1,289 research outputs found
Formation of an Icosahedral Structure during the Freezing of Gold Nanoclusters: Surface-Induced Mechanism
The freezing behavior of gold nanoclusters was studied by employing molecular
dynamics simulations based on a semi-empirical embedded-atom method.
Investigations of the gold nanoclusters revealed that, just after freezing,
ordered nano-surfaces with a fivefold symmetry were formed with interior atoms
remaining in the disordered state. Further lowering of temperatures induced
nano-crystallization of the interior atoms that proceeded from the surface
towards the core region, finally leading to an icosahedral structure. These
dynamic processes explain why the icosahedral cluster structure is dominantly
formed in spite of its energetic metastability.Comment: 9 pages, 4 figures(including 14 eps-files
Solid-liquid phase coexistence and structural transitions in palladium clusters
We use molecular dynamics with an embedded atom potential to study the
behavior of palladium nanoclusters near the melting point in the microcanonical
ensemble. We see transitions from both fcc and decahedral ground state
structures to icosahedral structures prior to melting over a range of cluster
sizes. In all cases this transition occurs during solid-liquid phase
coexistence and the mechanism for the transition appears to be fluctuations in
the molten fraction of the cluster and subsequent recrystallization into the
icosahedral structure.Comment: 8 pages, 6 figure
Critical Level Statistics of the Fibonacci Model
We numerically analyze spectral properties of the Fibonacci model which is a
one-dimensional quasiperiodic system. We find that the energy levels of this
model have the distribution of the band widths obeys and , the gap
distribution () .
We also compare the results with those of multi-scale Cantor sets. We find
qualitative differences between the spectra of the Fibonacci model and the
multi-scale Cantor sets.Comment: 7 page
Theoretical study of the thermal behavior of free and alumina-supported Fe-C nanoparticles
The thermal behavior of free and alumina-supported iron-carbon nanoparticles
is investigated via molecular dynamics simulations, in which the effect of the
substrate is treated with a simple Morse potential fitted to ab initio data. We
observe that the presence of the substrate raises the melting temperature of
medium and large nanoparticles ( = 0-0.16, = 80-1000, non-
magic numbers) by 40-60 K; it also plays an important role in defining the
ground state of smaller Fe nanoparticles ( = 50-80). The main focus of our
study is the investigation of Fe-C phase diagrams as a function of the
nanoparticle size. We find that as the cluster size decreases in the
1.1-1.6-nm-diameter range the eutectic point shifts significantly not only
toward lower temperatures, as expected from the Gibbs-Thomson law, but also
toward lower concentrations of C. The strong dependence of the maximum C
solubility on the Fe-C cluster size may have important implications for the
catalytic growth of carbon nanotubes by chemical vapor deposition.Comment: 13 pages, 11 figures, higher quality figures can be seen in article 9
at http://alpha.mems.duke.edu/wahyu
Slave-Boson Three-Band Model with O-O Hopping for High-Tc Superconductors
Slave boson mean-field approximation is carried out analytically for weakly
doped CuO_2 conduction planes, characterized by Cu-O charge transfer energy
\Delta_{pd}, Cu-O hopping t_0, O-O hopping t' and repulsion U_d between holes
on Cu site taken as infinite. At zero doping \delta, finite negative
t',|t'|<t_0/2, expands the range of stability of the covalent, conducting state
on the expense of the insulating state which, however, remains stable at larger
\Delta_{pd}. For sufficiently large \Delta_{pd} the renormalized charge
transfer energy saturates at 4|t'| instead of decreasing to zero, as at t'=0
case. In contrast to t', finite \delta suppresses the insulating state nearly
symmetrically with respect to the sign of \delta. The regime with charge
transfer energy renormalized close to 4|t'| fits remarkably well the ARPES
spectra of Bi2212 and LSCO, and, in the latter case, explains the observed
strong doping dependence of the Cu-O hopping.Comment: 4 pages, 2 figure
Effect of Stripes on Electronic States in Underdoped La_{2-x}Sr_xCuO_4
We investigate the electronic states of underdoped La_{2-x}Sr_xCuO_4 (LSCO)
by using a microscopic model, i.e., t-t'-t''-J model, containing vertical
charge stripes. The numerically exact diagonalization calculation on small
clusters shows the consistent explanation of the physical properties in the
angle-resolved photoemission, neutron magnetic scattering and optical
conductivity experiments such as the antiphase domain and quasi-one-dimensional
charge transport. The pair correlation function of the d-channel is suppressed
by the stripes. These results demonstrate a crucial role of the stripes in LSCOComment: 4 pages, 4 EPS figures, revised version, to appear in Phys. Rev.
Lett. Vol.82, No.25, 199
Electron-Like Fermi Surface and Remnant (pi,0) Feature in Overdoped La1.78Sr0.22CuO4
We have performed an angle-resolved photoemission study of overdoped
La1.78Sr0.22CuO4, and have observed sharp nodal quasiparticle peaks in the
second Brillouin zone that are comparable to data from Bi2Sr2CaCu2O8+d. The
data analysis using energy distribution curves, momentum distribution curves
and intensity maps all show evidence of an electron-like Fermi surface, which
is well explained by band structure calculations. Evidence for many-body
effects are also found in the substantial spectral weight remaining below the
Fermi level around (pi,0), where the band is predicted to lie above EF.Comment: 4 pages, 4 figure
Theory of Electron Differentiation, Flat Dispersion and Pseudogap Phenomena
Aspects of electron critical differentiation are clarified in the proximity
of the Mott insulator. The flattening of the quasiparticle dispersion appears
around momenta and on square lattices and determines the
criticality of the metal-insulator transition with the suppressed coherence in
that momentum region of quasiparticles. Such coherence suppression at the same
time causes an instability to the superconducting state if a proper incoherent
process is retained. The d-wave pairing interaction is generated from such
retained processes without disturbance from the coherent single-particle
excitations. Pseudogap phenomena widely observed in the underdoped cuprates are
then naturally understood from the mode-mode coupling of d-wave
superconducting(dSC) fluctuations with antiferromagnetic ones. When we assume
the existence of a strong d-wave pairing force repulsively competing with
antiferromagnetic(AFM) fluctuations under the formation of flat and damped
single-particle dispersion, we reproduce basic properties of the pseudogap seen
in the magnetic resonance, neutron scattering, angle resolved photoemission and
tunneling measurements in the cuprates.Comment: 9 pages including 2 figures, to appear in J. Phys. Chem. Solid
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