We report proton radiation enhanced self-diffusion (RESD) studies on Si-isotope heterostructures. Self-diffusion experiments under irradiation were performed at temperatures between 780 degrees C and 872 degrees C for various times and proton fluxes. Detailed modeling of RESD provides direct evidence that vacancies at high temperatures diffuse with a migration enthalpy of H(m)(V)=(1.8+/-0.5) eV significantly more slowly than expected from their diffusion at low temperatures, which is described by H(m)(V)<0.5 eV. We conclude that this diffusion behavior is a consequence of the microscopic configuration of the vacancy whose entropy and enthalpy of migration increase with increasing temperature.

We consider crystal formation of particles with dipole-dipole interactions that are confined to move in a one-dimensional helical geometry with their dipole moments oriented along the symmetry axis of the confining helix. The stable classical lowest energy configurations are found to be chain structures for a large range of pitch-to-radius ratios for relatively low density of dipoles and a moderate total number of particles. The classical normal mode spectra support the chain interpretation both through structure and the distinct degeneracies depending discretely on the number of dipoles per revolution. A larger total number of dipoles leads to a clusterization where the dipolar chains move closer to each other. This implies a change in the local density and the emergence of two length scales, one for the cluster size and one for the inter-cluster distance along the helix. Starting from three dipoles per revolution, this implies a breaking of the initial periodicity to form a cluster of two chains close together and a third chain removed from the cluster. This is driven by the competition between in-chain and out-of-chain interactions, or alternatively the side-by-side repulsion and the head-totail attraction in the system. The speed of sound propagates along the chains. It is independent of the number of chains although depending on geometry.

We study a quantum mechanical system consisting of up to three identical dipoles confined to move along a helical shaped trap. The long-range interactions between particles confined to move in this one dimension leads to an interesting effective two-particle potential with an oscillating behaviour. For this system we calculate the spectrum and the wave functions of the bound states. The full quantum solutions show clear imprints of the tendency for the system to form chains of dipoles along the helix, i.e. a configuration in which the dipoles are sitting approximately one winding of the helix apart so that they can take maximal advantage of the strong head-to-tail attraction that is a generic feature of the dipole-dipole interaction.

We consider one particle confined to a deformed one-dimensional wire. The quantum mechanical equivalent of the classical problem is not uniquely defined. We describe several possible hamiltonians and corresponding solutions for a finite wire with fixed endpoints and non-vanishing curvature. We compute and compare the disparate eigenvalues and eigenfunctions obtained from different quantization prescriptions. The JWKB approximation without potential leads precisely to the square well spectrum and the coordinate dependent stretched or compressed box related eigenfunctions. The geometric potential arising from an adiabatic expansion in terms of curvature is at best only valid for very small curvature.

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