3,505 research outputs found
Phase diagram of H2 adsorbed on graphene
The phase diagram of the first layer of H 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 He, the
ground state of molecular hydrogen is a 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
Supersolidity in quantum films adsorbed on graphene and graphite
Using quantum Monte Carlo we have studied the superfluid density of the first
layer of He and H adsorbed on graphene and graphite. Our main focus has
been on the equilibrium ground state of the system, which corresponds to a
registered phase. The perfect solid phase of H shows
no superfluid signal whereas 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 He without destroying the
spatial solid order.Comment: 5 pages, accepted for publication in PR
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
Minimal Flavor Violation and the Scale of Supersymmetry Breaking
In this paper we explore the constraints from B-physics observables in SUSY
models of Minimal Flavor Violation, in the large tan beta regime, for both low
and high scale supersymmetry breaking scenarios. We find that the rare B-decays
b -> s gamma and B_s -> mu+ mu- can be quite sensitive to the scale M at which
supersymmetry breaking is communicated to the visible sector. In the case of
high scale supersymmetry breaking, we show that the additional gluino
contribution to the b -> s gamma and B_s -> mu+ mu- rare decay rates can be
significant for large tan beta, mu and M_3. The constraints on B_u -> tau nu
are relatively insensitive to the precise scale of M. We also consider the
additional constraints from the present direct Higgs searches at the Tevatron
in the inclusive H/A -> tau tau channel, and the latest CDMS direct dark matter
detection experiments. We find that altogether the constraints from B-physics,
Higgs physics and direct dark matter searches can be extremely powerful in
probing regions of SUSY parameter space for low M_A and large tan beta, leading
to a preference for models with a lightest CP-even Higgs mass close to the
current experimental limit. We find interesting regions of parameter space that
satisfy all constraints and can be probed by Higgs searches at the Tevatron and
the LHC and by direct dark matter searches in the near future.Comment: 24 pages, 6 figures. Added citations. Published in PR
Cross Sections for the Production of He+ (np) 2P0 States by 50 to 150 keV Proton Impact on Helium
Cross sections have been measured for the production of He+ (np) 2Po states, n=2,3,4, by proton impact on helium over a projectile velocity range of 1.42–2.45 a.u. (50 ≤E≤150 keV). Cross sections were determined by measuring the extreme ultraviolet photons emitted from excited He1 ions. The data indicate a lower energy than expected for the maximum cross section. A comparison of the present results in terms of projectile energy dependance with the cross sections for excitation to He (1snp) 1Po, ionization, and total electron capture suggests the primary mechanism for the production of excited He+ at low energies is transfer excitation, with ionization excitation being the dominant mechanism at higher energies
FUSE Observations of the Dwarf Novae UU Aql, BV Cen, and CH UMa in Quiescence
We report on FUSE spectra of three U Gem-type, long period, dwarf novae, UU
Aql, BV Cen and CH UMa taken during their quiescence intervals. We discuss the
line identifications in their spectra and attempt to characterize the source(s)
of their FUV flux distribution. Archival IUE spectrum of CH UMa and BV Cen in
quiescence were identified as having a matching flux level with the FUSE
spectra and these were combined with each FUSE spectrum to broaden the
wavelength coverage and further constrain model fits. Multi-component synthetic
spectral fits from our model grids, consisting of single temperature white
dwarfs, two-temperature white dwarfs, accretion disks and white dwarfs plus
accretion disks, were applied to the FUSE spectra alone and to the combined
FUSE + IUE spectra. We present the results of our model analyses and their
implications.Comment: accepted in AJ, 26 pages, 6 tables, 8 figures (5 color, 3 b/w
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 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
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.
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/
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