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623 results on '"Ashcroft, N. W."'

211. Symmetry and Higher Superconductivity in the Lower Elements.

Author

Bianconi, Antonio and Ashcroft, N. W.

Abstract

At one atmosphere 29 elements are classified as superconductors; at high pressures there are to date an additional 23, many of these being drawn from the lighter elements. The number of superconductors for the elements in combination appears to be illimitable. Observed symmetries in these systems generally include orderings in both nuclear and electronic degrees of freedom. The fluctuations impelling order in the electron sub-system include those originating with the nuclear degrees of freedom but also with the electrons themselves, both itinerant and localized. For the elements in combination coherent multipole fluctuations in localized states may arise, and the relative contributions of such excitations to electron-pairing is then of some especial interest. When the elements are placed in combination the effects of external pressure may be replicated in part by an equivalent internal pressure, this resulting from a form of chemical pre-compression. [ABSTRACT FROM AUTHOR]

Published
2006
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242. The Unusual and the Expected in the Si/C Phase Diagram.

Author

Guoying Gao, Ashcroft, N. W., and Hoffmann, Roald

Subjects
*PHASE diagrams, *SILICON, *CARBON, *STOICHIOMETRY, *METASTABLE states, *GRAPHITE, *CYCLOPHANES, *POLYENES
Abstract

In the Si/C phase diagram, the only stable phases at P = 1 atm are the numerous polytypes of the simplest 1:1 stoichiometry, SiC. However, many metastable yet likely to be kinetically persistent phases can be found for almost any composition. Given the instability of simple graphite-type structures with considerable Si content, we thought these metastables would be only of the diamondoid class. Indeed they are for Si3C, a stoichiometry we studied on the silicon-rich side of the phase diagram. Yet on the carbon-rich side, which we chose to explore computationally with SiC3, there was a surprise in store, a series of unusual metastable structures. The most striking of these had the appearance of a collapsed graphite structure, with benzenoid C6 units and SiSi bridges between layers. This SiC3 structure is related to known meta-(1,3,5)cyclophanes. Three other metastable structures featured layers with all carbon polyene and polyphenylene arrays. Some of these can be metallic, as we have found. [ABSTRACT FROM AUTHOR]

Published
2013
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250. Making Sense of Boron-Rich Binary Be-B Phases.

Author

Hermann, Andreas, Ashcroft, N. W., and Hoffmann, Roald

Subjects
*BORON, *PHASE diagrams, *SOLID state chemistry, *BORANES, *ELECTRONIC structure, *STOICHIOMETRY, *GROUND state (Quantum mechanics)
Abstract

There is much uncertainty in the literature about the structure of several Be--B phases between 20 and 33 atom % Be. We clarify the structural choices in this region of the phase diagram, proposing structural candidates obtained from a combination of chemical intuition and unbiased solid-state structure searches. In particular, we discuss the structural, dynamical, and electronic properties of the ground states of the BeB2, BeB3, and BeB4 phases, as well as those of the complex (and superconducting) "BeB275" phase. For the latter, we find the polyhedral borane cluster electron-counting approach very useful to explain its electronic structure. We can also make sense of the partial and mixed occupancies in the structure by looking at the cavities in a parent structure. A Be29B81, stoichiometry seems most reasonable for the ground state of this phase. The electronic structure points to a region of stability for three additional electrons per unit cell, a 1% difference in total electron count For BeB2, which is usually studied computationally in the A1B2 structure type, we find several other structure types that are more stable, all essentially Zintl phases with 4-connected boron networks. New structure types are also predicted for BeB3 and BeB4 as well. [ABSTRACT FROM AUTHOR]

Published
2012
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