27 research outputs found
Adsorption of Xe and Ar on Quasicrystalline Al-Ni-Co
An interaction potential energy between and adsorbate (Xe and Ar) and the
10-fold Al-Ni-Co quasicrystal is computed by summing over all
adsorbate-substrate interatomic interactions. The quasicrystal atoms'
coordinates are obtained from LEED experiments and the Lennard-Jones parameters
of Xe-Al, Xe-Ni and Xe-Co are found using semiempirical combining rules. The
resulting potential energy function of position is highly corrugated.
Monolayer adsorption of Xe and Ar on the quasicrystal surface is investigated
in two cases: 1) in the limit of low coverage (Henry's law regime), and 2) at
somewhat larger coverage, when interactions between adatoms are considered
through the second virial coefficient, C_{AAS}. A comparison with adsorption on
a flat surface indicates that the corrugation enhances the effect on Xe-Xe
(Ar-Ar) interactions. The theoretical results for the low coverage adsorption
regime are compared to experimental (LEED isobar) data.Comment: 12 pages, 8figure
Evolution of topological order in Xe films on a quasicrystal surface
We report results of the first computer simulation studies of a physically
adsorbed gas on a quasicrystalline surface, Xe on decagonal Al-Ni-Co. The grand
canonical Monte Carlo method is employed, using a semi-empirical gas-surface
interaction, based on conventional combining rules, and the usual Lennard-Jones
Xe-Xe interaction. The resulting adsorption isotherms and calculated structures
are consistent with the results of LEED experimental data. The evolution of the
bulk film begins in the second layer, while the low coverage behavior is
epitaxial. This transition from 5-fold to 6-fold ordering is temperature
dependent, occurring earlier (at lower coverage) for the higher temperatures
Structures and topological transitions of hydrocarbon films on quasicrystalline surfaces
Lubricants can affect quasicrystalline coatings surfaces by modifying
commensurability of the interfaces. We report results of the first computer
simulation studies of physically adsorbed hydrocarbons on a quasicrystalline
surface: methane, propane, and benzene on decagonal Al-Ni-Co. The grand
canonical Monte Carlo method is employed, using novel Embedded Atom Method
potentials generated from it ab initio calculations, and standard hydrocarbon
interactions. The resulting adsorption isotherms and calculated structures show
the films' evolution from submonolayer to condensation. We discover the
presence and absence of the 5- to 6-fold topological transition, for benzene
and methane, respectively, in agreement with a previsouly formulated
phenomenological rule based on adsorbate-substrate size mismatch.Comment: 5 pages, 5 figure, 1 EPAPS-material.pd
Xe films on a decagonal Al-Ni-Co quasicrystal surface
The grand canonical Monte Carlo method is employed to study the adsorption of
Xe on a quasicrystalline Al-Ni-Co surface. The calculation uses a semiempirical
gas-surface interaction, based on conventional combining rules and the usual
Lennard-Jones Xe-Xe interaction. The resulting adsorption isotherms and
calculated structures are consistent with the results of LEED experimental
data. In this paper we focus on five features not discussed earlier (Phys. Rev.
Lett. 95, 136104 (2005)): the range of the average density of the adsorbate,
the order of the transition, the orientational degeneracy of the ground state,
the isosteric heat of adsorption of the system, and the effect of the vertical
cell dimension.Comment: 6 pages, 5 pic
Hidden phase in a two-dimensional Sn layer stabilized by modulation hole doping
Semiconductor surfaces and ultrathin interfaces exhibit an interesting variety of two-dimensional quantum matter phases, such as charge density waves, spin density waves and superconducting condensates. Yet, the electronic properties of these broken symmetry phases are extremely difficult to control due to the inherent difficulty of doping a strictly two-dimensional material without introducing chemical disorder. Here we successfully exploit a modulation doping scheme to uncover, in conjunction with a scanning tunnelling microscope tip-assist, a hidden equilibrium phase in a hole-doped bilayer of Sn on Si(111). This new phase is intrinsically phase separated into insulating domains with polar and nonpolar symmetries. Its formation involves a spontaneous symmetry breaking process that appears to be electronically driven, notwithstanding the lack of metallicity in this system. This modulation doping approach allows access to novel phases of matter, promising new avenues for exploring competing quantum matter phases on a silicon platform
Combined Forward-Backward Asymmetry Measurements in Top-Antitop Quark Production at the Tevatron
The CDF and D0 experiments at the Fermilab Tevatron have measured the asymmetry between yields of forward- and backward-produced top and antitop quarks based on their rapidity difference and the asymmetry between their decay leptons. These measurements use the full data sets collected in proton-antiproton collisions at a center-of-mass energy of TeV. We report the results of combinations of the inclusive asymmetries and their differential dependencies on relevant kinematic quantities. The combined inclusive asymmetry is . The combined inclusive and differential asymmetries are consistent with recent standard model predictions
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The forbidden beauty of quasicrystals
Twenty years ago last month Danny Shechtman of the Technion Institute in Israel announced the discovery of a new metallic alloy. At the time Shechtman had been on sabbatical leave at the National Bureau of Standards in Washington DC, investigating the properties of mixtures of metals that had been melted together and rapidly cooled. He found that one of these alloys – aluminium manganese – displayed a diffraction pattern with 10-fold rotational symmetry. However, such symmetries were supposed to be forbidden by the laws of crystallography