Self-directed growth of AlGaAs core-shell nanowires for visible light applications

Al(0.37)Ga(0.63)As nanowires (NWs) were grown in a molecular beam epitaxy system on GaAs(111)B substrates. Micro-photoluminescence measurements and energy dispersive X-ray spectroscopy indicated a core-shell structure and Al composition gradient along the NW axis, producing a potential minimum for carrier confinement. The core-shell structure formed during the growth as a consequence of the different Al and Ga adatom diffusion lengths.Comment: 20 pages, 7 figure

Analytical solution of generalized Burton--Cabrera--Frank equations for growth and post--growth equilibration on vicinal surfaces

We investigate growth on vicinal surfaces by molecular beam epitaxy making use of a generalized Burton--Cabrera--Frank model. Our primary aim is to propose and implement a novel analytical program based on a perturbative solution of the non--linear equations describing the coupled adatom and dimer kinetics. These equations are considered as originating from a fully microscopic description that allows the step boundary conditions to be directly formulated in terms of the sticking coefficients at each step. As an example, we study the importance of diffusion barriers for adatoms hopping down descending steps (Schwoebel effect) during growth and post-growth equilibration of the surface.Comment: 16 pages, REVTeX 3.0, IC-DDV-94-00

Novel diffusion mechanism on the GaAs(001) surface: the role of adatom-dimer interaction

Employing first principles total energy calculations we have studied the behavior of Ga and Al adatoms on the GaAs(001)-beta2 surface. The adsorption site and two relevant diffusion channels are identified. The channels are characterized by different adatom-surface dimer interaction. Both affect in a novel way the adatom migration: in one channel the diffusing adatom jumps across the surface dimers and leaves the dimer bonds intact, in the other one the surface dimer bonds are broken. The two channels are taken into account to derive effective adatom diffusion barriers. From the diffusion barriers we conclude a strong diffusion anisotropy for both Al and Ga adatoms with the direction of fastest diffusion parallel to the surface dimers. In agreement with experimental observations we find higher diffusion barriers for Al than for Ga.Comment: 4 pages, 2 figures, Phys. Rev. Lett. 79 (1997). Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

Dynamics of a deformable self-propelled particle under external forcing

We investigate dynamics of a self-propelled deformable particle under external field in two dimensions based on the model equations for the center of mass and a tensor variable characterizing deformations. We consider two kinds of external force. One is a gravitational-like force which enters additively in the time-evolution equation for the center of mass. The other is an electric-like force supposing that a dipole moment is induced in the particle. This force is added to the equation for the deformation tensor. It is shown that a rich variety of dynamics appears by changing the strength of the forces and the migration velocity of self-propelled particle

Re-entrant Layer-by-Layer Etching of GaAs(001)

We report the first observation of re-entrant layer-by-layer etching based on {\it in situ\/} reflection high-energy electron-diffraction measurements. With AsBr$_3$ used to etch GaAs(001), sustained specular-beam intensity oscillations are seen at high substrate temperatures, a decaying intensity with no oscillations at intermediate temperatures, but oscillations reappearing at still lower temperatures. Simulations of an atomistic model for the etching kinetics reproduce the temperature ranges of these three regimes and support an interpretation of the origin of this phenomenon as the site-selectivity of the etching process combined with activation barriers to interlayer adatom migration.Comment: 11 pages, REVTeX 3.0. Physical Review Letters, in press

X-ray diffraction study of spontaneous strain in superconducting Ba0.6 K0.4 BiO3

金沢大学理工研究域数物科学系The lattice parameter of Ba0.6 K0.4 BiO3, which is a well-studied conventional superconductor, was measured by the X-ray Rietveld method between 10 K and room temperature. A very small change in the lattice parameter could be detected in the superconducting phase. The change could be attributed to a spontaneous strain generated in the superconducting phase by the coupling between a superconducting order parameter and the strain. Previously published data on Y Ba2 Cu3 O6.5 and MgB2 were analyzed, and the present results were compared with our previously obtained results on La1.85 Sr0.15 CuO4. It was concluded that this coupling phenomenon is common to all superconductors. © 2010 Elsevier Ltd. All rights reserved

Classification and comparison of snow fences for the protection of transport infrastructures

Blowing snow or sand transport generates serious problems such as transport infrastructures buried under snow or sand in many parts of the world. Some of the most important problems that snow and sand storms can cause include drivers getting trapped on the roads, traffic being held up indefinitely, accidents occurring and populations being isolated. Snow fences provide a solution to this problem as they can hold back the snow, preventing displacement and wind-induced drifting. In this way, they reduce these problems on transport infrastructures and improve visibility, providing safer driving conditions. In this review, a classification is proposed of snow fences based on three basic types: earth, structural and living snow fences. Among the structural ones, non-porous and porous snow fences are distinguished. The different possibilities in terms of the placement of snow fences are also analyzed. Finally, different types of snow fences have been compared under design, construction and operation criteria. This review can provide initial guidelines for technicians to choose the best snow fence for blizzard conditions