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The relation between the bond defect, which is a topological defect, and structural transformations between crystalline and amorphous silicon, is studied by molecular-dynamics simulations. The investigation of 1-keV boron implantation into crystalline silicon proves that the bond defect can also be generated directly by collisional-induced bond switching in addition to its formation by incomplete recombination of primary defects. This supports the assumption that the bond defect may play an important role in the amorphization process of silicon by light ions. The analysis of the interface between ͑001͒ silicon and amorphous silicon shows that there are two typical defect configurations at the interface which result from two different orientations of the bond defect with respect to the interface. Thus the bond defect appears to be a characteristic structural feature of the interface. Moreover, annealing results indicate that the bond defect acts as a growth site for interface-mediated crystallization.

In this paper we present a model which allows numerical studies of ferrofluid dynamics taking into account the internal magnetic degrees of freedom of the ferrofluid particles. In standard ferrofluid models the magnetic moment of a ferrofluid particle is supposed to be fixed with respect to the particle itself, which corresponds to the limit of an infinitely high single-particle magnetic anisotropy. In contrast to this strongly simplifying assumption, we take into account that in real ferrofluids the magnetic moments of ferrofluid particles are allowed to rotate with respect to the particles themselves. Our model results in a system of equations of motion describing both magnetic and mechanical degrees of freedom, where the ‘magnetic’ equations are coupled with the ‘mechanical’ equations via (i) the interparticle distances determining the magnetodipolar interaction fields and (ii) orientations of the particle anisotropy axes with respect to their magnetic moments which define the mechanical torque on the particle. Using our model we have studied the ferrofluid magnetization dynamics for various particle concentrations, i.e., for various magnetodipolar interaction strengths. In particular, we present numerical results (a) the magnetization relaxation of a ferrofluid after the external field is switched off and (b) the frequency dependence of the ferrofluid AC susceptibility. Comparing these results with the corresponding dependences obtained for the rigid dipoles model, we demonstrate that for magnetic anisotropy values typical for commonly used ferrofluid materials (like magnetite) the inclusion of ‘magnetic’ degrees of freedom is qualitatively essential to obtain a correct description of the ferrofluid dynamics.

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We have studied the evolution of cellular structures in Ge 1-x Si x single-crystal growth as a function of process parameters. Because these structures are much larger than those occurring during the solidification of metals, we developed a modified phase-field method, which is able to handle these structure within reasonable computer times using the real material parameters. The model has been tested for computing equilibrium shapes of crystals, dendritic growth, and cellular growth of Ni x Cu 1-x. We also performed classical molecular dynamics calculations in order to compute the diffusion coefficients of Si and Ge in melts of various compositions.