This figure shows the core of a dislocation (the so-called 30 degree partial) in crystalline silicon as seen from the perspective of its electrons, as calculated within the DFT++ formulation of density functional theory, a highly flexible formulation which allows the rapid development of extremely efficient, portable scalar and parallel software for the ab initio calculation of material and chemical properties. (See Secs. III(B-F) of ``Multiresolution analysis of electronic structure: semicardinal and orthogonal wavelet bases,'' T.A. Arias, Reviews of Modern Physics 71, 267 (1999).

The spheres (white and light green -- sometimes they take on a brownish shade due to shadowing in the figure) represent the atomic cores, whereas the translucent gold and red surfaces represent contours (lower and higher, respectively) of the electron density. The build up of charge corresponding to bonds appear as red regions outlined in gold between each pair of atoms. The figure shows that the dislocation core maintains the expected four-fold, tetrahedral bonding network of silicon.

The atoms highlighted in green make up the core of the dislocation, which prefer to have atoms bonded together in pairs as do the two green atoms in the left half of the figure. The green atom on the right represents the most common defect of the dislocation, a soliton. Rather than pairing with another core atom, it "reaches" up and to the left to bond with a white, non-core atom, which already had four bonds and now forms a "floating-bond" defect, a silicon atom with five bonds.

Floating bonds are usually only found in amorphous silicon, and the unusual structure of the this soliton defect explains the unusual symmetry of the primary component of the magentic resonance observed in plastically deformed silicon. This resonance had remained a mystery for several decades and was oft noted to resemble resonance signatures found in amorphous silicon. (See ``Paramagnetic structure of the soliton of the 30 partial dislocation in silicon,'' by Gábor Csányi, Sohrab Ismail-Beigi and T.A. Arias, Physical Review Letters 80, 3984 (1998).)

Study of the formation and migration of this defect prompted the development and was the first application of the multiscale sampling approach (MSA) for computing the effects of configurational entropy at non-zero temperatures. (See ``A Multiscale Approach to Determination of Thermal Properties and Changes in Free Energy: Application to Reconstruction of Dislocations in Silicon,'' T.D. Engeness and T.A. Arias, Physical Review Letters, 79, 3006 (1997).) MSA has since been generalized to the study of dynamics, resulting in the ab initio hyperdynamics approach.

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