Vacancies exist throughout nature and determine the physical properties of materials. By manipulating the density and distribution of vacancies, it is possible to influence their physical properties ...such as band-gap, conductivity, magnetism,
etc.
This can generate exciting applications in the fields of water treatment, energy storage, and physical devices such as resistance-change memories. In this review, we focus on recent progress in vacancy engineering for the design of materials for energy harvesting applications. A brief discription of the concept of vacancies, the way to create and control them, as well as their fundamental properties, is first provided. Then, emphasis is placed on the strategies used to tailor vacancies for metal-insulator transitions, electronic structures, and introducing magnetism in non-magnetic materials. Finally, we present representative applications of different structures with vacancies as active electrode materials of lithium or sodium ion batteries, catalysts for water splitting, and hydrogen evolution.
Manipulating vacancies in functional materials offers scientists a powerful tool to design advanced materials for clean energy applications.
The ability to discriminate signal from noise plays a key role in the analysis and interpretation of functional magnetic resonance imaging (fMRI) measures of brain activity. Over the past two ...decades, a number of major sources of noise have been identified, including system-related instabilities, subject motion, and physiological fluctuations. This article reviews the characteristics of the various noise sources as well as the mechanisms through which they affect the fMRI signal. Approaches for distinguishing signal from noise and the associated challenges are also reviewed. These challenges reflect the fact that some noise sources, such as respiratory activity, are generated by the same underlying brain networks that give rise to functional signals that are of interest.
Exploring 2-group global symmetries Córdova, Clay; Dumitrescu, Thomas T.; Intriligator, Kenneth
The journal of high energy physics,
02/2019, Letnik:
2019, Številka:
2
Journal Article
Recenzirano
Odprti dostop
A
bstract
We analyze four-dimensional quantum field theories with continuous 2-group global symmetries. At the level of their charges, such symmetries are identical to a product of continuous flavor ...or spacetime symmetries with a 1-form global symmetry
U
(1)
B
(1)
, which arises from a conserved 2-form current
J
B
(2)
. Rather, 2-group symmetries are characterized by deformed current algebras, with quantized structure constants, which allow two flavor currents or stress tensors to fuse into
J
B
(2)
. This leads to unconventional Ward identities, which constrain the allowed patterns of spontaneous 2-group symmetry breaking and other aspects of the renormalization group flow. If
J
B
(2)
is coupled to a 2-form background gauge field
B
(2)
, the 2-group current algebra modifies the behavior of
B
(2)
under background gauge transformations. Its transformation rule takes the same form as in the Green-Schwarz mechanism, but only involves the background gauge or gravity fields that couple to the other 2-group currents. This makes it possible to partially cancel reducible ’t Hooft anomalies using Green-Schwarz counterterms for the 2-group background gauge fields. The parts that cannot be cancelled are reinterpreted as mixed, global anomalies involving
U
(1)
B
(1)
, which receive contributions from topological, as well as massless, degrees of freedom. Theories with 2-group symmetry are constructed by gauging an abelian flavor symmetry with suitable mixed ’t Hooft anomalies, which leads to many simple and explicit examples. Some of them have dynamical string excitations that carry
U
(1)
B
(1)
charge, and 2-group symmetry determines certain ’t Hooft anomalies on the world sheets of these strings. Finally, we point out that holographic theories with 2-group global symmetries have a bulk description in terms of dynamical gauge fields that participate in a conventional Green-Schwarz mechanism.
A
bstract
We systematically analyze the operator content of unitary superconformal multiplets in
d
≥ 3 spacetime dimensions. We present a simple, general, and efficient algorithm that generates all ...of these multiplets by correctly eliminating possible null states. The algorithm is conjectural, but passes a vast web of consistency checks. We apply it to tabulate a large variety of superconformal multiplets. In particular, we classify and construct all multiplets that contain conserved currents or free fields, which play an important role in superconformal field theories (SCFTs). Some currents that are allowed in conformal field theories cannot be embedded in superconformal multiplets, and hence they are absent in SCFTs. We use the structure of superconformal stress tensor multiplets to show that SCFTs with more than 16 Poincaré supercharges cannot arise in
d
≥ 4, even when the corresponding superconformal algebras exist. We also show that such theories do arise in
d
= 3, but are necessarily free.
There are well-understood psychological limits on our capacity to process information. As information proliferation—the consumption and sharing of information—increases through social media and other ...communications technology, these limits create an attentional bottleneck, favoring information that is more likely to be searched for, attended to, comprehended, encoded, and later reproduced. In information-rich environments, this bottleneck influences the evolution of information via four forces of cognitive selection, selecting for information that is belief-consistent, negative, social, and predictive. Selection for belief-consistent information leads balanced information to support increasingly polarized views. Selection for negative information amplifies information about downside risks and crowds out potential benefits. Selection for social information drives herding, impairs objective assessments, and reduces exploration for solutions to hard problems. Selection for predictive patterns drives overfitting, the replication crisis, and risk seeking. This article summarizes the negative implications of these forces of cognitive selection and presents eight warnings that represent severe pitfalls for the naive “informavore,” accelerating extremism, hysteria, herding, and the proliferation of misinformation.
Functional coupling across distributed brain regions varies across task contexts, yet there are stable features. To better understand the range and central tendencies of network configurations, ...coupling patterns were explored using functional MRI (fMRI) across 14 distinct continuously performed task states ranging from passive fixation to increasingly demanding classification tasks. Mean global correlation profiles across the cortex ranged from 0.69 to 0.82 between task states. Network configurations from both passive fixation and classification tasks similarly predicted task coactivation patterns estimated from meta-analysis of the literature. Thus, even across markedly different task states, central tendencies dominate the coupling configurations. Beyond these shared components, distinct task states displayed significant differences in coupling patterns in response to their varied demands. One possibility is that anatomical connectivity provides constraints that act as attractors pulling network configurations towards a limited number of robust states. Reconfigurable coupling modes emerge as significant modifications to a core functional architecture.
The global signal is widely used as a regressor or normalization factor for removing the effects of global variations in the analysis of functional magnetic resonance imaging (fMRI) studies. However, ...there is considerable controversy over its use because of the potential bias that can be introduced when it is applied to the analysis of both task-related and resting-state fMRI studies. In this paper we take a closer look at the global signal, examining in detail the various sources that can contribute to the signal. For the most part, the global signal has been treated as a nuisance term, but there is growing evidence that it may also contain valuable information. We also examine the various ways that the global signal has been used in the analysis of fMRI data, including global signal regression, global signal subtraction, and global signal normalization. Furthermore, we describe new ways for understanding the effects of global signal regression and its relation to the other approaches.
ABSTRACT
Star formation in galaxies appears to be self-regulated by energetic feedback processes. Among the most promising agents of feedback are cosmic rays (CRs), the relativistic ion population of ...interstellar and intergalactic plasmas. In these environments, energetic CRs are virtually collisionless and interact via collective phenomena mediated by kinetic-scale plasma waves and large-scale magnetic fields. The enormous separation of kinetic and global astrophysical scales requires a hydrodynamic description. Here, we develop a new macroscopic theory for CR transport in the self-confinement picture, which includes CR diffusion and streaming. The interaction between CRs and electromagnetic fields of Alfvénic turbulence provides the main source of CR scattering, and causes CRs to stream along the magnetic field with the Alfvén velocity if resonant waves are sufficiently energetic. However, numerical simulations struggle to capture this effect with current transport formalisms and adopt regularization schemes to ensure numerical stability. We extent the theory by deriving an equation for the CR momentum density along the mean magnetic field and include a transport equation for the Alfvén-wave energy. We account for energy exchange of CRs and Alfvén waves via the gyroresonant instability and include other wave damping mechanisms. Using numerical simulations, we demonstrate that our new theory enables stable, self-regulated CR transport. The theory is coupled to magnetohydrodynamics, conserves the total energy and momentum, and correctly recovers previous macroscopic CR transport formalisms in the steady-state flux limit. Because it is free of tunable parameters, it holds the promise to provide predictable simulations of CR feedback in galaxy formation.
There is an ever-increasing demand for higher-performing polymeric materials counterbalanced by the need for sustainability throughout the life cycle. Copolymers comprising ester, carbonate, or ether ...linkages could fulfill some of this demand as their monomer–polymer chemistry is closer to equilibrium, facilitating (bio)degradation and recycling; many monomers are or could be sourced from renewables or waste. Here, an efficient and broadly applicable route to make such copolymers is discussed, a form of switchable polymerization catalysis which exploits a single catalyst, switched between different catalytic cycles, to prepare block sequence selective copolymers from monomer mixtures. This perspective presents the principles of this catalysis, catalyst design criteria, the selectivity and structural copolymer characterization tools, and the properties of the resulting copolymers. Uses as thermoplastic elastomers, toughened plastics, adhesives, and self-assembled nanostructures, and for programmed degradation, among others, are discussed. The state-of-the-art research into both catalysis and products, as well as future challenges and directions, are presented.
Chronic diseases are a major health problem in the United States. Accumulated data suggest that consumption of vegetables can significantly reduce the risk of many chronic diseases. Dietary ...guidelines for 2015–2020 from the U.S. Department of Agriculture and the U.S. Department of Health and Human Services recommend 1–4 cups of vegetables per day for males and 1–3 cups of vegetables per day for females, depending on their age. However, the average intake of vegetables is below the recommended levels. Microgreens are young vegetable greens. Although they are small, microgreens have delicate textures, distinctive flavors, and various nutrients. In general, microgreens contain greater amounts of nutrients and health-promoting micronutrients than their mature counterparts. Because microgreens are rich in nutrients, smaller amounts may provide similar nutritional effects compared to larger quantities of mature vegetables. However, literature on microgreens remains limited. In this Review, we discuss chemical compositions, growing conditions, and biological efficacies of microgreens. We seek to stimulate interest in further study of microgreens as a promising dietary component for potential use in diet-based disease prevention.
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