1,023 research outputs found
Homogeneous Gold Catalysis through Relativistic Effects: Addition of Water to Propyne
In the catalytic addition of water to propyne the Au(III) catalyst is not
stable under non-relativistic conditions and dissociates into a Au(I) compound
and Cl2. This implies that one link in the chain of events in the catalytic
cycle is broken and relativity may well be seen as the reason why Au(III)
compounds are effective catalysts.Comment: 12 pages, 3 figures, 1 tabl
Electronic structure and dimerization of a single monatomic gold wire
The electronic structure of a single monatomic gold wire is presented for the
first time. It has been obtained with state-of-the-art ab-initio full-potential
density-functional (DFT) LMTO (linearized muffin-tin orbital) calculations
taking into account relativistic effects. For stretched structures in the
experimentally accessible range the conduction band is exactly half-filled,
whereas the band structures are more complex for the optimized structure. By
studying the total energy as a function of unit-cell length and of a possible
bond-length alternation we find that the system can lower its total energy by
letting the bond lengths alternate leading to a structure containing separated
dimers with bond lengths of about 2.5 \AA, largely independent of the
stretching. However, first for fairly large unit cells (above roughly 7 \AA),
is the total-energy gain upon this dimerization comparable with the energy
costs upon stretching. We propose that this together with band-structure
effects is the reason for the larger interatomic distances observed in recent
experiments. We find also that although spin-orbit couplings lead to
significant effects on the band structure, the overall conclusions are not
altered, and that finite Au_2, Au_4, and Au_6 chains possess electronic
properties very similar to those of the infinite chain.Comment: (14 pages, 5 figures; Elsevier Preprint style elsart.sty
Higher order Schrodinger and Hartree-Fock equations
The domain of validity of the higher-order Schrodinger equations is analyzed
for harmonic-oscillator and Coulomb potentials as typical examples. Then the
Cauchy theory for higher-order Hartree-Fock equations with bounded and Coulomb
potentials is developed. Finally, the existence of associated ground states for
the odd-order equations is proved. This renders these quantum equations
relevant for physics.Comment: 19 pages, to appear in J. Math. Phy
Embedding Transition-Metal Atoms in Graphene: Structure, Bonding, and Magnetism
We present a density-functional-theory study of transition-metal atoms (Sc–Zn, Pt, and Au) embedded in single and double vacancies (SV and DV) in a graphene sheet. We show that for most metals, the bonding is strong and the metal-vacancy complexes exhibit interesting magnetic behavior. In particular, an Fe atom on a SV is not magnetic, while the Fe@DV complex has a high magnetic moment. Surprisingly, Au and Cu atoms at SV are magnetic. Both bond strengths and magnetic moments can be understood within a simple local-orbital picture, involving carbon sp2 hybrids and the metal spd orbitals. We further calculate the barriers for impurity-atom migration, and they agree well with available experimental data. We discuss the experimental realization of such systems in the context of spintronics and nanocatalysis.Peer reviewe
Common Origin for Surface Reconstruction and the Formation of Chains of Metal Atoms
During the fracture of nanocontacts gold spontaneously forms freely suspended
chains of atoms, which is not observed for the iso-electronic noble metals Ag
and Cu. Au also differs from Ag and Cu in forming reconstructions at its
low-index surfaces. Using mechanically controllable break junctions we show
that all the 5d metals that show similar reconstructions (Ir, Pt and Au) also
form chains of atoms, while both properties are absent in the 4d neighbor
elements (Rh, Pd, Ag), indicating a common origin for these two phenomena. A
competition between s and d bonding is proposed as an explanation
g factor of Li-like ions with nonzero nuclear spin
The fully relativistic theory of the g factor of Li-like ions with nonzero
nuclear spin is considered for the (1s)^2 2s state. The magnetic-dipole
hyperfine-interaction correction to the atomic g factor is calculated including
the one-electron contributions as well as the interelectronic-interaction
effects of order 1/Z. This correction is combined with the
interelectronic-interaction, QED, nuclear recoil, and nuclear size corrections
to obtain high-precision theoretical values for the g factor of Li-like ions
with nonzero nuclear spin. The results can be used for a precise determination
of nuclear magnetic moments from g factor experiments.Comment: 20 pages, 5 figure
Origin of anomalously long interatomic distances in suspended gold chains
The discovery of long bonds in gold atom chains has represented a challenge
for physical interpretation. In fact, interatomic distances frequently attain
3.0-3.6 A values and, distances as large as 5.0 A may be seldom observed. Here,
we studied gold chains by transmission electron microscopy and performed
theoretical calculations using cluster ab initio density functional formalism.
We show that the insertion of two carbon atoms is required to account for the
longest bonds, while distances above 3 A may be due to a mixture of clean and
one C atom contaminated bonds.Comment: 4 pages, 4 Postscript figures, to be published in Physical Review
Letter
Cross-cultural analysis of attention disengagement times supports the dissociation of faces and patterns in the infant brain
CITATION: Pyykko, J., et al. 2019. Cross-cultural analysis of attention disengagement times supports the dissociation of faces and patterns in the infant brain. Scientific Reports, 9:14414, doi:10.1038/s41598-019-51034-x.The original publication is available at https://www.nature.comInfants are slower to disengage from faces than non-face patterns when distracted by novel competing stimuli. While this perceptual predilection for faces is well documented, its universality and mechanisms in relation to other aspects of attention are poorly understood. We analysed attention disengagement times for faces and non-face patterns in a large sample of 6-to 9-month-old infants (N = 637), pooled from eye tracking studies in socioculturally diverse settings (Finland, Malawi, South Africa). Disengagement times were classified into distinct groups of quick and delayed/censored responses by unsupervised clustering. Delayed disengagement was frequent for faces (52.1% of trials), but almost negligible for patterns (3.9% of trials) in all populations. The magnitude of this attentional bias varied by individuals, whereas the impact of situational factors and facial expression was small. Individual variations in disengagement from faces were moderately stable within testing sessions and independent from variations in disengagement times for patterns. These results point to a fundamental dissociation of face and pattern processing in infants and demonstrate that the bias for faces can be robust against distractors and habituation. The results raise the possibility that attention to faces varies as an independent, early-emerging social trait in populations.https://www.nature.com/articles/s41598-019-51034-xPublisher's versio
Study protocol: the impact of growth charts and nutritional supplements on child growth in Zambia (ZamCharts): a cluster randomized controlled trial
https://www.researchsquare.com/article/rs-2816403/v1First author draf
Cohesive properties of alkali halides
We calculate cohesive properties of LiF, NaF, KF, LiCl, NaCl, and KCl with
ab-initio quantum chemical methods. The coupled-cluster approach is used to
correct the Hartree-Fock crystal results for correlations and to systematically
improve cohesive energies, lattice constants and bulk moduli. After inclusion
of correlations, we recover 95-98 % of the total cohesive energies. The lattice
constants deviate from experiment by at most 1.1 %, bulk moduli by at most 8 %.
We also find good agreement for spectroscopic properties of the corresponding
diatomic molecules.Comment: LaTeX, 10 pages, 1 figure, accepted by Phys. Rev.
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