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Phase Change Materials: the Future of Computer Memory?

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Phase change (PC) materials remain leading candidates for future computer random access memory (RAM) and rewritable storage devices (CD-RW, DVD-RW, Blu-ray Discs, etc.). The battle to replace the digital versatile disk (DVD) was decided in 2008 in favour of the Blu-ray Disc, and the recording media of all BD products involve PC materials. Information is stored in these devices in the form of microscopic bits (each less than 100 nanometers in size) in an ultrathin layer of a polycrystalline alloy containing several elements. The bits may have a disordered (amorphous) or an ordered (crystalline) structure, and the transition between the two phases is not only extremely rapid (some tens of nanoseconds) but also reversible. Amorphous bits are formed by quenching after a localized and short (~ 1 nanosecond) pulse to heat them to a temperature above their own melting point. Longer laser heating (of the order of 10 ns) to above the glass transition but below the melting point returns the bit to the metastable crystalline form. The state can be identified by monitoring the optical or electrical properties..

The physical requirements of PC materials, particularly the rapid crystallization, are satisfied by relatively few materials. The focus for some years now has been on alloys of three or four elements, many of which contain germanium (Ge), antimony (Sb) and tellurium (Te) ["GST alloys", common in BD applications] or alloys of Sb (70%) and Te (30%). With small amounts of silver (Ag) and indium (In), "AIST" alloys are in widespread use in DVD-RW devices.

Although both alloy families contain Sb and Te, the phase change mechanisms are very different. In GST materials, the amorphous bit crystallizes via nucleation, i.e. small crystallites formed in the interior grow rapidly until they cover the whole bit. The phase change in AIST alloys proceeds from the rim of the bit adjoining the crystalline surroundings, towards its interior.

We have performed simulations of the amorphous structure of prototype members of both families and have presented plausible scenarios for the crystallization of each. In AIST, we propose a "bond exchange" model in which the local environment in the amorphous bit is changed by small movements of an Sb atom that result in the exchange of a "short" and a "long" bond. A sequence or avalanche of many such steps results in reorientation (crystallization) without requiring large atomic motions.

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R. O. Jones, J. Akola

 

Biological and Organic Reactions

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The combination of biology with computer science is one of the great growth areas of science, and the availability of massively parallel computers is making an essential contribution. However, in spite of many advances in drug design, the identification of genes in DNA sequences, and the structure of proteins from their sequences, the atomistic study of reactions in biological molecules is still at an early stage.

In principle, the density functional formalism provides a means for studying such problems, but even the most powerful computers limit simulations to around 1000 atoms and time scales of some hundreds of picoseconds. These severe limitations make them unsuitable for most problems of biological interest, where systems of tens of thousands of atoms must be studied over time scales of microseconds or even much longer.

Such studies require simulations using classical force fields, and we have implemented them to study ATP (adenosine 5'-triphosphate) binding cassette transporters. ABC transporters are membrane proteins that actively transport substrates through lipid bilayers, and the protein Sav1866 was discovered from the analysis of antibiotic-resistant staphylococcus aureus. We have studied its structure in a lipid bilayer in several simulations of around 200000 atoms over more than 1 microsecond.

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R. O. Jones, J. Akola, J.-H. Lin (Taipei)

 

Organic Molecules on Surfaces - Molecular Electronics

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The idea of exploring and monitoring new possibilities of incorporating organic molecules into existing technologies and building molecule-based nanoscale electronic circuits with rectifying, logic, and switching functions has stimulated experimental and theoretical efforts to study and predict their properties. The development of organic/inorganic interfaces depends critically on understanding the bonding and lateral interactions that govern the orientation, conformation, and two-dimensional arrangement of molecules at surfaces. Density functional (DFT) calculations provide new insight into this problem, particularly the interactions between such molecules, and we have used the EstCoMPP program, a projector augmented plane-wave package, to investigate the geometrical and electronic structure of several molecular layers that use the carboxylate group as an anchor to metal surfaces.

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Mechanical & Thermo-mechanical Properties of Materials

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The DF method is capable of determining energies as a function of atomic positions, and it should be able in principle to predict mechanical and thermo-mechanical properties, such as thermal expansion coefficients or elastic constants of crystalline materials. The theory of thermal expansion developed by Born and Grüneisen requires knowledge of the phonon frequencies and the way they change with changing unit cell size. This is a great computational challenge and our success in explaining the thermal expansion of ß-eucryptite - the basic material from which all cooktops are made - is quite remarkable.

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R. O. Jones


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