7,516 research outputs found
The Importance of Charge Redistribution during Electrochemical Reactions: A Density Functional Theory Study of Silver Orthophosphate (Ag3PO4)
The structural sensitivity of silver orthophosphate (Ag 3 PO 4 ) for photo-electrochemical water oxidation on (100), (110) and (111) surfaces has recently been reported by experimental studies (D. J. Martin et al., Energy Environ. Sci., 2013, 6, 3380-3386). The (111) surface showed the highest performance with an oxygen evolution rate of 10 times higher than the other surfaces. The high performance of the (111) surface was attributed to high hole mobility, high surface energy and, in a recent theoretical study (Z. Ma et al., RSC Adv., 2017, 7, 23994-24003), to a lower OH adsorption energy and the band structure. The investigations are based on a few structures and a full atomistic picture of the Ag 3 PO 4 under electrochemical reactions is still missing. Therefore, we report here a systematic study of the oxygen evolution reaction (OER) of Ag 3 PO 4 (100), (110), and (111) surfaces by density functional theory (DFT) calculations. Through a detailed investigation of the reaction energies and the overpotentials of OER on all possible surface orientations with all possible terminations and different involvement of Ag adsorption sites, we can confirm that (111) surfaces are highly active. However, surface orientation was not found to exclusively determine the electrochemical activity; neither did the number of Ag atoms involved in the adsorption of the intermediate species nor the type of surface termination or the different potential determining reaction steps. By using Bader charge analysis and investigation of the charge redistribution during OER, we found that the highest activity, i.e. lowest overpotential, is related to the charge redistribution of two OER steps, namely the O ad and the HOO ad formation. If the charge redistribution between these steps is small, then the overpotential is small and, hence, the activity is high. Charge redistributions are usually small for the (111) surface and therefore the (111) surface is usually the most active one. The concept of charge redistribution being decisive for the high activity of Ag 3 PO 4 may open a new design strategy for materials with highly efficient electrochemical surfaces.</p
One-dimensional quasicrystals with power-law hopping
One-dimensional quasi-periodic systems with power-law hopping, ,
differ from both the standard Aubry-Azbel-Harper (AAH) model and from power-law
systems with uncorrelated disorder. Whereas in the AAH model all
single-particle states undergo a transition from ergodic to localized at a
critical quasi-disorder strength, short-range power-law hops with can
result in mobility edges. Interestingly, there is no localization for
long-range hops with , in contrast to the case of uncorrelated
disorder. Systems with long-range hops are rather characterized by
ergodic-to-multifractal edges and a phase transition from ergodic to
multifractal (extended but non ergodic) states. We show that both mobility and
ergodic-to-multifractal edges may be clearly revealed in experiments on
expansion dynamics.Comment: 5 pages, 5 figures, plus Supplementary material, revise
Fermi-surface induced modulation in an optimally doped YBCO superconductor
We have observed a Fermi-surface (FS) induced lattice modulation in a YBCO
superconductor with a wavevector along CuO chains, {\it i.e.} =(0,,0). The value of is twice the Fermi
wavevector () along {\bf b*} connecting nearly nested FS `ridges'.
The modulation exists only within O-vacancy-ordered islands
(characterized by = and persists well above and
below . Our results are consistent with the presence of a FS-induced
charge-density wave
Effect of borehole stress concentration on compressional wave velocity measurements
Formation elastic properties near a borehole may be altered from their original state due to the stress concentration around the borehole. This could lead to a biased estimation of formation elastic properties measured from sonic logging data. To study the effect of stress concentration around a borehole on sonic logging, we first use an iterative approach, which combines a rock physics model and a finite-element method, to calculate the stress-dependent elastic properties of the rock around a borehole when it is subjected to an anisotropic stress loading. Then we use the anisotropic elastic model obtained from the first step and a finite-difference method to simulate the acoustic response in a borehole. Our numerical results are consistent with published laboratory measurements of the azimuthal velocity variations caused by borehole stress concentration. Both numerical and experimental results show that the variation of P-wave velocity versus azimuth has broad maxima and cusped minima, which is different from the presumed cosine behavior. This is caused by the preference of the wavefield to propagate through a higher velocity region
Synthesizing Skyrmion Molecules in Fe-Gd Thin Films
We show that properly engineered amorphous Fe-Gd alloy thin films with
perpendicular magnetic anisotropy exhibit room-temperature skyrmion molecules,
or a pair of like-polarity, opposite-helicity skyrmions. Magnetic mirror
symmetry planes present in the stripe phase, instead of chiral exchange,
determine the internal skyrmion structure and the net achirality of the
skyrmion phase. Our study shows that stripe domain engineering in amorphous
alloy thin films may enable the creation of skyrmion phases with
technologically desirable properties.Comment: 15 pages, 6 figures. Accepted for publication in Applied Physics
Letter
Robust nonparametric detection of objects in noisy images
We propose a novel statistical hypothesis testing method for detection of
objects in noisy images. The method uses results from percolation theory and
random graph theory. We present an algorithm that allows to detect objects of
unknown shapes in the presence of nonparametric noise of unknown level and of
unknown distribution. No boundary shape constraints are imposed on the object,
only a weak bulk condition for the object's interior is required. The algorithm
has linear complexity and exponential accuracy and is appropriate for real-time
systems. In this paper, we develop further the mathematical formalism of our
method and explore important connections to the mathematical theory of
percolation and statistical physics. We prove results on consistency and
algorithmic complexity of our testing procedure. In addition, we address not
only an asymptotic behavior of the method, but also a finite sample performance
of our test.Comment: This paper initially appeared in 2010 as EURANDOM Report 2010-049.
Link to the abstract at EURANDOM repository:
http://www.eurandom.tue.nl/reports/2010/049-abstract.pdf Link to the paper at
EURANDOM repository: http://www.eurandom.tue.nl/reports/2010/049-report.pd
Multiple dynamical time-scales in networks with hierarchically nested modular organization
Many natural and engineered complex networks have intricate mesoscopic
organization, e.g., the clustering of the constituent nodes into several
communities or modules. Often, such modularity is manifested at several
different hierarchical levels, where the clusters defined at one level appear
as elementary entities at the next higher level. Using a simple model of a
hierarchical modular network, we show that such a topological structure gives
rise to characteristic time-scale separation between dynamics occurring at
different levels of the hierarchy. This generalizes our earlier result for
simple modular networks, where fast intra-modular and slow inter-modular
processes were clearly distinguished. Investigating the process of
synchronization of oscillators in a hierarchical modular network, we show the
existence of as many distinct time-scales as there are hierarchical levels in
the system. This suggests a possible functional role of such mesoscopic
organization principle in natural systems, viz., in the dynamical separation of
events occurring at different spatial scales.Comment: 10 pages, 4 figure
Bubbling and bistability in two parameter discrete systems
We present a graphical analysis of the mechanisms underlying the occurrences
of bubbling sequences and bistability regions in the bifurcation scenario of a
special class of one dimensional two parameter maps. The main result of the
analysis is that whether it is bubbling or bistability is decided by the sign
of the third derivative at the inflection point of the map function.Comment: LaTeX v2.09, 14 pages with 4 PNG figure
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