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, $1/r^a$, 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 $a>1$ can result in mobility edges. Interestingly, there is no localization for long-range hops with $a\leq 1$, 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.} ${\bf q}_1$=(0,$\delta$,0). The value of $\delta\sim0.21$ is twice the Fermi wavevector ($2{\bf k}_F$) along {\bf b*} connecting nearly nested FS `ridges'. The ${\bf q}_1$ modulation exists only within O-vacancy-ordered islands (characterized by ${\bf q}_0$=$(\frac14,0,0))$ and persists well above and below $T_c$. 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