James E. Bernard

Extended shape evolution of low mismatch Si1−xGex alloy islands on Si(100)

The sequence of shape transitions in low mismatch, dilute coherent Si1−xGex (x<0.2) alloy islands was documented by scanning tunneling microscopy and cross-sectional transmission electron microscopy. In dilute Si1−xGex islands we observe an extended shape evolution involving a new “barn” shape formed by introduction of steep {111} facets not observed at higher mismatch strain. This extended shape evolution implies a delayed onset of plastic deformation as a result of an altered competition between strain relaxation via coherent islands and the introduction of dislocations in this regime.

Is there an elastic anomaly for a (001) monolayer of InAs embedded in GaAs

When a coherently grown (001)‐oriented layer of InAs is embedded in a GaAs host, the coherency strain induces a perpendicular distortion of the embedded layer, predicted by continuum elasticity theory to be e⊥=7.3%. Brandt, Ploog, Bierwolf, and Hohenstein, [Phys. Rev. Lett. 68, 1339 (1992)] have described a high‐resolution electron microscopic analysis of such buried layers that appears to reveal a breakdown of continuum elasticity theory in the limit of monolayer films. In particular, they found for a single monolayer of InAs a lattice distortion that corresponds to e⊥=12.5%. Here we report on an investigation into whether a first‐principles local‐density total energy minimization shows such an elastic anomaly in the monolayer limit. We find that it does not.

Ordering in semiconductor alloys

A thermodynamic first‐principles theory of stability, including charge transfer, elastic forces, and atomic relaxations reveals the physical origins of stable and metastable ordering in bulk and epitaxial semiconductor alloys and superlattices.

Thermodynamics of surface‐induced ordering in the Ga1−xInxP alloy

The occurrence of spontaneous CuPt‐like ordering in epitaxial Ga1−xInxP alloys is examined by means of a thermodynamic theory. We treat both cation‐ and anion‐terminated surfaces. We find that the order parameter of the observed CuPtB variant is significant for reconstructed cation‐terminated surfaces even at preparation temperatures. A possible mechanism for the formation of the three‐dimensional CuPtB structure involves partial equilibration of the cation‐terminated reconstructed surface followed by freezing of the cations as the surface is buried by newly deposited layers.

Exact Dynamics of Multicomponent Bose-Einstein Condensates in Optical Lattices in One, Two and Three Dimensions

Numerous exact solutions to the nonlinear mean-field equations of motion are constructed for multicomponent Bose-Einstein condensates on one-, two-, and three-dimensional optical lattices. We find both stationary and nonstationary solutions, which are given in closed form. Among these solutions are a vortex-antivortex array on the square optical lattice and modes in which two or more components slosh back and forth between neighboring potential wells. We obtain a variety of solutions for multicomponent condensates on the simple cubic lattice, including a solution in which one condensate is at rest and the other flows in a complex three-dimensional array of intersecting vortex lines. A number of physically important solutions are stable for a range of parameter values, as we show by direct numerical integration of the equations of motion.