584 research outputs found
CO oxidation on a single Pd atom supported on magnesia
The oxidation of CO on single Pd atoms anchored to MgO(100) surface oxygen
vacancies is studied with temperature-programmed-reaction mass-spectrometry and
infrared spectroscopy. In one-heating-cycle experiments CO, formed from
O and CO preadsorbed at 90 K, is detected at 260 K and 500 K. Ab-initio
simulations suggest two reaction routes, with Pd(CO)O and Pd(CO)CO
found as precursors for the low and high temperature channels, respectively.
Both reactions result in annealing of the vacancy and induce migration and
coalescence of the remaining Pd-CO to form larger clusters.Comment: 4 pages, 3 figures, scheduled for publication in PRL 18 June 200
Supported magnetic nanoclusters: Softlanding of Pd clusters on a MgO surface
Low-energy deposition of neutral Pd_N clusters (N=2-7 and 13) on a MgO(001)
surface F-center (FC) was studied by spin-density-functional molecular dynamics
simulations. The incident clusters are steered by an attractive "funnel"
created by the FC, resulting in adsorption of the cluster, with one of its
atoms bonded atop of the FC. The deposited Pd_2-Pd_6 clusters retain their
gas-phase structures, while for N>6 surface-commensurate isomers are
energetically more favorable. Adsorbed clusters with N > 3 are found to remain
magnetic at the surface.Comment: 5 pages, 2 figs, Phys.Rev.Lett., accepte
Oxygen clamps in gold nanowires
We investigate how the insertion of an oxygen atom in an atomically thin gold
nanowire can affect its rupture. We find, using ab initio total energy density
functional theory calculations, that O atoms when inserted in gold nanowires
form not only stable but also very strong bonds, in such a way that they can
extract atoms from a stable tip, serving in this way as a clamp that could be
used to pull a string of gold atoms.Comment: 4 pages; 4 figure
Electron and boson clusters in confined geometries: symmetry breaking in quantum dots and harmonic traps
We discuss the formation of crystalline electron clusters in semiconductor
quantum dots and of crystalline patterns of neutral bosons in harmonic traps.
In a first example, we use calculations for two electrons in an elliptic
quantum dot to show that the electrons can localize and form a molecular dimer.
The calculated singlet-triplet splitting (J) as a function of the magnetic
field (B) agrees with cotunneling measurements, with its behavior reflecting
the effective dissociation of the dimer for large B. Knowledge of the dot shape
and of J(B) allows determination of the degree of entanglement. In a second
example, we study strongly repelling neutral bosons in two-dimensional harmonic
traps. Going beyond the Gross-Pitaevskii (GP) mean-field approximation, we show
that bosons can localize and form polygonal-ring-like crystalline patterns. The
total energy of the crystalline phase saturates in contrast to the GP solution,
and its spatial extent becomes smaller than that of the GP condensate.Comment: LATEX, 9 pages with 6 figures. To appear in Proc. Natl. Acad. Sci.
(USA). For related papers, see http://www.prism.gatech.edu/~ph274cy
Phase-Controlled Force and Magnetization Oscillations in Superconducting Ballistic Nanowires
The emergence of superconductivity-induced phase-controlled forces in the
(0.01-0.1) nN range, and of magnetization oscillations, in nanowire junctions,
is discussed. A giant magnetic response to applied weak magnetic fields, is
predicted in the ballistic Josephson junction formed by a superconducting tip
and a surface, bridged by a normal metal nanowire where Andreev states form.Comment: 5 pages, 3 figure
Premelting of Thin Wires
Recent work has raised considerable interest on the nature of thin metallic
wires. We have investigated the melting behavior of thin cylindrical Pb wires
with the axis along a (110) direction, using molecular dynamics and a
well-tested many-body potential. We find that---in analogy with cluster
melting---the melting temperature of a wire with radius is lower
than that of a bulk solid, , by . Surface melting
effects, with formation of a thin skin of highly diffusive atoms at the wire
surface, is observed. The diffusivity is lower where the wire surface has a
flat, local (111) orientation, and higher at (110) and (100) rounded areas. The
possible relevance to recent results on non-rupturing thin necks between an STM
tip and a warm surface is addressed.Comment: 10 pages, 4 postscript figures are appended, RevTeX, SISSA Ref.
131/94/CM/S
Energetics, forces, and quantized conductance in jellium modeled metallic nanowires
Energetics and quantized conductance in jellium modeled nanowires are
investigated using the local density functional based shell correction method,
extending our previous study of uniform in shape wires [C. Yannouleas and U.
Landman, J. Phys. Chem. B 101, 5780 (1997)] to wires containing a variable
shaped constricted region. The energetics of the wire (sodium) as a function of
the length of the volume conserving, adiabatically shaped constriction leads to
formation of self selecting magic wire configurations. The variations in the
energy result in oscillations in the force required to elongate the wire and
are directly correlated with the stepwise variations of the conductance of the
nanowire in units of 2e^2/h. The oscillatory patterns in the energetics and
forces, and the correlated stepwise variation in the conductance are shown,
numerically and through a semiclassical analysis, to be dominated by the
quantized spectrum of the transverse states at the narrowmost part of the
constriction in the wire.Comment: Latex/Revtex, 11 pages with 5 Postscript figure
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