On the possible "supersolid" character of parahydrogen clusters

We present results of a theoretical study of structural and superfluid properties of parahydrogen clusters comprising 25, 26 and 27 molecules at low temperature. The microscopic model utilized here is based on the Silvera-Goldman pair potential. Numerical results are obtained by means of Quantum Monte Carlo simulations, making use of the continuous-space Worm Algorithm. The clusters are superfluid in the low temperature limit, but display markedly different physical behaviours. For N=25 and 27, superfluidity at low temperature arises as clusters melt, i.e., become progressively liquid-like as a result of quantum effects. On the other hand, for N = 26 the cluster remains rigid and solid-like. We argue that this cluster can be regarded as a mesoscopic "supersolid". This physical picture is supported by results of simulations in which a single parahydrogen molecule in the cluster is isotopically substituted.Comment: 18 pages, 7 figure

Adsorption of para-Hydrogen on Krypton pre-plated graphite

Adsorption of para-Hydrogen on the surface of graphite pre-plated with a single layer of atomic krypton is studied thoretically by means of Path Integral Ground State Monte Carlo simulations. We compute energetics and density profiles of para-hydrogen, and determine the structure of the adsorbed film for various coverages. Results show that there are two thermodynamically stable monolayer phases of para-hydrogen, both solid. One is commensurate with the krypton layer, the other is incommensurate. No evidence is seen of a thermodynamically stable liquid phase, at zero temperature. These results are qualitatively similar to what is seen for for para-hydrogen on bare graphite. Quantum exchanges of hydrogen molecules are suppressed in this system.Comment: 12 pages, 6 figures, to appear in the proceedings of "Advances in Computational Many-Body Physics", Banff, Alberta (Canada), January 13-16 200

Variational Monte Carlo study of the ground state properties and vacancy formation energy of solid para-H2 using a shadow wave function

A Shadow Wave Function (SWF) is employed along with Variational Monte Carlo techniques to describe the ground state properties of solid molecular para-hydrogen. The study has been extended to densities below the equilibrium value, to obtain a parameterization of the SWF useful for the description of inhomogeneous phases. We also present an estimate of the vacancy formation energy as a function of the density, and discuss the importance of relaxation effects near the vacant site

Interaction of H2with a Double-Walled Armchair Nanotube by First-Principles Calculations

We have studied, by first-principles methods, the interaction of molecular hydrogen with a double-walled (2,10) carbon nanotube (DWCNT). This combination of the smallest possible diameter for the inner nanotube with a significantly larger outer tube allows for substantial space between the nanotube walls, in which molecular hydrogen can adsorb. We performed classical force field molecular dynamics simulations of the infinitely extended (periodic) DWCNT with varying amounts of hydrogen, which showed that, depending on the H-2 loading, both a coaxial and a noncoaxial DWCNT configuration can be stable. We then carried out the electronic structure calculations on both the coaxial and the noncowdal geometries of the nanotubes to accurately compute the H-2 adsorption pathways inside the DWCNT. Interestingly, the noncoaxial DWCNT (2,10) shows a barrierless reaction path to dissociate the H-2 molecule. We also investigated the case of a DWCNT of finite length, whose edges are either clean or terminated with H atoms, and we searched for the favorite adsorption sites for a H-2 molecule in the interstitial region between the inner and the outer tube. The finite DWCNT whose edges are passivated by H atoms can be suggested as a potential candidate for hydrogen storage. The H-2 molecules, in fact, may enter in the cavity between the two nanotubes without reacting with the dangling bonds of the C atoms and can be physisorbed with a binding energy of about 0.06 eV, suitable for hydrogen storage. We emphasize the important role played in the physisorbed states for all the systems studied by the van der Waals interactions, which are properly included in the present study. Compared to the interaction of H-2 with graphene and the single-walled carbon nanotube (SWCNT), the existence of another carbon layer, for the coaxial DWCNT, does not significantly lower the energy barriers for chemisorption and instead enhances the binding energy of the H-2 molecule to the inner tube up to 0.1 eV