The shape of epitaxially grown silicon nanowires and the influence of line tension

Silicon nanowires grown epitaxially via the vapor–liquid–solid mechanism show a larger diameter at the base of the nanowire, which cannot be explained by an overgrowth of the nanowire alone. By considering the equilibrium condition for the contact angle of the droplet, the Neumann quadrilateral relation, a quasi-static model of epitaxial nanowire growth is derived. It is found that a change of the contact angle of the droplet is responsible for the larger diameter of the nanowire base, so that the expansion has to be considered a fundamental aspect of epitaxial vapor–liquid–solid growth. By comparison of experimental results with theoretical calculations, an estimate for the line tension is obtained. In addition, the growth model predicts the existence of two different growth modes. Only within a certain range of line-tension values is the mode corresponding to ordinary nanowire growth realized, whereas nanowire growth stops at a relatively small height if the line tension exceeds an upper boundary. An approximate analytic expression for the upper boundary as a function of the surface tensions is given. PACS 68.65.-k; 61.46.+w; 81.10.Bk     

Toxikologie

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Theory of nuclear quadrupole interactions in aluminum and copper metals in presence of muon

The electric field gradients at²⁷Al and⁶³Cu nuclei which are nearest neighbors to the muon in the face-centered cubic metals aluminum and copper, with muon at an octahedral interstitial site, are studied. The electron density fluctuations needed for the evaluation of the electronic, or valence, contributions to field gradients are taken from earlier investigations involving the spherical solid approximation. The enhancement factors a that have to be applied to the electric field-gradients obtained from these approximations, due to the departures of the electronic wave-functions from plane-wave character, and the incorporation of antishielding effects, have been obtained for both APW and OPW approaches to the conduction electron wave-functions and good agreement is found between the results by the two approaches. Size effects due to the lattice distortion associated with the presence of the muon are included through actual point ion summations using available calculated displacements of the ions surrounding the muon. The valence contributions are the dominant ones but the size effect contributions are also significant. The net field-gradient obtained for the²⁷Al site is significantly smaller than experiment while that for⁶³Cu is substantially larger than experiment. Possible sources that could lead to better agreement with experiment are discussed and it is concluded that major improvement is needed in the valence effect contributions in both metals.     

Nonlinear transport in p-type SiGe quantum well structure containing Ge quantum dots

We use magneto-transport spectroscopy to study a dramatic instability between a low and high conductivity mode in Si/SiGe-based resonant tunneling diodes with an embedded layer of self-assembled Ge hut cluster quantum dots in the Si barrier. In the low current regime a simple activation-type behavior with an astonishingly low activation energy in the order of 0.1 meV is determined. In the high current regime a region of negative differential conduction can be observed. We discuss the influence of different layer structures and magnetic fields.     

Phase control in a coherent superposition of degenerate quantum states through frequency control

We demonstrate a controlled phase change of π in a degenerate superposition by altering a laser frequency by only 10 MHz. The method relies on the preparation of an adiabatic state involving the M = ±2 and M = 0 states of the ³P₂ (J = 2) level of metastable neon. Dependent on the frequency, the preparation proceeds either by stimulated Raman adiabatic passage (STIRAP) or by coherent population trapping (CPT). In the former case the superposition is prepared by adiabatic transfer induced in an extended tripod linkage scheme. In the latter case population is optically pumped into the Zeeman manifold of the level ³P₂. The population which does not reach a dark state decays to the ground state of neon. The amplitudes and relative phases of the dark states differ for the two cases. The phase change is monitored using the method of phase-to-population mapping.     

Interaction of He Atoms with XeF2 Observed in Crossed Molecular Beams

The interactional shape of XeF₂ with respect to energy and distance was observed by He atom scattering in molecular beam experiments. The heterogeneous second virial coefficient and the binary coefficient of diffusion were calculated from the resulting interaction potential.