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

Universal Amplitude Ratios in the Ising Model in Three Dimensions

We use a high-precision Monte Carlo simulation to determine the universal specific-heat amplitude ratio A+/A- in the three-dimensional Ising model via the impact angle \phi of complex temperature zeros. We also measure the correlation-length critical exponent \nu from finite-size scaling, and the specific-heat exponent \alpha through hyperscaling. Extrapolations to the thermodynamic limit yield \phi = 59.2(1.0) degrees, A+/A- = 0.56(3), \nu = 0.63048(32) and \alpha = 0.1086(10). These results are compatible with some previous estimates from a variety of sources and rule out recently conjectured exact values.Comment: 17 pages, 5 figure

Self-Averaging in the Three Dimensional Site Diluted Heisenberg Model at the critical point

We study the self-averaging properties of the three dimensional site diluted Heisenberg model. The Harris criterion \cite{critharris} states that disorder is irrelevant since the specific heat critical exponent of the pure model is negative. According with some analytical approaches \cite{harris}, this implies that the susceptibility should be self-averaging at the critical temperature ($R_\chi=0$). We have checked this theoretical prediction for a large range of dilution (including strong dilution) at critically and we have found that the introduction of scaling corrections is crucial in order to obtain self-averageness in this model. Finally we have computed critical exponents and cumulants which compare very well with those of the pure model supporting the Universality predicted by the Harris criterion.Comment: 11 pages, 11 figures, 14 tables. New analysis (scaling corrections in the g2=0 scenario) and new numerical simulations. Title and conclusions chang

H2 in the interstitial channels of nanotube bundles

The equation of state of H2 adsorbed in the interstitial channels of a carbon nanotube bundle has been calculated using the diffusion Monte Carlo method. The possibility of a lattice dilation, induced by H2 adsorption, has been analyzed by modeling the cohesion energy of the bundle. The influence of factors like the interatomic potentials, the nanotube radius and the geometry of the channel on the bundle swelling is systematically analyzed. The most critical input is proved to be the C-H2 potential. Using the same model than in planar graphite, which is expected to be also accurate in nanotubes, the dilation is observed to be smaller than in previous estimations or even inexistent. H2 is highly unidimensional near the equilibrium density, the radial degree of freedom appearing progressively at higher densities.Comment: Accepted for publication in PR

“Caracterización antropométrica infanto - juvenil Y su relación nutricional en el sector de monte sinai, Guayaquil - Ecuador”

This study determines the anthropometric characterization in relation to the nutritional status of the child population - Youth. The project was carried out in the Sector of Sinai´s Mount. Guayaquil - Guayas, Ecuador; in the period from May 2014 to September 2016, based on focus groups Subcentro Health, CNH, CIBV the MIES and Educational Units of Sinai´s Mount. Through training habits and eating habits were advised - nutritional correct, a sample of 787 children and adolescents (neonates, young infants and elderly, preschool, children and adolescents) was determined by taking anthropometric measurements: weight, height and perimeter cephalic in children up to 12 years and BMI in adolescents aged 13-19 years. This investigation revealed that according to the age indicator / weight / height / head circumference and Body Mass Index 333 (42.31%) of the infanto - youth sector, presented some degree of malnutrition, 358 (45.49%) normal weight and 96 (12.19%) overweight and obesity. It becomes visible that the child population - youth accused a high prevalence of mild chronic malnutrition in relation to severe chronic malnutrition, which shows animprovement in the nutritional status of the sector. In addition it was found that overweight and obesity increased compared with previous studies.El estudio determina la caracterización antropométrica en relación al estado nutricional de una población infanto-juvenil. El proyecto se realizó en el Sector de Monte Sinaí. Guayaquil, Guayas, Ecuador; en el período comprendido entre Mayo del 2014 a Septiembre del 2016, tomando como grupos focales el Subcentro de Salud, CNH, CIBV del MIES y las Unidades Educativas de Monte Sinaí. Mediante capacitaciones se aconsejaron hábitos y costumbres alimentarias - nutricionales correctas, se determinó una muestra de 787 infantes y adolescentes (neonatos, lactantes menores y mayores, preescolares, escolares y adolescentes), a través de la toma de mediciones antropométricas: peso, talla y perímetro cefálico en niños hasta los 12 años, e índice de masa corporal en adolescentes de 13 a 19 años. Esta investigación reveló que de acuerdo con el indicador edad/peso/talla/perímetro cefálico e Índice de Masa Corporal 333 (42,31%) de los infanto-juveniles del sector, presentó algún grado de desnutrición, 358 (45,49%) Normopeso y 96 (12,19%) con sobrepeso y Obesidad. Se hace visible que la población infanto-juvenil acusa un alto predominio de la Desnutrición Leve Crónica en relación con la Desnutrición Grave Crónica, lo que demuestra una mejora en el estado nutricional del sector.Además se constató que el sobrepeso y la obesidad aumentaron en comparación con estudios anteriores

Microcanonical finite-size scaling in specific heat diverging 2nd order phase transitions

A Microcanonical Finite Site Ansatz in terms of quantities measurable in a Finite Lattice allows to extend phenomenological renormalization (the so called quotients method) to the microcanonical ensemble. The Ansatz is tested numerically in two models where the canonical specific-heat diverges at criticality, thus implying Fisher-renormalization of the critical exponents: the 3D ferromagnetic Ising model and the 2D four-states Potts model (where large logarithmic corrections are known to occur in the canonical ensemble). A recently proposed microcanonical cluster method allows to simulate systems as large as L=1024 (Potts) or L=128 (Ising). The quotients method provides extremely accurate determinations of the anomalous dimension and of the (Fisher-renormalized) thermal $\nu$ exponent. While in the Ising model the numerical agreement with our theoretical expectations is impressive, in the Potts case we need to carefully incorporate logarithmic corrections to the microcanonical Ansatz in order to rationalize our data.Comment: 13 pages, 8 figure

Geopolímeros de tipo binario basados en una puzolana natural y escoria siderúrgica de alto horno

This study describes the synthesis at ambient temperature (25±3 °C) of binary geopolymer systems based on natural volcanic pozzolan and granulated blast furnace slag. Na2SiO3 and NaOH were used as alkaline activators. The effects of the SiO2/Al2O3, Na2O/Al2O3 ratio and the amount of slag added (from 0 to 30%) on the reaction kinetics, compressive strength and microstructure of the final product were studied. To characterise the geopolymer pastes, techniques such as X-ray diffraction (XRD), infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used. The results indicate the possibility of obtaining a geopolymer cement with a compressive strength of up to 48.11 MPa after 28 days of curing at ambient temperature whose characteristics are comparable to those of commercial portland cement.Este trabajo describe la síntesis a temperatura ambiente (25±3 °C) de sistemas geopoliméricos de tipo binario basados en una puzolana natural de origen volcánico y escoria siderúrgica de alto horno usando activadores alcalinos basados en la combinación de Na2SiO3 y NaOH. Se estudió el efecto de la relación SiO2/Al2O3, Na2O/Al2O3 y la cantidad de escoria adicionada en niveles entre el 0 y 30% sobre la cinética de reacción, la resistencia a la compresión y la microestructura del producto final. Para la caracterización de las pastas geopoliméricas se utilizaron técnicas como difracción de rayos X (DRX), espectroscopia infrarroja (FTIR) y microscopia electrónica de barrido (MEB). Los resultados conseguidos revelan la posibilidad de obtener un cementante geopolimérico con una resistencia a la compresión de hasta 48,11 MPa a los 28 días de curado a temperatura ambiente cuyas características son comparables a las de un cemento portland comercial