877 research outputs found
Initial stage of the 2D-3D transition of a strained SiGe layer on a pit-patterned Si(001) template
We investigate the initial stage of the 2D-3D transition of strained Ge
layers deposited on pit-patterned Si(001) templates. Within the pits, which
assume the shape of inverted, truncated pyramids after optimized growth of a Si
buffer layer, the Ge wetting layer develops a complex morphology consisting
exclusively of {105} and (001) facets. These results are attributed to a
strain-driven step-meandering instability on the facetted side-walls of the
pits, and a step-bunching instability at the sharp concave intersections of
these facets. Although both instabilities are strain-driven, their coexistence
becomes mainly possible by the geometrical restrictions in the pits. It is
shown that the morphological transformation of the pit surface into low-energy
facets has strong influence on the preferential nucleation of Ge islands at the
flat bottom of the pits.Comment: 19 pages, 7 figure
Development of the ALMA-North America Sideband-Separating SIS Mixers
As the Atacama Large Millimeter/submillimeter Array (ALMA) nears completion,
73 dual-polarization receivers have been delivered for each of Bands 3 (84-116
GHz) and 6 (211-275 GHz). The receivers use sideband-separating superconducting
Nb/Al-AlOx/Nb tunnel-junction (SIS) mixers, developed for ALMA to suppress
atmospheric noise in the image band. The mixers were designed taking into
account dynamic range, input return loss, and signal-to-image conversion (which
can be significant in SIS mixers). Typical SSB receiver noise temperatures in
Bands 3 and 6 are 30 K and 60 K, resp., and the image rejection is typically 15
dB.Comment: Submitted to IEEE Trans. Microwave Theory Tech., June 2013. 10 pages,
21 figure
Fluorescent visualization of a spreading surfactant
The spreading of surfactants on thin films is an industrially and medically
important phenomenon, but the dynamics are highly nonlinear and visualization
of the surfactant dynamics has been a long-standing experimental challenge. We
perform the first quantitative, spatiotemporally-resolved measurements of the
spreading of an insoluble surfactant on a thin fluid layer. During the
spreading process, we directly observe both the radial height profile of the
spreading droplet and the spatial distribution of the fluorescently-tagged
surfactant. We find that the leading edge of spreading circular layer of
surfactant forms a Marangoni ridge in the underlying fluid, with a trough
trailing the ridge as expected. However, several novel features are observed
using the fluorescence technique, including a peak in the surfactant
concentration which trails the leading edge, and a flat, monolayer-scale
spreading film which differs from concentration profiles predicted by current
models. Both the Marangoni ridge and surfactant leading edge can be described
to spread as . We find spreading exponents, and for the ridge peak and
surfactant leading edge, respectively, which are in good agreement with
theoretical predictions of . In addition, we observe that the
surfactant leading edge initially leads the peak of the Marangoni ridge, with
the peak later catching up to the leading edge
The Nonsteroidal Anti-Inflammatory Drug Indomethacin Induces Heterogeneity in Lipid Membranes: Potential Implication for Its Diverse Biological Action
The nonsteroidal anti-inflammatory drug (NSAID), indomethacin (Indo), has a large number of divergent biological effects, the molecular mechanism(s) for which have yet to be fully elucidated. Interestingly, Indo is highly amphiphilic and associates strongly with lipid membranes, which influence localization, structure and function of membrane-associating proteins and actively regulate cell signaling events. Thus, it is possible that Indo regulates diverse cell functions by altering micro-environments within the membrane. Here we explored the effect of Indo on the nature of the segregated domains in a mixed model membrane composed of dipalmitoyl phosphatidyl-choline (di16:0 PC, or DPPC) and dioleoyl phosphatidyl-choline (di18:1 PC or DOPC) and cholesterol that mimics biomembranes.Using a series of fluorescent probes in a fluorescence resonance energy transfer (FRET) study, we found that Indo induced separation between gel domains and fluid domains in the mixed model membrane, possibly by enhancing the formation of gel-phase domains. This effect originated from the ability of Indo to specifically target the ordered domains in the mixed membrane. These findings were further confirmed by measuring the ability of Indo to affect the fluidity-dependent fluorescence quenching and the level of detergent resistance of membranes.Because the tested lipids are the main lipid constituents in cell membranes, the observed formation of gel phase domains induced by Indo potentially occurs in biomembranes. This marked Indo-induced change in phase behavior potentially alters membrane protein functions, which contribute to the wide variety of biological activities of Indo and other NSAIDs
SuperGaN: Synthesis of NbTiN/GaN/NbTiN Tunnel Junctions
Nb-based circuits have broad applications in quantum-limited photon
detectors, low-noise parametric amplifiers, superconducting digital logic
circuits, and low-loss circuits for quantum computing. The current
state-of-the-art approach for superconductor-insulator-superconductor (SIS)
junction material is the Gurvitch trilayer process based on magnetron
sputtering of Nb electrodes with Al-Oxide or AlN tunnel barriers grown on an Al
overlayer. However, a current limitation of elemental Nb-based circuits is the
low-loss operation of THz circuits operating above the 670 GHz gap frequency of
Nb and operation at higher temperatures for projects with a strict power
budget, such as space-based applications.
NbTiN is an alternative higher energy gap material and we have previously
reported on the first NbTiN/AlN/NbTiN
superconducting-insulating-superconducting (SIS) junctions with an epitaxially
grown AlN tunnel barrier. One drawback of a directly grown tunnel barrier
compared to thermal oxidation or plasma nitridation is control of the barrier
thickness and uniformity across a substrate, leading to variations in current
density (Jc). Semiconductor barriers with smaller barrier heights enable
thicker tunnel barriers for a given Jc. GaN is an alternative semiconductor
material with a closed-packed Wurtzite crystal structure similar to AlN and it
can be epitaxially grown as a tunnel barrier using the Reactive Bias Target Ion
Beam Deposition (RBTIBD) technique. This work presents the preliminary results
of the first reported high-quality NbTiN/GaN/NbTiN heterojunctions with
underdamped SIS I(V) characteristics.Comment: Presented at the 16th EUROPEAN CONFERENCE ON APPLIED
SUPERCONDUCTIVIT
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