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Breakdown of the Stokes-Einstein relation above the melting temperature in a liquid phase-change material

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      Abstract

      Failure of classic equation linking diffusion and viscosity points to new thinking on switch kinetics of phase-change materials.

      Abstract

      The dynamic properties of liquid phase-change materials (PCMs), such as viscosity η and the atomic self-diffusion coefficient D, play an essential role in the ultrafast phase switching behavior of novel nonvolatile phase-change memory applications. To connect η to D, the Stokes-Einstein relation (SER) is commonly assumed to be valid at high temperatures near or above the melting temperature Tm and is often used for assessing liquid fragility (or crystal growth velocity) of technologically important PCMs. However, using quasi-elastic neutron scattering, we provide experimental evidence for a breakdown of the SER even at temperatures above Tm in the high–atomic mobility state of a PCM, Ge1Sb2Te4. This implies that although viscosity may have strongly increased during cooling, diffusivity can remain high owing to early decoupling, being a favorable feature for the fast phase switching behavior of the high-fluidity PCM. We discuss the origin of the observation and propose the possible connection to a metal-semiconductor and fragile-strong transition hidden below Tm.

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      Formation of glasses from liquids and biopolymers.

       C Angell (1995)
      Glasses can be formed by many routes. In some cases, distinct polyamorphic forms are found. The normal mode of glass formation is cooling of a viscous liquid. Liquid behavior during cooling is classified between "strong" and "fragile," and the three canonical characteristics of relaxing liquids are correlated through the fragility. Strong liquids become fragile liquids on compression. In some cases, such conversions occur during cooling by a weak first-order transition. This behavior can be related to the polymorphism in a glass state through a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The sudden loss of some liquid degrees of freedom through such first-order transitions is suggestive of the polyamorphic transition between native and denatured hydrated proteins, which can be interpreted as single-chain glass-forming polymers plasticized by water and cross-linked by hydrogen bonds. The onset of a sharp change in d dT( is the Debye-Waller factor and T is temperature) in proteins, which is controversially indentified with the glass transition in liquids, is shown to be general for glass formers and observable in computer simulations of strong and fragile ionic liquids, where it proves to be close to the experimental glass transition temperature. The latter may originate in strong anharmonicity in modes ("bosons"), which permits the system to access multiple minima of its configuration space. These modes, the Kauzmann temperature T(K), and the fragility of the liquid, may thus be connected.
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        Phase-change materials for rewriteable data storage.

        Phase-change materials are some of the most promising materials for data-storage applications. They are already used in rewriteable optical data storage and offer great potential as an emerging non-volatile electronic memory. This review looks at the unique property combination that characterizes phase-change materials. The crystalline state often shows an octahedral-like atomic arrangement, frequently accompanied by pronounced lattice distortions and huge vacancy concentrations. This can be attributed to the chemical bonding in phase-change alloys, which is promoted by p-orbitals. From this insight, phase-change alloys with desired properties can be designed. This is demonstrated for the optical properties of phase-change alloys, in particular the contrast between the amorphous and crystalline states. The origin of the fast crystallization kinetics is also discussed.
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          Enhancement of Protein Crystal Nucleation by Critical Density Fluctuations

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            Author and article information

            Affiliations
            [1 ]I. Institute of Physics (IA), RWTH Aachen University, Aachen, Germany.
            [2 ]Heinz Maier-Leibnitz Zentrum (MLZ) and Physik Department, Technische Universität München, Lichtenbergstrasse 1, 85747 Garching, Germany.
            [3 ]Heraeus Holding GmbH, Heraeusstr.12-14, 63450 Hanau, Germany.
            [4 ]Chair of Metallic Materials, Saarland University, Campus C6.3, 66123 Saarbrücken, Germany.
            [5 ]Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85712, USA.
            [6 ]School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA.
            Author notes
            [* ]Corresponding author. Email: austenangell@ 123456gmail.com (C.A.A.); shuai.wei@ 123456physik.rwth-aachen.de (S.W.)
            Journal
            Sci Adv
            Sci Adv
            SciAdv
            advances
            Science Advances
            American Association for the Advancement of Science
            2375-2548
            November 2018
            30 November 2018
            : 4
            : 11
            30515453
            6269161
            aat8632
            10.1126/sciadv.aat8632
            Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

            This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

            Funding
            Funded by: doi http://dx.doi.org/10.13039/100000001, National Science Foundation;
            Award ID: 1640860
            Funded by: doi http://dx.doi.org/10.13039/100000001, National Science Foundation;
            Award ID: CHE-1213265
            Funded by: doi http://dx.doi.org/10.13039/100005156, Alexander von Humboldt-Stiftung;
            Award ID: Feoder-Lynen Research Fellowship
            Funded by: doi http://dx.doi.org/10.13039/501100007210, RWTH Aachen University;
            Award ID: Place-to-be RWTH Start-Up
            Categories
            Research Article
            Research Articles
            SciAdv r-articles
            Materials Science
            Materials Science
            Custom metadata
            Eunice Ann Alesin

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