1,721,036 research outputs found
Density-functional theory guided advances in phase-change materials and memories
No full textPhase-change materials (PCMs) are promising candidates for novel data-storage and memory applications. They encode digital information by exploiting the optical and electronic contrast between amorphous and crystalline states. Rapid and reversible switching between the two states can be induced by voltage or laser pulses. Here, we review how density-functional theory (DFT) is advancing our understanding of PCMs. We describe key DFT insights into structural, electronic, and bonding properties of PCMs and into technologically relevant processes such as fast crystallization and relaxation of the amorphous state. We also comment on the leading role played by predictive DFT simulations in new potential applications of PCMs, including topological properties, switching between different topological states, and magnetic properties of doped PCMs. Such DFT-based approaches are also projected to be powerful in guiding advances in other materials-science fields
A chemical link between Ge-Sb-Te and In-Sb-Te phase-change materials
No full textWe identify a similar feature in the chemical-bonding nature of seemingly different phase-change materials (PCMs) for data storage. This affords new insight into the "next-generation'' material In3SbTe2, establishes a hitherto missing link to the more ubiquitous Ge-Sb-Te alloys, and encourages the search for new PCMs beyond established electron-counting schemes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
From Atomistic Surface Chemistry to Nanocrystals of Functional Chalcogenides
No full textSynthesis and utilization of nanocrystals are highly active fields of current research, but they require a thorough understanding of the underlying crystal surfaces. In this Minireview, we span the arc from surfaces to free nanocrystals, and onward to their chemical synthesis, using as examples lead selenide (PbSe), tin telluride (SnTe), and their direct chemical relatives. Besides experimental insights, we highlight the increasingly influential role played by quantum-chemical simulations of surfaces and nanocrystals. What can theory do today, or possibly tomorrow; where are its limits? Answering these questions, and skillfully linking them to experiments, could open up new atomistically (that is, chemically) guided perspectives for nanosynthesis
Machine learning based interatomic potential for amorphous carbon
No full textWe introduce a Gaussian approximation potential (GAP) for atomistic simulations of liquid and amorphous elemental carbon. Based on a machine learning representation of the density-functional theory (DFT) potential-energy surface, such interatomic potentials enable materials simulations with close-to DFT accuracy but at much lower computational cost. We first determine the maximum accuracy that any finite-range potential can achieve in carbon structures; then, using a hierarchical set of two-, three-, and many-body structural descriptors, we construct a GAP model that can indeed reach the target accuracy. The potential yields accurate energetic and structural properties over a wide range of densities; it also correctly captures the structure of the liquid phases, at variance with a state-of-the-art empirical potential. Exemplary applications of the GAP model to surfaces of “diamondlike” tetrahedral amorphous carbon (-C) are presented, including an estimate of the amorphous material’s surface energy and simulations of high-temperature surface reconstructions (“graphitization”). The presented interatomic potential appears to be promising for realistic and accurate simulations of nanoscale amorphous carbon structures
Many-Body Dispersion Correction Effects on Bulk and Surface Properties of Rutile and Anatase TiO
No full textTitanium dioxide (titania, TiO) is a widely studied material with diverse applications. Here, we explore how pairwise and many-body descriptions of van der Waals dispersion interactions perform in atomistic modeling of the two most important TiO polymorphs, rutile and anatase. In particular, we obtain an excellent description of both bulk structures from density-functional theory (DFT) computations with the many-body dispersion (MBD) method of Tkatchenko and co-workers coupled to an iterative Hirshfeld partitioning scheme ("Hirshfeld-I"). Beyond the bulk, we investigate the most important crystal surfaces, namely, rutile (110), (101), and (100) and anatase (101), (100), and (001). Dispersion has a highly anisotropic effect on the different () surfaces; this directly changes the predicted nanocrystal morphology as determined from Wulff constructions. The periodic DFT+MBD method combined with Hirshfeld-I partitioning appears to be promising for future large-scale atomistic studies of this technologically important material
Computational Methods for Solids
No full textToday's scientific progress would be unthinkable without theoretical and computational assistance. This holds true also for the solid-state sciences - which are without doubt a fundamental part of modern inorganic chemistry. This chapter is concerned mainly with first principles or ab initio quantum-chemical methods; the fundamental goal of solving Schrödinger's equation does not change upon going to extended systems, but there are some very important new ideas to consider. First, we describe these essential concepts; however, we do not, nor attempt to, provide an exhaustive overview of electronic-structure theory. Subsequently, we deal with simplifications, which are necessary to make quantum-chemical computations tractable and which possess special importance in the solid state. Simplifying 'well' is thus a vital part of any theorist's work. Finally, we describe applications - how chemists 'see' bonds in complicated structures, and how the computational toolkit may complement and enhance chemical concepts. They illustrate our most important message: how beautifully rock-solid theories and chemists' ingenious models blend in the solid state. © 2013 Elsevier Ltd. All rights reserved
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