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

Quantized vortices around wavefront nodes, 2

Quantized vortices can occur around nodal points in wavefunctions. The derivation depends only on the wavefunction being single valued, continuous, and having continuous first derivatives. Since the derivation does not depend upon the dynamical equations, the quantized vortices are expected to occur for many types of waves such as electromagnetic and acoustic. Such vortices have appeared in the calculations of the H + H2 molecular collisions and play a role in the chemical kinetics. In a companion paper, it is shown that quantized vortices occur when optical waves are internally reflected from the face of a prism or particle beams are reflected from potential energy barriers

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

Noble gas films on a decagonal AlNiCo quasicrystal

Thermodynamic properties of Ne, Ar, Kr, and Xe adsorbed on an Al-Ni-Co quasicrystalline surface (QC) are studied with Grand Canonical Monte Carlo by employing Lennard-Jones interactions with parameter values derived from experiments and traditional combining rules. In all the gas/QC systems, a layer-by-layer film growth is observed at low temperature. The monolayers have regular epitaxial fivefold arrangements which evolve toward sixfold close-packed structures as the pressure is increased. The final states can contain either considerable or negligible amounts of defects. In the latter case, there occurs a structural transition from five to sixfold symmetry which can be described by introducing an order parameter, whose evolution characterizes the transition to be continuous or discontinuous as in the case of Xe/QC (first-order transition with associated latent heat). By simulating fictitious noble gases, we find that the existence of the transition is correlated with the size mismatch between adsorbate and substrate's characteristic lengths. A simple rule is proposed to predict the phenomenon.Comment: 19 pages. 8 figures. (color figures can be seen at http://alpha.mems.duke.edu/wahyu/ or http://www.iop.org/EJ/abstract/0953-8984/19/1/016007/

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.

Flickering in FU Orionis

We analyze new and published optical photometric data of FU Orionis, an eruptive pre-main sequence star. The outburst consists of a 5.5 mag rise at B with an e-folding timescale of roughly 50 days. The rates of decline at B and V are identical, 0.015 +- 0.001 mag per yr. Random fluctuations superimposed on this decline have an amplitude of 0.035 +- 0.005 mag at V and occur on timescales of 1 day or less. Correlations between V and the color indices U-B, B-V, and V-R indicate that the variable source has the optical colors of a G0 supergiant. We associate this behavior with small amplitude flickering of the inner accretion disk.Comment: 19 pages of text, 3 tables, and 6 figures to be published in the Astrophysical Journal, 10 March 200

Possible role of 3He impurities in solid 4He

We use a quantum lattice gas model to describe essential aspects of the motion of 4He atoms and of 3He impurities in solid 4He. This study suggests that 3He impurities bind to defects and promote 4He atoms to interstitial sites which can turn the bosonic quantum disordered crystal into a metastable supersolid. It is suggested that defects and interstitial atoms are produced during the solid 4He nucleation process where the role of 3He impurities (in addition to the cooling rate) is known to be important even at very small (1 ppm) impurity concentration. It is also proposed that such defects can form a glass phase during the 4He solid growth by rapid cooling.Comment: 4 two-column Revtex pages, 4 figures. Europhysics Letters (in Press

On Quartet Superfluidity of Fermionic Atomic Gas

Possibility of a quartet superfluidity in fermionic systems is studied as a new aspect of atomic gas at ultra low temperatures. The four-fold degeneracy of hyperfine state and moderate coupling is indispensable for the quartet superfluidity to occur. Possible superconductivity with quartet condensation in electron systems is discussed.Comment: 7 pages, 1 figure. J. Phys. Soc. Jpn. vol.74 (2005) No.7, in press; Note added for related previous works; some typographic errors revise

Second layer of H2 and D2 adsorbed on graphene

We report diffusion Monte Carlo calculations on the phase diagrams of para-H2 and ortho-D2 adsorbed on top of a first layer of the same substances on graphene. We found that the ground state of the second layer is a triangular incommensurate solid for both isotopes. The densities for promotion to a second layer and for the onset of a two-dimensional solid on that second layer compare favorably with available experimental data in both cases.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito