Phase diagram of H2 adsorbed on graphene

The phase diagram of the first layer of H$_2$ adsorbed on top of a single graphene sheet has been calculated by means of a series of diffusion Monte Carlo (DMC) simulations. We have found that, as in the case of $^4$He, the ground state of molecular hydrogen is a $\sqrt3 \times \sqrt3$ commensurate structure, followed, upon a pressure increase, by an incommensurate triangular solid. A striped phase of intermediate density was also considered, and found lying on top of the equilibrium curve separating both commensurate and incommensurate solids.Comment: 5 pages, 3 figure

Numerical test of the Cardy-Jacobsen conjecture in the site-diluted Potts model in three dimensions

We present a microcanonical Monte Carlo simulation of the site-diluted Potts model in three dimensions with eight internal states, partly carried out in the citizen supercomputer Ibercivis. Upon dilution, the pure model's first-order transition becomes of the second-order at a tricritical point. We compute accurately the critical exponents at the tricritical point. As expected from the Cardy-Jacobsen conjecture, they are compatible with their Random Field Ising Model counterpart. The conclusion is further reinforced by comparison with older data for the Potts model with four states.Comment: Final version. 9 pages, 9 figure

Supersolidity in quantum films adsorbed on graphene and graphite

Using quantum Monte Carlo we have studied the superfluid density of the first layer of $^4$He and H$_2$ adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered $\sqrt3 \times \sqrt3$ phase. The perfect solid phase of H$_2$ shows no superfluid signal whereas $^4$He has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in $^4$He without destroying the spatial solid order.Comment: 5 pages, accepted for publication in PR

Estructura y dinámica de la biota asociada al alga Macrocystis pyrifera (Phaeophyceae) en el canal del Beagle, Tierra del Fuego.

The community associated to the kelp Macrocystis pyrifera from the Beagle Channel (54°00´S; 68°20´W) was studied. Sixty-eight taxa including Algae (5), Porifera (indet.), Bryozoa (7), Nemertea (2), Annelida (10), Mollusca (22), Crustacea (15) and Echinodermata (7) were recognized. A seasonal sampling during one year showed differences in taxa composition when comparing (a) two different environments; (b) the spring-summer period and the autumn-winter period; and (c) the parts of the alga (fronds and holdfast).Se estudió la comunidad asociada al alga Macrocystis pyrifera del Canal del Beagle (54°00’S; 68°20’O). Se reconocieron 68 taxones que incluyen Algae (5), Porifera (indet.), Bryozoa (7), Nemertina (2), Annelida (10), Mollusca (22), Crustacea (15) y Echinodermata (7). Se realizó un seguimiento anual de la comunidad detectándose diferencias en la composición de los taxones al comparar (a) dos localidades con características ambientales diferentes; (b) los períodos estacionales; y (c) las partes del alga (frondes y disco de fijación)

Low temperature phase diagram of condensed para-Hydrogen in two dimensions

Extensive Path Integral Monte Carlo simulations of condensed para-Hydrogen in two dimensions at low temperature have been carried out. In the zero temperature limit, the system is a crystal at equilibrium, with a triangular lattice structure. No metastable liquid phase is observed, as the system remains a solid down to the spinodal density, and breaks down into solid clusters at lower densities. The equilibrium crystal is found to melt at a temperature close to 7 K

Phase transitions of H2 adsorbed on the surface of single carbon nanotubes

By means of Diffusion Monte Carlo calculations, we obtained the complete phase diagrams of H$_2$ adsorbed on the outer surface of isolated armchair carbon nanotubes of radii ranging from 3.42 to 10.85 \AA. We only considered density ranges corresponding to the filling of the first adsorption layer in these curved structures. In all cases, the zero-temperature ground state was found to be an incommensurate solid, except in the widest tube, in which the structure with lowest energy is an analogous of the $\sqrt{3} \times \sqrt{3}$ phase found in planar substrates. Those incommensurate solids result form the arrangement of the hydrogen molecules in circumferences whose plane is perpendicular to the main axis of the carbon nanotube. For each tube, there is only one of such phases stable in the density range considered, except in the case of the (5,5) and (6,6) tubes, in which two of these incommensurate solids are separated by novel first order phase transitions.Comment: 5 pages. to appear in Phys. Rev.

Control de aditivo tiourea en la solución de electrodeposición de cinc por EIS

En este trabajo se analiza la influencia de la concentración de tiourea en el proceso de deposición de cinc por medio de técnicas electroquímicas. Se ha encontrado una concentración óptima del aditivo en cuanto a que los depósitos son de más calidad y resultan ser más refinados y nivelados. Como se sabe, una forma de protección contra la corrosión de los metales utilizados como sustratos, es mediante recubrimientos metálicos que le dan al sustrato o metal de base protección o más valor (en el caso de metales preciosos como el oro o la plata o el cromado decorativo) o le mejoran notoriamente sus propiedades de resistencia mecánica y / o la resistencia contra la corrosión en servicio (cincados, cromados ingenieriles). Una de las técnicas de obtención de recubrimientos es mediante la electrodeposición. Los aditivos, en general son sustancias orgánicas que se agregan a la solución de electrodeposición para modificar la calidad de los acabados metálicos. Son fórmulas bajo patente y se desconoce cual es la función del aditivo para cada proceso. El uso correcto del aditivo puede resultar en la producción de un recubrimiento nivelado, brillante y con buena resistencia contra la corrosión. En los estudios voltamétricos hemos encontrado que la reducción del Zn 2+ se acelera en presencia de tiourea. Mediante la Espectroscopia de Impedancia Electroquímica, EIS, hemos hallado la cantidad óptima de aditivo para un acabado más decorativo. Se ha encontrado que la resistencia a la corrosión disminuye en presencia detiourea. Mientras que la resistencia a la corrosión se incrementa, cuando las concentraciones de tiourea son próximas al valor óptimo.Fil:Mahmud, Z. Instituto Nacional de Tecnología Industrial (INTI). Procesos SuperficialesFil:Gordillo, G. Universidad de Buenos Aires (UBA). Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química FísicaFil:Gassa, L. Universidad Nacional de La Plata (UNLP). Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFil:Ventura D'Alkaine, C. Universidade Federal de São Carlos (UFSCar

Control of zinc plating solutions, determining the optimum concentration of thiourea additive by electrochemical techniques

In this work we have analyzed the effect of thiourea concentration in the acid zinc deposition process. For these purpose, we have used the electrochemical techniques (cathodic voltammetry and Electrochemistry Impedance Spectroscopy, EIS). It allows determining the optimal concentration thiourea additive. In this range, we have obtained better finishes, more refined and decoratives layers. We have found in voltammetric studies that the electroreduction of Zn 2+ in the presence of thiourea is accelerated. The EIS studies has shown the “electrodeposition resistance” decrease in the presence of thiourea. The addition of more thiourea in the solution, in the optimum concentration, the electrodeposition resistance to the electrodeposition process is increased. The main contribution of this research and its scientific contribution is the use of electrochemical techniques for determining the optimal concentration of thiourea additive in the zinc electroplating solution in chloride acid mediaFil:Mahmud, Z. Instituto Nacional de Tecnología Industrial (INTI). Procesos SuperficialesFil:Gordillo, G. Universidad de Buenos Aires (UBA). Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química FísicaFil:Gassa, L. Universidad Nacional de La Plata (UNLP). Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFil:Ventura D'Alkaine, C. Universidade Federal de São Carlos (UFSCar