Melting behavior of large disordered sodium clusters

The melting-like transition in disordered sodium clusters Na_N, with N=92 and 142 is studied by using a first-principles constant-energy molecular dynamics simulation method. Na_142, whose atoms are distributed in two (surface and inner) main shells with different radial distances to the center of mass of the cluster, melts in two steps: the first one, at approx. 130 K, is characterized by a high intrashell mobility of the atoms, and the second, homogeneous melting, at approx. 270 K, involves diffusive motion of all the atoms across the whole cluster volume (both intrashell and intershell displacements are allowed). On the contrary, the melting of Na_92 proceeds gradually over a very wide temperature interval, without any abrupt step visible in the thermal or structural melting indicators. The occurrence of well defined steps in the melting transition is then shown to be related to the existence of a distribution of the atoms in shells. Thereby we propose a necessary condition for a cluster to be considered rigorously amorphouslike (totally disordered), namely that there are no space regions of the cluster where the local value of the atomic density is considerably reduced. Na_92 is the only cluster from the two considered that verifies this condition, so its thermal behavior can be considered as representative of that expected for amorphous clusters. Na_142, on the other hand, has a discernible atomic shell structure and should be considered instead as just partially disordered. The thermal behavior of these two clusters is also compared to that of icosahedral (totally ordered) sodium clusters of the same sizes.Comment: LaTeX file. 7 pages with 12 picture

Anomalous Roughening in Experiments of Interfaces in Hele-Shaw Flows with Strong Quenched Disorder

We report experimental evidences of anomalous kinetic roughening in the stable displacement of an oil-air interface in a Hele-Shaw cell with strong quenched disorder. The disorder consists on a random modulation of the gap spacing transverse to the growth direction (tracks). We have performed experiments varying average interface velocity and gap spacing, and measured the scaling exponents. We have obtained beta=0.50, beta*=0.25, alpha=1.0, alpha_l=0.5, and z=2. When there is no fluid injection, the interface is driven solely by capillary forces, and a higher value of beta around beta=0.65 is measured. The presence of multiscaling and the particular morphology of the interfaces, characterized by high slopes that follow a L\'evy distribution, confirms the existence of anomalous scaling. From a detailed study of the motion of the oil--air interface we show that the anomaly is a consequence of different local velocities over tracks plus the coupling in the motion between neighboring tracks. The anomaly disappears at high interface velocities, weak capillary forces, or when the disorder is not sufficiently persistent in the growth direction. We have also observed the absence of scaling when the disorder is very strong or when a regular modulation of the gap spacing is introduced.Comment: 14 pages, 17 figure

Estudio de la anisotropía de la dureza

In the present work a study of the variations in the hardness of the minerals has been made in fonction of the crystallographic direction in which it has been measured. Experimental data has been obtained using a Vickers instru- ment. The minerala choosen for study are cinnabar and enargite

Determinación de la dureza de los minerales mediante la medida de micro-huellas

In the prescnt work the nietliod to deterniine the hardiiess by taking measuretrients at microscopical scale is described. The relationship between the Mohs scale of hardness and the VHN is stablished and also the four types of microscope equipment which are used in this method is described

Anomalous roughening of wood fractured surfaces

Scaling properties of wood fractured surfaces are obtained from samples of three different sizes. Two different woods are studied: Norway spruce and Maritime pine. Fracture surfaces are shown to display an anomalous dynamic scaling of the crack roughness. This anomalous scaling behavior involves the existence of two different and independent roughness exponents. We determine the local roughness exponents ${\zeta}_{loc}$ to be 0.87 for spruce and 0.88 for pine. These results are consistent with the conjecture of a universal local roughness exponent. The global roughness exponent is different for both woods, $\zeta$ = 1.60 for spruce and $\zeta$ = 1.35 for pine. We argue that the global roughness exponent $\zeta$ is a good index for material characterization.Comment: 7 two columns pages plus 8 ps figures, uses psfig. To appear in Physical Review

Obtención de las secciones pulidas utilizadas en los métodos cuantitativos

The conditions in what one has to perfom the different processes are described in detail: sowing of the sections, bruting or prepolish, polish and final finish, in order to obtain oriented polished sections, in which measurements of reflectance are. The most suitable abrasives and easily ob- tained are mentioned

Medida de reflectancia de una superficie pulida: estudio microscópico de minerales opacos

After a short historical introduction, we describe the method to study opaque minerals in reflected light quantitati- ve microscopy. Physical properties are calculated: refractive index and coefficient of absortion, from experimental data. Recommendations of the nomenclature for defining the op- tical properties in absorbent crystals are stablished

Descripción de los equipos microfotométricos de reflexión con incidencia normal

We stablish the necessary working conditions for the equipments used in the quantitative methods of reflected light microscopy. The principal characteristics common to al1 the rnicrophotometric equipments of the most prestige in the world are described

Estudio óptico del cinabrio

By means of a microphotometer the principal reflectances at normal incidence have been measured, in air and in oil, on oriented sections (OOOl), (1010) and (1120) of cinnabar. Measurements were made at intervals of 10 nm from 400 nm to 850 nm. From 550 nm a Nicol prism was used as the polariser; as monochromator a glass prism was used. Dispersion curves are given for the reflectances (in air and in oil) and also for the values of the refractive index and the absorption coefficien

Determinación y cálculo de las constantes ópticas en un cristal opaco biáxico: enargita

A partir de los valores de reflectancia para longitudes de onda comprendidas entre 440 nm y 660 nm obtenidos por el método microfotométrico con incidencia normal, ernpleanclo dos medios de índice de refracción conocidos, se han calculado el índice de refracción y el coeficiente de absorción en las tres direcciones principales de vibración de la enargita. Las mediciones se han efectuado en las secciones orientadas(001) y (110) de la enargita. El mineral se ha identificado mediante la difracción de rayos X