[en] Arrays of GaAs pyramids with square (001) bases of length 1 - 5 μm have been fabricated by molecular beam epitaxy regrowth on pre-patterned GaAs (001) substrates. The optical properties of the pyramid faces have been studied by microreflection and microtransmission imaging measurements with light (λ=900 - 1000nm) incident through the pyramid base. Digitized charge coupled device images indicate that total internal reflection occurs at the {110} pyramid facets and that their reflectivities are greater than 80%, provided overgrowth of the facets does not occur. These properties suggest that such structures may be suitable as the top mirror in novel micron-scale vertical microcavity devices. [copyright] 2001 American Institute of Physics

[en] We measured valence band offsets in Ta₂O₅–WO₃, Ta₂O₅–Nb₂O₅ and WO₃–Nb₂O₅ heterostructure couples by in situ x-ray photoelectron spectroscopy, immediately following the bi-layer growth in ultra-high vacuum. Conduction band offsets were estimated using the measured valence band offsets in conjunction with the literature values for the respective band gaps. The offsets between Ta₂O₅ and WO₃ and between Ta₂O₅ and Nb₂O₅ layers were strongly asymmetric, with 0.8–1.1 eV (0.1–0.2 eV) barriers for the conduction (valence) bands, depending on the particular couple and the stacking sequence. Such asymmetry can be very useful in switching devices. (paper)

[en] We study electron hopping in thin metal-insulator-metal structures which involves two defect centres with a strong electron-phonon coupling. We calculate the dependences of the current, J, on voltage, V, and temperature, T, and show that they are consistent with those observed in molecular monolayers of fatty acids. We analyse in detail an unusual, near-exponential temperature dependence of the current: J(T) ∝ exp (T/T₀) T ≥ 50 K in eicosanoic acid (C20) organic monolayers sandwiched between Pt electrodes, where the parameter T₀ increases with the bias voltage. We show that at relatively high voltages the two-defect small-polaron hopping results in N-shape current-voltage characteristics which were observed in some organic molecular monolayers

[en] We report on the recent progress at Jefferson Lab in developing ultra high gradient and high Q₀ superconducting radio frequency (SRF) cavities for future SRF based machines. A new 1300 MHz 9-cell prototype cavity is being fabricated. This cavity has an optimized shape in terms of the ratio of the peak surface field (both magnetic and electric) to the acceleration gradient, hence the name low surface field (LSF) shape. The goal of the effort is to demonstrate an acceleration gradient of 50 MV/m with Q₀ of 10¹⁰ at 2 K in a 9-cell SRF cavity. Fine-grain niobium material is used. Conventional forming, machining and electron beam welding method are used for cavity fabrication. New techniques are adopted to ensure repeatable, accurate and inexpensive fabrication of components and the full assembly. The completed cavity is to be first mechanically polished to a mirror-finish, a newly acquired in-house capability at JLab, followed by the proven ILC-style processing recipe established already at JLab. In parallel, new single-cell cavities made from large-grain niobium material are made to further advance the cavity treatment and processing procedures, aiming for the demonstration of an acceleration gradient of 50 MV/m with Q₀ of 2пїЅ10¹⁰ at 2K