this way to very small magnetic moment as the cluster size increases.

Se estudia el magnetismo de cúmulos libres de vanadio (V N, 2 ≤ N ≤ 169) con estructura bcc usando un hamiltoniano de amarre fuerte considerando solo electrones tipo d. La densidad de estados de espín se calcula de forma auto-consistente dentro de la aproximación no-restringida de Hartree-Fock. Encontramos que los cúmulos libres de vanadio presentan comportamientos magnéticos muy diferentes dependiendo del tamaño. Resultados para el momento local, µi, la magnetización promedio, ū, y el orden magnético a T = 0, se obtienen como función del tamaño N. En los cúmulos de vanadio ū varia como función del tamaño debido a la interrelación entre la coordinación y el entorno local; para N ≤ 4 el estado base es ferromagnético, mientras que para N ≥ 9 el estado base es primordialmente antiferromagnético, en concordancia con cálculos ab initio y de amarre fuerte, respectivamente. En el caso de cúmulos grandes los momentos magnéticos locales de las capas internas se desvanecen para evitar la frustración, conduciendo de esta manera a valores muy pequeños del momento magnético en los cúmulos al aumentar su tamaño.

There is strong evidence that the patched (PTCH) gene is a gene for susceptibility to the nevoid basal cell carcinoma syndrome. PTCH has also been shown to mutate in both familial and sporadic basal cell carcinomas. However, mutations of the gene seem to be rare in squamous cell carcinomas. In order to characterize the role of the gene in the broader spectrum of sporadic skin malignant and pre-malignant lesions, we performed a polymerase chain reaction-single-strand conformation polymorphism (PCR-SSCP) analysis of genomic DNA extracted from 105 adult patients (46 females and 59 males). There were 66 patients with basal cell carcinomas, 30 with squamous cell carcinomas, 2 with malignant melanomas and 7 patients with precancerous lesions. Two tissue samples were collected from each patient, one from the central portion of the tumor and another from normal skin. Using primers that encompass the entire exon 1, exon 8 and exon 18, where most of the mutations have been detected, we were unable to demonstrate any band shift. Three samples suspected to present aberrant migrating bands were excised from the gel and sequenced directly. In addition, we sequenced 12 other cases, including tumors and corresponding normal samples. A wild-type sequence was found in all 15 cases. Although our results do not exclude the presence of clonal alterations of the PTCH gene in skin cancers or mutations in other exons that were not screened, the present data do not support the presence of frequent mutations reported for non-melanoma skin cancer of other populations.

Avaliou-se a influência do N e K na concentração e acúmulo de nutrientes no pimentão, cultivar Mayata, irrigado por gotejamento. O experimento foi instalado em cultivo protegido telado, em Latossolo Vermelho mesoférrico, entre o período de novembro de 1996 a agosto de 1997, durante 34 semanas. Os tratamentos constaram de três doses de N (13,3; 26,6 e 39,9 g m-2), como uréia, e três doses de K (5,5; 11,0 e 16,6 g m-2), como KCl, distribuídas em esquema fatorial, utilizando o delineamento em blocos casualizados. Todas as plantas foram irrigadas por gotejamento tendo sido a uréia e o cloreto de potássio aplicados logo abaixo dos gotejadores. As doses de N não afetaram a concentração dos nutrientes em folhas recém-maduras, no início do florescimento e início da frutificação, enquanto o K diminuiu a concentração de P e S, no início da frutificação, exceto na dose de 5,5 g m-2 de K. O N aumentou o acúmulo de nutrientes na parte aérea, ao final do ciclo de cultivo, quando aplicado na dose de 26,6 g m-2, porém não influenciou a produção de frutos. O K contribuiu para a absorção de nutrientes e produção de frutos, somente quando presente em baixas concentrações no solo. Doses de K de 16,6 g m-2, relacionaram-se com a alta condutividade elétrica (1.400 µS cm-1), até a camada de 40 cm, e com a alta concentração de cloreto na parte aérea e frutos de pimentão.

Nitrogen and potassium applications affected the nutrient concentration and accumulation of sweet pepper, cultivar Mayata, grown in a protected environment. An experiment was conducted in a protected environment in Eutrostox, during 34 weeks from November, 1996 to August, 1997, with the following treatments: control and combination of three rates of N (13.3; 26.6 and 39.9 g m-2) and three rates of K (5.5; 11.0 and 16.6 g m-2). All plants were watered by trickle irrigation and the urea and KCl were applied below each trickler. The results led to the following conclusions: N did not affect the concentration of nutrients in recently-ripened leaves, in the beginning of flowering and fruiting, although K, in high rates, decreased the concentration of P and S, in the beginning of fruiting; N increased the absorption of nutrients on the top part at the end of the cultivation period at the rate of 26.6 g m-2, but did not influence fruit production. Potassium contributed to absorption of nutrient and fruit production, only when concentrations in the soil were low. Patassium, in high rates (16.6 g m-2 ), related with high electric conductivity (1.400 µS cm-1), in a layer of 40 cm and high Cl concentration in the shoot and fruits.