775 research outputs found
The strange-quark mass from QCD sum rules in the pseudoscalar channel
QCD Laplace transform sum rules, involving the axial-vector current
divergences, are used in order to determine the strange quark mass. The
two-point function is known in QCD up to four loops in perturbation theory, and
up to dimension-six in the non-perturbative sector. The hadronic spectral
function is reconstructed using threshold normalization from chiral symmetry,
together with experimental data for the two radial excitations of the kaon. The
result for the running strange quark mass, in the scheme at a scale
of 1 is: .Comment: 10 pages. Latex file. 2 Figures obtained from author CAD upon reques
Frequency Dielectric Spectroscopy and Dissipation Factor Measurements during Thermal Cycles on Different Types of MV Cable Joints
Results of periodic measurements of Dissipation Factor (DF) and Frequency Dielectric Spectroscopy (FDS) on Medium Voltage (MV) cables during thermal cycles, are reported and discussed in this paper. Twelve cables equipped with different types of joints were connected together to form a ring in short-circuit. The ring was energized at rated voltage and current. Temperature cycles with a period of one day (11 hours of heating) were applied by controlling the current. Periodically, each cable was tested separately using both DF and FDS techniques to monitor the ageing progression. This paper focuses on the experimental results obtained testing six new cables provided by different types of new joints while the other six coming from the field, are not considered here. Results obtained testing the six new cables indicate that FDS evaluation is a reliable tool to distinguish the aging progression of the insulation from the seasonal and reversible changes while DF values recorded at different voltage levels show a non-regular behavior particularly in XLPE cables. These results clearly suggest that the systematic use of FDS in condition assessment of underground cables can provide more information on the ageing progression than the traditional DF measurements
Effect of the surface model on the theoretical description of the chemisorption of atomic hydrogen on Cu(001)
Adsorption at surfaces can be modelled using a periodic supercell approach or using finite clusters. For many systems and properties these models are complementary and often the most productive way to work is to use a combination of these techniques. If reliable data is to be obtained it is essential that convergence is achieved with respect to the size of supercell and cluster. This work discusses the convergence of chemisorption properties of H on Cu(001) with respect to the cluster size. To this end calculations of the H binding energy and equilibrium distance, are reported for cluster models of increasing size containing up to 77 metal atoms. Likewise, periodic slab model calculations are used to provide the corresponding values towards which the cluster approach should converge. In many previous studies of a wide variety of systems it has been established that computed equilibrium distances converge rapidly with respect to cluster size. Here, a systematic study of the dependence on cluster size shows that, for adsorption in the 4-fold site, convergence is not achieved even for very large clusters. The reason for this poor convergence is seen to be the inability of the cluster model to reproduce accurately the charge density and electrostatic potential of the crystalline surface
Metastable precursors during the oxidation of the Ru(0001) surface
Using density-functional theory, we predict that the oxidation of the
Ru(0001) surface proceeds via the accumulation of sub-surface oxygen in
two-dimensional islands between the first and second substrate layer. This
leads locally to a decoupling of an O-Ru-O trilayer from the underlying metal.
Continued oxidation results in the formation and stacking of more of these
trilayers, which unfold into the RuO_2(110) rutile structure once a critical
film thickness is exceeded. Along this oxidation pathway, we identify various
metastable configurations. These are found to be rather close in energy,
indicating a likely lively dynamics between them at elevated temperatures,
which will affect the surface chemical and mechanical properties of the
material.Comment: 11 pages including 9 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Adlayer core-level shifts of random metal overlayers on transition-metal substrates
We calculate the difference of the ionization energies of a core-electron of
a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a
fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using
density-functional-theory. We analyze the initial-state contributions and the
screening effects induced by the core hole, and study the influence of the
alloy composition for a number of noble metal-transition metal systems. Data
are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x)
Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our
analysis clearly indicates the importance of final-state screening effects for
the interpretation of measured core-level shifts. Calculated deviations from
the initial-state trends are explained in terms of the change of inter- and
intra-atomic screening upon alloying. A possible role of alloying on the
chemical reactivity of metal surfaces is discussed.Comment: 4 pages, 2 figures, Phys. Rev. Letters, to appear in Feb. 199
The structure of epitaxial V2O3 films and their surfaces : a medium energy ion scattering study
Medium energy ion scattering, using 100 keV H+ incident ions, has been used to investigate the growth of epitaxial films, up to thicknesses of ~200 Å, of V2O3 on both Pd(111) and Au(111). Scattered-ion energy spectra provide a measure of the average film thickness and the variations in this thickness, and show that, with suitable annealing, the crystalline quality is good. Plots of the scattering yield as a function of scattering angle, so-called blocking curves, have been measured for two different incidence directions and have been used to determine the surface structure. Specifically, scattering simulations for a range of different model structures show poor agreement with experiment for half-metal (….V’O3V) and vanadyl (….V’O3V=O) terminations, with and without surface interlayer relaxations. However, good agreement with experiment is found for the modified oxygen-termination structure, first proposed by Kresse et al., in which a subsurface V half-metal layer is moved up into the outermost V buckled metal layer to produce a VO2 overlayer on the underlying V2O3, with an associated layer structure of ….O3VV’’V’O3
Stability of sub-surface oxygen at Rh(111)
Using density-functional theory (DFT) we investigate the incorporation of
oxygen directly below the Rh(111) surface. We show that oxygen incorporation
will only commence after nearly completion of a dense O adlayer (\theta_tot =
1.0 monolayer) with O in the fcc on-surface sites. The experimentally suggested
octahedral sub-surface site occupancy, inducing a site-switch of the on-surface
species from fcc to hcp sites, is indeed found to be a rather low energy
structure. Our results indicate that at even higher coverages oxygen
incorporation is followed by oxygen agglomeration in two-dimensional
sub-surface islands directly below the first metal layer. Inside these islands,
the metastable hcp/octahedral (on-surface/sub-surface) site combination will
undergo a barrierless displacement, introducing a stacking fault of the first
metal layer with respect to the underlying substrate and leading to a stable
fcc/tetrahedral site occupation. We suggest that these elementary steps,
namely, oxygen incorporation, aggregation into sub-surface islands and
destabilization of the metal surface may be more general and precede the
formation of a surface oxide at close-packed late transition metal surfaces.Comment: 9 pages including 9 figure files. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Surface Core Level Shifts of Clean and Oxygen Covered Ru(0001)
We have performed high resolution XPS experiments of the Ru(0001) surface,
both clean and covered with well-defined amounts of oxygen up to 1 ML coverage.
For the clean surface we detected two distinct components in the Ru 3d_{5/2}
core level spectra, for which a definite assignment was made using the high
resolution Angle-Scan Photoelectron Diffraction approach. For the p(2x2),
p(2x1), (2x2)-3O and (1x1)-O oxygen structures we found Ru 3d_{5/2} core level
peaks which are shifted up to 1 eV to higher binding energies. Very good
agreement with density functional theory calculations of these Surface Core
Level Shifts (SCLS) is reported. The overriding parameter for the resulting Ru
SCLSs turns out to be the number of directly coordinated O atoms. Since the
calculations permit the separation of initial and final state effects, our
results give valuable information for the understanding of bonding and
screening at the surface, otherwise not accessible in the measurement of the
core level energies alone.Comment: 16 pages including 10 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
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