Murray S. Daw

The embedded-atom method: a review of theory and applications

Abstract The embedded-atom method is a semi-empirical method for performing calculations of defects in metals. The EAM incorporates a picture of metallic bonding, for which there is some fundamental basis. The limitations of the EAM are fairly well characterized: it works best for purely metallic systems with no directional bonding; it does not treat covalency or significant charge transfer; and it does not handle Fermi-surface effects. The main physical property incorporated in the EAM is the moderation of bond strength by other bonds (coordination-dependent bond strength). Within these constraints, the EAM provides a very useful and robust means of calculating approximate structure and energetics, from which many interesting properties of metals can be obtained. We believe that atomistic calculations will continue to play an important role in the development of materials theory. Where the EAM can be useful, there is a tremendous number of interesting projects that have yet to be carried out. The understanding of mechanical properties on an atomistic level has only just begun. For materials where the EAM is not expected to work well, there are recent developments which may allow calculations similar to those presented here. We have mentioned already the problem of treating directional bonding in semiconductors and elements from the transition series. One approach which promises to be useful for treating directional bonding is reviewed by Carlsson [70]; the interested reader is encouraged to start there.

Silicon optical fiber

Described herein are initial experimental details and properties of a silicon core, silica glass-clad optical fiber fabricated using conventional optical fiber draw methods. Such semiconductor core fibers have potential to greatly influence the fields of nonlinear fiber optics, infrared and THz power delivery. More specifically, x-ray diffraction and Raman spectroscopy showed the core to be highly crystalline silicon. The measured propagation losses were 4.3 dB/m at 2.936 microm, which likely are caused by either microcracks in the core arising from the large thermal expansion mismatch with the cladding or to SiO(2) precipitates formed from oxygen dissolved in the silicon melt. Suggestions for enhancing the performance of these semiconductor core fibers are provided. Here we show that lengths of an optical fiber containing a highly crystalline semiconducting core can be produced using scalable fiber fabrication techniques.

Application of the embedded atom method to phonons in transition metals

Abstract The lattice dynamical matrix has been derived from a new model of metallic cohesion, the Embedded Atom Method, based on approximations in density functional theory. The predictions of previously published empirical functions for Ni and Pd are compared to experimental phonon dispersions. The agreement shows that the Embedded Atom Method provides a reasonable description of transition metal lattice dynamics.

Calculations of the energetics and structure of Pt(110) reconstruction using the embedded atom method

The energetics and structure of reconstructed Pt(110)-(1×2) are calculated using the embedded atom method. Several models of the reconstruction are considered, including the missing-row model and the sawtooth (Bonzel-Ferrer) model. The missing-row structure is calculated to be lower in energy than the unreconstructed surface, which is in turn much lower than the sawtooth structure. Multi-layer relaxations are found to occur for all models. In particular, the optimized missing-row geometry shows top interlayer contraction, second-row pairing, third-layer buckling, and fourth-row paining.

The (1 × 2) missing-row phase of Au(110): energetics determined from an extended embedded atom method

The genesis of order in the missing-row phase of the Au(110) surface and the disordering of that phase have been investigated via examination of the underlying static energetics. We determined the total cohesive energy of various clusters of Au “adatoms” on an Au(110) surface and that of various extended defects on the surface using the embedded atom method with an extension to improve the accuracy of treatment in regions where charge density gradients are large. From the energetics of the clusters we have extracted lattice-gas “adatom-interaction” parameters and studied the resulting disordering phase transition using transfer-matrix scaling. We find Tc = 670 K, in reasonable agreement with experiment. (Experimental Tcs reported range from 650 to 770 K.) From our study of the energy of steps on the (1 × 1) surface and roughening excitations of the reconstructed surface two important results emerge: the energy of steps whose edges are parallel to the rows is very small suggesting the latter play an important role in initiating ordering; certain defects which roughen the (1 × 2) surface are also found to be low in energy, suggesting that the disordering transition includes intrinsic roughening.

On the fabrication of all-glass optical fibers from crystals

The highly nonequilibrium conditions under which optical fibers conventionally are drawn afford considerable, yet underappreciated, opportunities to realize fibers comprised of novel materials or materials that themselves cannot be directly fabricated into fiber form using commercial scalable methods. Presented here is an in-depth analysis of the physical, compositional, and selected optical properties of silica-clad erbium-doped yttrium aluminosilicate glass optical fibers derived from undoped, 0.25, and 50 wt % Er3+-doped yttrium aluminum garnet (YAG) crystals. The YAG-derived fibers were found to be noncrystalline as evidenced by x-ray diffraction and corroborated by spectroscopic measurements. Elemental analysis across the core/clad interface strongly suggests that diffusion plays a large role in this amorphization. Despite the noncrystalline nature of the fibers, they do exhibit acceptable low losses (∼0.15–0.2 dB/m) for many applications, broad-band emissions in the near-infrared, and enhanced thermal conductivity along their length while maintaining equivalent mechanical strength with respect to conventional silica optical fibers. Further, considerably higher rare-earth doping levels are realized than can be achieved by conventional solution or vapor-phase doping schemes. A discussion of opportunities for such approaches to nontraditional fiber materials is presented.The highly nonequilibrium conditions under which optical fibers conventionally are drawn afford considerable, yet underappreciated, opportunities to realize fibers comprised of novel materials or materials that themselves cannot be directly fabricated into fiber form using commercial scalable methods. Presented here is an in-depth analysis of the physical, compositional, and selected optical properties of silica-clad erbium-doped yttrium aluminosilicate glass optical fibers derived from undoped, 0.25, and 50 wt % Er3+-doped yttrium aluminum garnet (YAG) crystals. The YAG-derived fibers were found to be noncrystalline as evidenced by x-ray diffraction and corroborated by spectroscopic measurements. Elemental analysis across the core/clad interface strongly suggests that diffusion plays a large role in this amorphization. Despite the noncrystalline nature of the fibers, they do exhibit acceptable low losses (∼0.15–0.2 dB/m) for many applications, broad-band emissions in the near-infrared, and enhanced therm...

Surface vacancies in InP and GaAlAs

We present calculations of the bound‐state energy levels of ideal vacancies near the (110) surface of InP and GaAlAs. We find that there is a strong correlation between the the calculated position of the highest filled anion vacancy level in the neutral vacancy and the measured Fermi level at the surface. This correlation suggests that the recently proposed defect model of Schottky barrier formation is capable of accounting for the observed trends in Schottky barrier formation and that the states responsible in III‐V semiconductors are related to defects which introduce dangling cation bonds.

Some aspects of forces and fields in atomic models of crack tips

This paper examines the stresses and displacement gradients in atomistic models of cracks based on an EAM potential devised for aluminum. Methods for computing these quantities are described. Results are presented for two models differing in terms of the orientations of the crack relative to the crystal, a [100] (010) orientation that behaves in a brittle fashion and a [111] (110) orientation that emits partial dislocations prior to extending. Both models display lattice trapping. The stresses in the brittle crack model are compared with the linear elastic prediction and found to be in remarkably good agreement to within distances of about one lattice parameter of the crack tip and at the free surface where contributions from sources other than strain energy (e.g., surface tension) influence the results. Similar results are observed for the ductile model until dislocation emission occurs. The largest stresses that develop just prior to crack extension or dislocation emission are used to estimate the ratio of theoretical tensile strength to shear strength in this material. Eshelbys conservation integrals, F and M , are also computed. F is found to be essentially contour independent and in agreement with the linear elastic prediction in both models until dislocation emission occurs, at which point a large screening contribution arises from the emitted partials. The contour size dependence of M reveals some interesting features of the crack tip including a slight wobble of the crack tip inside its potential well with changing applied K and the existence of forces acting to move the crack faces apart as blunting occurs.

Atomic structure of a Σ99 grain boundary in aluminium: A comparison between atomic-resolution observation and pair-potential and embedded-atom simulations

Abstract An atomic-resolution image of a symmetrical Σ99 {557} 〈110〉 tilt boundary in aluminium is compared with images simulated from models based on atomistic calculations using pair potentials and the embedded-atom method. The two methods for atomistic modelling result in very similar structures, and image simulations based on these structures closely match the experimental results. This study shows that high-resolution electron microscopy can now be used to assess quantitatively the degree of coincidence between experimental and theoretical atomic structures of high-Σ grain boundaries.

High-resolution transmission electron microscopy of grain boundaries in aluminum and correlation with atomistic calculations

Abstract The application of high-resolution transmission electron microscopy (HRTEM) to the study of several symmetric tilt boundaries in aluminum is presented. The observed images are correlated with simulated images based on molecular statics calculations using the embedded atom method. In general, good agreement is obtained between experiment and theory. The complementary aspects of comparing HRTEM results with atomistic calculations are also discussed with respect to obtaining three-dimensional information about the boundaries and accounting for thin-foil effects on the observed structures.