1,379 research outputs found
Software Engineering Laboratory (SEL). Data base organization and user's guide, revision 1
The structure of the Software Engineering Laboratory (SEL) data base is described. It defines each data base file in detail and provides information about how to access and use the data for programmers and other users. Several data base reporting programs are described also
Activated O2 dissociation and formation of oxide islands on the Be(0001) surface: Another atomistic model for metal oxidation
By simulating the dissociation of O2 molecules on the Be(0001) surface using
the first-principles molecular dynamics approach, we propose a new atomistic
model for the surface oxidation of sp metals. In our model, only the
dissociation of the first oxygen molecule needs to overcome an energy barrier,
while the subsequent oxygen molecules dissociate barrierlessly around the
adsorption area. Consequently, oxide islands form on the metal surface, and
grow up in a lateral way. We also discover that the firstly dissociated oxygen
atoms are not so mobile on the Be(0001) surface, as on the Al(111) surface. Our
atomistic model enlarges the knowledge on metal surface oxidations by perfectly
explaining the initial stage during the surface oxidation of Be, and might be
applicable to some other sp metal surfaces.Comment: 5 pages, 4 figure
First-principles calculations for the adsorption of water molecules on the Cu(100) surface
First-principles density-functional theory and supercell models are employed
to calculate the adsorption of water molecules on the Cu(100) surface. In
agreement with the experimental observations, the calculations show that a H2O
molecule prefers to bond at a one-fold on-top (T1) surface site with a tilted
geometry. At low temperatures, rotational diffusion of the molecular axis of
the water molecules around the surface normal is predicted to occur at much
higher rates than lateral diffusion of the molecules. In addition, the
calculated binding energy of an adsorbed water molecule on the surfaces is
significantly smaller than the water sublimation energy, indicating a tendency
for the formation of water clusters on the Cu(100) surface.Comment: 5 pages, 3 figures, submitted to Phys. Rev.
Path integral Monte Carlo simulations of silicates
We investigate the thermal expansion of crystalline SiO in the --
cristobalite and the -quartz structure with path integral Monte Carlo
(PIMC) techniques. This simulation method allows to treat low-temperature
quantum effects properly. At temperatures below the Debye temperature, thermal
properties obtained with PIMC agree better with experimental results than those
obtained with classical Monte Carlo methods.Comment: 27 pages, 10 figures, Phys. Rev. B (in press
Spin-Orbit Coupling in Iridium-Based 5d Compounds Probed by X-ray Absorption Spectroscopy
We have performed x-ray absorption spectroscopy (XAS) measurements on a
series of Ir-based 5d transition metal compounds, including Ir, IrCl3, IrO2,
Na2IrO3, Sr2IrO4, and Y2Ir2O7. By comparing the intensity of the "white-line"
features observed at the Ir L2 and L3 absorption edges, it is possible to
extract valuable information about the strength of the spin-orbit coupling in
these systems. We observe remarkably large, non-statistical branching ratios in
all Ir compounds studied, with little or no dependence on chemical composition,
crystal structure, or electronic state. This result confirms the presence of
strong spin-orbit coupling effects in novel iridates such as Sr2IrO4, Na2IrO3,
and Y2Ir2O7, and suggests that even simple Ir-based compounds such as IrO2 and
IrCl3 may warrant further study. In contrast, XAS measurements on Re-based 5d
compounds, such as Re, ReO2, ReO3, and Ba2FeReO6, reveal statistical branching
ratios and negligible spin-orbit coupling effects.Comment: 9 pages, 4 figure
Phonon dispersion and electron-phonon coupling in MgB_2 and AlB_2
We present a first principles investigation of the lattice dynamics and
electron-phonon coupling of the superconductor MgB_2 and the isostructural
AlB_2 within the framework of density functional perturbation theory using a
mixed-basis pseudopotential method. Complete phonon dispersion curves and
Eliashberg functions \alpha^2F are calculated for both systems. We also report
on Raman measurements, which support the theoretical findings. The calculated
generalized density-of-states for MgB_2 is in excellent agreement with recent
neutron-scattering experiments. The main differences in the calculated phonon
spectra and \alpha^2F are related to high frequency in-plane boron vibrations.
As compared to AlB_2, they are strongly softened in MgB_2 and exhibit an
exceptionally strong coupling to electronic states at the Fermi energy. The
total coupling constants are \lambda_{MgB_2}=0.73 and \lambda_{AlB_2}=0.43.
Implications for the superconducting transition temperature are briefly
discussed.Comment: 10 pages, 4 figures, to appear in Phys. Rev. Let
Low Temperature Measurements by Infrared Spectroscopy in CoFeO Ceramic
In this paper results of new far-infrared and middle-infrared measurements
(wavenumber range of 4000cm-1 - 100cm-1) in the range of the temperature from
300K to 8K of the CoFe2O4 ceramic are presented. The bands positions and their
shapes are the same in the wide temperature range. The quality of the sample
was investigated by X-ray, EDS and EPMA studies. The CoFe2O4 reveals the cubic
structure (Fd-3m) in the temperature range from 85K to 360 K without any traces
of distortion. On the current level of knowledge the polycrystalline CoFe2O4
does not exhibit phase transition in the temperature range from 8 K to 300 K.Comment: 10 pages, 6 figure
Simulation of thermal conductivity and heat transport in solids
Using molecular dynamics (MD) with classical interaction potentials we
present calculations of thermal conductivity and heat transport in crystals and
glasses. Inducing shock waves and heat pulses into the systems we study the
spreading of energy and temperature over the configurations. Phonon decay is
investigated by exciting single modes in the structures and monitoring the time
evolution of the amplitude using MD in a microcanonical ensemble. As examples,
crystalline and amorphous modifications of Selenium and are
considered.Comment: Revtex, 8 pages, 11 postscript figures, accepted for publication in
PR
Phase Decomposition and Chemical Inhomogeneity in Nd2-xCexCuO4
Extensive X-ray and neutron scattering experiments and additional
transmission electron microscopy results reveal the partial decomposition of
Nd2-xCexCuO4 (NCCO) in a low-oxygen-fugacity environment such as that typically
realized during the annealing process required to create a superconducting
state. Unlike a typical situation in which a disordered secondary phase results
in diffuse powder scattering, a serendipitous match between the in-plane
lattice constant of NCCO and the lattice constant of one of the decomposition
products, (Nd,Ce)2O3, causes the secondary phase to form an oriented,
quasi-two-dimensional epitaxial structure. Consequently, diffraction peaks from
the secondary phase appear at rational positions (H,K,0) in the reciprocal
space of NCCO. Additionally, because of neodymium paramagnetism, the
application of a magnetic field increases the low-temperature intensity
observed at these positions via neutron scattering. Such effects may mimic the
formation of a structural superlattice or the strengthening of
antiferromagnetic order of NCCO, but the intrinsic mechanism may be identified
through careful and systematic experimentation. For typical reduction
conditions, the (Nd,Ce)2O3 volume fraction is ~1%, and the secondary-phase
layers exhibit long-range order parallel to the NCCO CuO2 sheets and are 50-100
angstromsthick. The presence of the secondary phase should also be taken into
account in the analysis of other experiments on NCCO, such as transport
measurements.Comment: 15 pages, 17 figures, submitted to Phys. Rev.
Structural stability of Fe5Si3 and Ni2Si studied by high-pressure x-ray diffraction and ab initio total-energy calculations
We performed high-pressure angle dispersive x-ray diffraction measurements on
Fe5Si3 and Ni2Si up to 75 GPa. Both materials were synthesized in bulk
quantities via a solid-state reaction. In the pressure range covered by the
experiments, no evidence of the occurrence of phase transitions was observed.
On top of that, Fe5Si3 was found to compress isotropically, whereas an
anisotropic compression was observed in Ni2Si. The linear incompressibility of
Ni2Si along the c-axis is similar in magnitude to the linear incompressibility
of diamond. This fact is related to the higher valence-electron charge density
of Ni2Si along the c-axis. The observed anisotropic compression of Ni2Si is
also related to the layered structure of Ni2Si where hexagonal layers of Ni2+
cations alternate with graphite-like layers formed by (NiSi)2- entities. The
experimental results are supported by ab initio total-energy calculations
carried out using density functional theory and the pseudopotential method. For
Fe5Si3, the calculations also predicted a phase transition at 283 GPa from the
hexagonal P63/mcm phase to the cubic structure adopted by Fe and Si in the
garnet Fe5Si3O12. The room-temperature equations of state for Fe5Si3 and Ni2Si
are also reported and a possible correlation between the bulk modulus of iron
silicides and the coordination number of their minority element is discussed.
Finally, we report novel descriptions of these structures, in particular of the
predicted high-pressure phase of Fe5Si3 (the cation subarray in the garnet
Fe5Si3O12), which can be derived from spinel Fe2SiO4 (Fe6Si3O12).Comment: 44 pages, 13 figures, 3 Table
- …