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N W Ashcroft

Publications and source records attributed to N W Ashcroft.

6 recordsLinked to original sources
PubMed2003

Enhanced Friedel structure and proton pairing in dense solid hydrogen.

The mechanism of proton pairing in dense solid hydrogen, and its progression with density, are both studied using effective potentials between protons which include electronic response up to quadratic terms. For high pressures nonlinear effects originating with different pairs are crucial in establishing the net attraction within a given pair, and in this picture Friedel oscillations are considerably enhanced by quadratic response, subsequently playing a very important role in the overall pairing. Calculated vibron frequencies also show substantial agreement with experiment, reflecting at the same time significant changes in the physical character of the pairing itself.

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PubMed2002

Low-energy linear structures in dense oxygen: implications for the epsilon phase.

Using density functional theory implemented within the generalized gradient approximation, a new nonmagnetic insulating ground state of solid oxygen is proposed and found to be energetically favored at pressures corresponding to the epsilon phase. The newly predicted static ground state is composed of linear herringbone-type chains of O2 molecules and has Cmcm symmetry (with an alternative monoclinic cell). Importantly, this phase supports IR-active zone-center phonons, and their computed frequencies are found to be in broad agreement with recent infrared absorption experiments.

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PubMed2001

Structure and effective interactions in three-component hard sphere liquids.

Complete and simple analytical expressions for the partial structure factors of the ternary hard sphere mixture are obtained within the Percus-Yevick approximation and presented as functions of relative packing fractions and relative hard sphere diameters. These solutions follow from the Laplace transform method as applied to multicomponent systems by Lebowitz [Phys. Rev. 133, A895 (1964)]. As an important application, we examine effective interactions in hard sphere liquid mixtures using the microscopic information contained in their partial structure factors. Thus the ensuring pair potential for an effective one-component system is obtained from the correlation functions by using an approximate inversion, and examples of effective potentials for three-component hard sphere mixtures are given. These mixtures may be of particular interest for the study of the packing aspects of melts that form glasses or quasicrystals, since noncrystalline solids often emerge from melts with at least three atomic constituents.

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PubMed2001

On the constitution of sodium at higher densities.

Using density functional theory the atomic and electronic structures of sodium are predicted to depart substantially from those expected of simple metals for r(s)<2.48 ( p>130 GPa). Newly predicted phases include those with low structural symmetry and semimetallic electronic properties (including zero-gap semiconducting limiting behavior), and even those that raise the possibility of superconductivity, all at currently achievable pressures. Important differences emerge between sodium and lithium at high densities, and these are attributable to corresponding differences in their respective cores.

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PubMed1999

Generalized density-functional theory: extended weighted density approaches.

A third-order density-functional theory is introduced by an approach that may be used to find density-functional theories to any higher-order accuracy provided only that known homogeneous state correlation functions are utilized as input. It is constructed from the required knowledge of a single weight function at each order. By way of application results are presented for the melting of classical hard spheres using functionals accurate to a third-order functional Taylor series in the homogeneous limit. Within the framework of the modified weighted density approximation, there is a uniform improvement in the solid phase-free energies, pressures and melting parameters, and further improvement also results when these functionals are optimized in a way that utilizes the close packing limit. The sensitivity of the results to existing and proposed models of the third-order direct correlation function is discussed.

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