Bond Paths Are Not Chemical Bonds Bader, Richard F. W
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
09/2009, Letnik:
113, Številka:
38
Journal Article
Recenzirano
This account takes to task papers that criticize the definition of a bond path as a criterion for the bonding between the atoms it links by mistakenly identifying it with a chemical bond. It is ...argued that the notion of a chemical bond is too restrictive to account for the physics underlying the broad spectrum of interactions between atoms and molecules that determine the properties of matter. A bond path on the other hand, as well as being accessible to experimental verification and subject to the theorems of quantum mechanics, is applicable to any and all of the interactions that account for the properties of matter. It is shown that one may define a bond path operator as a Dirac observable, making the bond path the measurable expectation value of a quantum mechanical operator. Particular attention is given to van der Waals interactions that traditionally are assumed to represent attractive interactions that are distinct from chemical bonding. They are assumed by some to act in concert with Pauli repulsions to account for the existence of condensed states of molecules. It is such dichotomies of interpretation that are resolved by the experimental detection of bond paths and the delineation of their properties in molecular crystals. Specific criticisms of the stabilization afforded by the presence of bond paths derived from spectroscopic measurements performed on dideuteriophenanthrene are shown to be physically unsound. The concept of a bond path as a “bridge of density” linking bonded atoms was introduced by London in 1928 following the definition of the electron density by Schrödinger in 1926. These papers marked the beginning of the theory of atoms in molecules linked by bond paths.
Purpose To measure macular choroidal thickness in normal eyes at different points using enhanced depth imaging (EDI) optical coherence tomography (OCT) and to evaluate the association of choroidal ...thickness and age. Design Retrospective, observational case series. Methods EDI OCT images were obtained in patients without significant retinal or choroidal pathologic features. The images were obtained by positioning a spectral-domain OCT device close enough to the eye to acquire an inverted image. Seven sections were obtained within a 5 × 30-degree area centered at the fovea, with 100 scans averaged for each section. The choroid was measured from the outer border of the retinal pigment epithelium to the inner scleral border at 500-μm intervals of a horizontal section from 3 mm temporal to the fovea to 3 mm nasal to the fovea. Statistical analysis was performed to evaluate variations of choroidal thickness at each location and to correlate choroidal thickness and patient age. Results The mean age of the 30 patients (54 eyes) was 50.4 years (range, 19 to 85 years), and 14 patients (46.7%) were female. The choroid was thickest underneath the fovea (mean, 287 μm; standard deviation, ± 76 μm). Choroidal thickness decreased rapidly in the nasal direction and averaged 145 μm (± 57 μm) at 3 mm nasal to the fovea. Increasing age was correlated significantly with decreasing choroidal thickness at all points measured. Regression analysis suggested that the subfoveal choroidal thickness decreased by 15.6 μm for each decade of life. Conclusions Choroidal thickness seems to vary topographically within the posterior pole. The thickness of the choroid showed a negative correlation with age. The decrease in the thickness of the choroid may play a role in the pathophysiologic features of various age-related ocular conditions.
The dynamical diquark picture asserts that exotic hadrons can be formed from widely separated colored diquark or triquark components. We use the Born-Oppenheimer (BO) approximation to study the ...spectrum of states thus constructed, both in the basis of diquark spins and in the basis of heavy quark-antiquark spins. We develop a compact notation for naming these states, and use the results of lattice simulations for hybrid mesons to predict the lowest expected BO potentials for both tetraquarks and pentaquarks. We then compare to the set of exotic candidates with experimentally determined quantum numbers, and find that all of them can be accommodated. Once decay modes are also considered, one can develop selection rules of both exact (JPC conservation) and approximate (within the context of the BO approximation) types and test their effectiveness. We find that the most appealing way to satisfy both sets of selection rules requires including additional low-lying BO potentials, a hypothesis that can be checked on the lattice.
Abstract
Merons are nontrivial topological spin textures highly relevant for many phenomena in solid state physics. Despite their importance, direct observation of such vortex quasiparticles is ...scarce and has been limited to a few complex materials. Here, we show the emergence of merons and antimerons in recently discovered two-dimensional (2D) CrCl
3
at zero magnetic field. We show their entire evolution from pair creation, their diffusion over metastable domain walls, and collision leading to large magnetic monodomains. Both quasiparticles are stabilized spontaneously during cooling at regions where in-plane magnetic frustration takes place. Their dynamics is determined by the interplay between the strong in-plane dipolar interactions and the weak out-of-plane magnetic anisotropy stabilising a vortex core within a radius of 8–10 nm. Our results push the boundary to what is currently known about non-trivial spin structures in 2D magnets and open exciting opportunities to control magnetic domains via topological quasiparticles.
Purpose To investigate the retinal vascular findings and associated anatomic abnormalities in the central macula of eyes with diabetic retinopathy using volume-rendered angiographic and structural ...optical coherence tomography. Study Design Observational case series. Methods In this retrospective study 25 eyes of 14 consecutive patients were imaged with optical coherence tomography (OCT) using split-spectrum amplitude decorrelation. The structural OCT data were segmented for cystoid spaces and integrated into the angiographic data for subsequent volume rendering. The opacity of various vascular layers could be decreased to improve visualization of deeper structures and the images could be rotated about 3 axes. The inner and deep vascular plexus were analyzed in relation to structural changes such as cystoid spaces and disorganization of the retinal inner layers. Results Retinal vascular flow abnormalities manifested by large flow voids in the inner vascular layer with retention of large vessels and confluent areas of flow loss in the deep plexus. Areas of cystoid edema were associated with topographically colocalizing flow voids in the deep vascular layer. In eyes with no edema isolated abnormalities of the deep plexus were associated with thinning of the inner nuclear layer and abnormalities of both layers were associated uniformly with the OCT findings of disorganization of the retinal inner layers. Conclusions Widespread vascular abnormalities in diabetic retinopathy could be delineated along with the corresponding anatomic changes in the retina using volume-rendered angiographic and structural OCT. This method of imaging offers potential to improve visualization of vascular disorders of the eye.
Mobile-edge computing (MEC) is a promising paradigm to improve the quality of computation experience of mobile devices because it allows mobile devices to offload computing tasks to MEC servers, ...benefiting from the powerful computing resources of MEC servers. However, the existing computation-offloading works have also some open issues: 1) security and privacy issues; 2) cooperative computation offloading; and 3) dynamic optimization. To address the security and privacy issues, we employ the blockchain technology that ensures the reliability and irreversibility of data in MEC systems. Meanwhile, we jointly design and optimize the performance of blockchain and MEC. In this article, we develop a cooperative computation offloading and resource allocation framework for blockchain-enabled MEC systems. In the framework, we design a multiobjective function to maximize the computation rate of MEC systems and the transaction throughput of blockchain systems by jointly optimizing offloading decision, power allocation, block size, and block interval. Due to the dynamic characteristics of the wireless fading channel and the processing queues at MEC servers, the joint optimization is formulated as a Markov decision process (MDP). To tackle the dynamics and complexity of the blockchain-enabled MEC system, we develop an asynchronous advantage actor-critic-based cooperation computation offloading and resource allocation algorithm to solve the MDP problem. In the algorithm, deep neural networks are optimized by utilizing asynchronous gradient descent and eliminating the correlation of data. The simulation results show that the proposed algorithm converges fast and achieves significant performance improvements over existing schemes in terms of total reward.
Blockchain, as the underlying technology of crypto-currencies, has attracted significant attention. It has been adopted in numerous applications, such as smart grid and Internet-of-Things. However, ...there is a significant scalability barrier for blockchain, which limits its ability to support services with frequent transactions. On the other side, edge computing is introduced to extend the cloud resources and services to be distributed at the edge of the network, but currently faces challenges in its decentralized management and security. The integration of blockchain and edge computing into one system can enable reliable access and control of the network, storage, and computation distributed at the edges, hence providing a large scale of network servers, data storage, and validity computation near the end in a secure manner. Despite the prospect of integrated blockchain and edge computing systems, its scalability enhancement, self organization, functions integration, resource management, and new security issues remain to be addressed before widespread deployment. In this survey, we investigate some of the work that has been done to enable the integrated blockchain and edge computing system and discuss the research challenges. We identify several vital aspects of the integration of blockchain and edge computing: motivations, frameworks, enabling functionalities, and challenges. Finally, some broader perspectives are explored.
Distributed Denial of Service (DDoS) attacks in cloud computing environments are growing due to the essential characteristics of cloud computing. With recent advances in software-defined networking ...(SDN), SDN-based cloud brings us new chances to defeat DDoS attacks in cloud computing environments. Nevertheless, there is a contradictory relationship between SDN and DDoS attacks. On one hand, the capabilities of SDN, including software-based traffic analysis, centralized control, global view of the network, dynamic updating of forwarding rules, make it easier to detect and react to DDoS attacks. On the other hand, the security of SDN itself remains to be addressed, and potential DDoS vulnerabilities exist across SDN platforms. In this paper, we discuss the new trends and characteristics of DDoS attacks in cloud computing, and provide a comprehensive survey of defense mechanisms against DDoS attacks using SDN. In addition, we review the studies about launching DDoS attacks on SDN, as well as the methods against DDoS attacks in SDN. To the best of our knowledge, the contradictory relationship between SDN and DDoS attacks has not been well addressed in previous works. This work can help to understand how to make full use of SDN's advantages to defeat DDoS attacks in cloud computing environments and how to prevent SDN itself from becoming a victim of DDoS attacks, which are important for the smooth evolution of SDN-based cloud without the distraction of DDoS attacks.
Multi-scale brain networks Betzel, Richard F.; Bassett, Danielle S.
NeuroImage,
10/2017, Letnik:
160
Journal Article
Recenzirano
Odprti dostop
The network architecture of the human brain has become a feature of increasing interest to the neuroscientific community, largely because of its potential to illuminate human cognition, its variation ...over development and aging, and its alteration in disease or injury. Traditional tools and approaches to study this architecture have largely focused on single scales—of topology, time, and space. Expanding beyond this narrow view, we focus this review on pertinent questions and novel methodological advances for the multi-scale brain. We separate our exposition into content related to multi-scale topological structure, multi-scale temporal structure, and multi-scale spatial structure. In each case, we recount empirical evidence for such structures, survey network-based methodological approaches to reveal these structures, and outline current frontiers and open questions. Although predominantly peppered with examples from human neuroimaging, we hope that this account will offer an accessible guide to any neuroscientist aiming to measure, characterize, and understand the full richness of the brain's multiscale network structure—irrespective of species, imaging modality, or spatial resolution.
•The human brain can be represented as a multi-scale network.•Characterizing the architecture of multi-scale networks requires new tools.•We review promising multilayer tools from network science.
Chemical and mechanical properties of lithology exert a first‐order control on landscape evolution and biological colonization of substrate. To quantify the influence of lithology on topography, ...vegetation density, and animal biodiversity, I compile lithologic, topographic, climatic, and biological data sets for mountainous regions globally. I find significant variations in the topographic steepness of regions underlain by different lithologies that, accounting for tectonic uplift, likely reflect lithologic differences in erosional resistance. These relative differences in erodibility are similar across different climate zones. To isolate the effect of lithology on vegetation and animal biodiversity, I account for the heterogeneous lithologic distribution among climate zones. I show that siliciclastic, plutonic, and, for some biological variables, metamorphic rocks exhibit elevated values of Normalized Difference Vegetation Index and tetrapod and amphibian species richness relative to carbonate rocks. These results likely reflect lithology‐related variation in soil nutrients and hydrology that promote or inhibit habitat suitability.
Plain Language Summary
A myriad of rock types are exposed at Earth's surface, all of which have different chemical and physical properties. These differences are important because rock properties affect processes on Earth's surface that shape topography and because rocks are the base material from which most soils form. Here, I investigate how the steepness of a landscape varies based on differences in rock type and show that rock type variations can partly explain Earth's topography. I also test whether the differences in rock type that lead to variations in soil properties and water availability influence plant cover and animal richness globally. I find that limestone areas have less vegetation and lower numbers of amphibian, bird, and mammalian species. This is likely related to low water availability and nutrient content in limestone areas.
Key Points
Erodibility differences between lithologies partially control mountainous topography globally
Vegetation density and biodiversity are globally affected by variations in lithology
Hydrologic and chemical properties of carbonates have a negative effect on vegetation density and animal biodiversity
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