1,321 research outputs found
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
Structural models for the Si(553)-Au atomic chain reconstruction
Recent photoemission experiments on the Si(553)-Au reconstruction show a
one-dimensional band with a peculiar ~1/4 filling. This band could provide an
opportunity for observing large spin-charge separation if electron-electron
interactions could be increased. To this end, it is necessary to understand in
detail the origin of this surface band. A first step is the determination of
the structure of the reconstruction. We present here a study of several
structural models using first-principles density functional calculations. Our
models are based on a plausible analogy with the similar and better known
Si(557)-Au surface, and compared against the sole structure proposed to date
for the Si(553)-Au system [Crain JN et al., 2004 Phys. Rev. B 69 125401 ].
Results for the energetics and the band structures are given. Lines for the
future investigation are also sketched
Diffusion of Pt dimers on Pt(111)
We report the results of a density-functional study of the diffusion of Pt
dimers on the (111) surface of Pt. The calculated activation energy of 0.37 eV
is in {\em exact} agreement with the recent experiment of Kyuno {\em et al.}
\protect{[}Surf. Sci. {\bf 397}, 191 (1998)\protect{]}. Our calculations
establish that the dimers are mobile at temperatures of interest for adatom
diffusion, and thus contribute to mass transport. They also indicate that the
diffusion path for dimers consists of a sequence of one-atom and (concerted)
two-atom jumps.Comment: Pour pages postscript formatted, including one figure; submitted to
Physical Review B; other papers of interest can be found at url
http://www.centrcn.umontreal.ca/~lewi
Strain-Dependence of Surface Diffusion: Ag on Ag(111) and Pt(111)
Using density-functional theory with the local-density approximation and the
generalized gradient approximation we compute the energy barriers for surface
diffusion for Ag on Pt(111), Ag on one monolayer of Ag on Pt(111), and Ag on
Ag(111). The diffusion barrier for Ag on Ag(111) is found to increase linearly
with increasing lattice constant. We also discuss the reconstruction that has
been found experimentally when two Ag layers are deposited on Pt(111). Our
calculations explain why this strain driven reconstruction occurs only after
two Ag layers have been deposited.Comment: 4 pages, 3 figures, Phys. Rev. B 55 (1997), in pres
Projection of plane-wave calculations into atomic orbitals
The projection of the eigenfunctions obtained in standard plane-wave
first-principle electronic-structure calculations into atomic-orbital basis
sets is proposed as a formal and practical link between the methods based on
plane waves and the ones based on atomic orbitals. Given a candidate atomic
basis, ({\it i}) its quality is evaluated by its projection into the plane-wave
eigenfunctions, ({\it ii}) it is optimized by maximizing that projection, ({\it
iii}) the associated tight-binding Hamiltonian and energy bands are obtained,
and ({\it iv}) population analysis is performed in a natural way. The proposed
method replaces the traditional trial-and-error procedures of finding
appropriate atomic bases and the fitting of bands to obtain tight-binding
Hamiltonians. Test calculations of some zincblende semiconductors are
presented.Comment: RevTex. 4 pages. 3 uuencoded compressed (tared) postscript figs. To
appear in Solid St. Commu
A mixed ultrasoft/normconserved pseudopotential scheme
A variant of the Vanderbilt ultrasoft pseudopotential scheme, where the
normconservation is released for only one or a few angular channels, is
presented. Within this scheme some difficulties of the truly ultrasoft
pseudopotentials are overcome without sacrificing the pseudopotential softness.
i) Ghost states are easily avoided without including semicore shells. ii) The
ultrasoft pseudo-charge-augmentation functions can be made more soft. iii) The
number of nonlocal operators is reduced. The scheme will be most useful for
transition metals, and the feasibility and accuracy of the scheme is
demonstrated for the 4d transition metal rhodium.Comment: 4 pages, 2 figure
Electrons in Dry DNA from Density Functional Calculations
The electronic structure of an infinite poly-guanine - poly-cytosine DNA
molecule in its dry A-helix structure is studied by means of density-functional
calculations. An extensive study of 30 nucleic base pairs is performed to
validate the method. The electronic energy bands of DNA close to the Fermi
level are then analyzed in order to clarify the electron transport properties
in this particularly simple DNA realization, probably the best suited candidate
for conduction. The energy scale found for the relevant band widths, as
compared with the energy fluctuations of vibrational or genetic-sequence
origin, makes highly implausible the coherent transport of electrons in this
system. The possibility of diffusive transport with sub-nanometer mean free
paths is, however, still open. Information for model Hamiltonians for
conduction is provided.Comment: 8 pages, 4 figure
First principles study of the adsorption of C60 on Si(111)
The adsorption of C60 on Si(111) has been studied by means of
first-principles density functional calculations.
A 2x2 adatom surface reconstruction was used to simulate the terraces of the
7x7 reconstruction.
The structure of several possible adsorption configurations was optimized
using the ab initio atomic forces, finding good candidates for two different
adsorption states observed experimentally.
While the C60 molecule remains closely spherical, the silicon substrate
appears quite soft, especially the adatoms, which move substantially to form
extra C-Si bonds, at the expense of breaking Si-Si bonds.
The structural relaxation has a much larger effect on the adsorption
energies, which strongly depend on the adsorption configuration, than on the
charge transfer.Comment: 4 pages with 3 postscript figures, to appear in Surf. Science.
(proceedings of the European Conference on Surface Science ECOSS-19, Sept
2000
Optical properties of the vibrations in charged C molecules
The transition strengths for the four infrared-active vibrations of charged
C molecules are evaluated in self-consistent density functional theory
using the local density approximation. The oscillator strengths for the second
and fourth modes are strongly enhanced relative to the neutral C
molecule, in good agreement with the experimental observation of ``giant
resonances'' for those two modes. Previous theory, based on a ``charged
phonon'' model, predicted a quadratic dependence of the oscillator strength on
doping, but this is not borne out in our calculations.Comment: 10 pages, RevTeX3.
Surface Magnetic Phase Diagram of Tetragonal Manganites
To gain insights into the fundamental and characteristic features of the
surface of doped manganites, we constructed a general magnetic phase diagram of
LaSrMnO (001) surfaces in the plane spanned by and the
bulk tetragonal distortion , from the first-principles calculations. We
found that the surfaces are quite different from the bulk in the sense that
both the (La, Sr)O and MnO terminated surfaces show strong tendency toward
antiferromagnetism (A-type and C-type respectively). The basic physics
governing the phase diagram can be understood in terms of the surface orbital
polarizations. It is also found that the strong surface segregation of Sr atoms
is mostly caused by the electrostatic interaction and will further enhance the
tendency to surface antiferromagnetism.Comment: 3 figure
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