398 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
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
Coulomb Blockade and Coherent Single-Cooper-Pair Tunneling in Single Josephson Junctions
We have measured the current-voltage characteristics of small-capacitance
single Josephson junctions at low temperatures (T < 0.04 K), where the strength
of the coupling between the single junction and the electromagnetic environment
was controlled with one-dimensional arrays of dc SQUIDs. We have clearly
observed Coulomb blockade of Cooper-pair tunneling and even a region of
negative differential resistance, when the zero-bias resistance of the SQUID
arrays is much higher than the quantum resistance h/e^2 = 26 kohm. The negative
differential resistance is evidence of coherent single-Cooper-pair tunneling in
the single Josephson junction.Comment: RevTeX, 4 pages with 6 embedded figure
Luminescence dating of soils and sediments from Jerash, Jordan
The urban site of Jerash, Jordan is recognised as one of the great cities of the classical Middle East and has been the subject of ongoing systematic archaeological investigations since the 1920s. Its significance lies in its location on limestone geology in one of the more fertile areas of the Ajlun Highlands in northern Jordan with a good water supply, a number of springs and its central position in regional trade routes. The hinterland context of the city is yet to be considered and is a significant omission given the importance of water and its management together with the agricultural systems dependent on water in supporting urban development. Landscape chronologies are vital to the establishment of city and hinterland relationships and in this working papers we assess the value and significance of optically stimulated luminescence (OSL) measurement in this endeavour. Our findings so far suggest a measurement cluster range of ca. 480 BC – 250 BC in landscapes underlying the city and a dominant trend of sediments infilling the adjacent Wadi Suf between 640 ± 240 AD and 1400 ± 60 AD reflecting land management changes in a soil environment sensitive to degradation
Integrated heterodyne array receivers for submillimeter astronomy
The advent of large format (~100 pixel) spectroscopic imaging cameras at submillimeter wavelengths would fundamentally change the way in which astronomy is performed in this important wavelength regime. While the possibility of such instruments has been discussed for more than two decades, only recently have advances in mixer technology, device fabrication, micromachining, digital signal processing, and telescope design made the construction of such an instrument possible and economical. In our paper, we will present the design concept for a 10×10 heterodyne camera
Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating
We model the photoelectric emission from and charging of interstellar dust
and obtain photoelectric gas heating efficiencies as a function of grain size
and the relevant ambient conditions. Using realistic grain size distributions,
we evaluate the net gas heating rate for various interstellar environments, and
find less heating for dense regions characterized by R_V=5.5 than for diffuse
regions with R_V=3.1. We provide fitting functions which reproduce our
numerical results for photoelectric heating and recombination cooling for a
wide range of interstellar conditions. In a separate paper we will examine the
implications of these results for the thermal structure of the interstellar
medium. Finally, we investigate the potential importance of photoelectric
heating in H II regions, including the warm ionized medium. We find that
photoelectric heating could be comparable to or exceed heating due to
photoionization of H for high ratios of the radiation intensity to the gas
density. We also find that photoelectric heating by dust can account for the
observed variation of temperature with distance from the galactic midplane in
the warm ionized medium.Comment: 50 pages, including 18 figures; corrected title and abstract field
Ultra-Thin Silicon Beam Lead Chips for Superconducting Terahertz Circuits
Abstract. We present a process for fabricating THz superconducting circuits on ultra-thin (4um and less) silicon chips. The chips feature gold beam-leads, and are designed to accommodate RF filter structures, and either SIS junctions or hot-electron bolometers as the non-linear circuit element. The beam leads provide electrical connections, thermal contact, and physical support for the chip within a waveguide. Our approach begins by fabricating the superconducting circuit and beam leads atop the device layer of a silicon-on-insulator (SOI) substrate. The chip is then mounted, device side down, atop a quartz carrier wafer. A combination of mechanical lapping and chemical etching removes the handle silicon. Using backside photolithographic alignment through the quartz carrier, a thick photoresist is patterned on the exposed device silicon. The individual chips are then defined in a reactive ion etch of the device silicon, which is terminated after the quartz carrier and gold beam leads are exposed. The combination of superconducting mixers technology and silicon-micromachining techniques promises to open up the THz regime to large format spectroscopic imaging arrays. The potential for such systems are multiple; examples include atmospheric research, astrophysics, and security systems
The number of transmission channels through a single-molecule junction
We calculate transmission eigenvalue distributions for Pt-benzene-Pt and
Pt-butadiene-Pt junctions using realistic state-of-the-art many-body
techniques. An effective field theory of interacting -electrons is used to
include screening and van der Waals interactions with the metal electrodes. We
find that the number of dominant transmission channels in a molecular junction
is equal to the degeneracy of the molecular orbital closest to the metal Fermi
level.Comment: 9 pages, 8 figure
First results from DesertSTAR: a 7-pixel 345-GHz heterodyne array receiver for the Heinrich Hertz Telescope
We present the first astronomical results from DesertSTAR, a 7 pixel heterodyne array receiver designed for operation in the astrophysically rich 345 GHz atmospheric window. DesertSTAR was constructed for the 10m Heinrich Hertz Telescope located at 3150m elevation on Mt. Graham, Arizona. This receiver promises to increase mapping speed at the HHT by a factor of ~15 over the facility's existing single beam, dual polarization receiver. DesertSTAR uses tunerless, single-ended waveguide SIS mixers to achieve uncorrected receiver noise temperatures of ~60K. The instantaneous bandwidth is 2 GHz, with a 5 GHz Intermediate Frequency, offering 1600 km/s of velocity coverage. Cryogenic isolators are employed between the mixers and low noise amplifiers to assure a flat IF passband. The system uses a Joule-Thompson closed-cycle refrigerator with 180W capacity at 70K and 1.8W capacity at 4K. A novel reflective phase grating is used for Local Oscillator multiplexing, while a simple Mylar beamsplitter is used as an LO diplexer. Optics include only polyethelene mixer lenses and a single, cold, flat mirror, maximizing simplicity for high efficiency and easy optical alignment. The computer controlled bias system provides low noise bias for the SIS junctions, magnets and LNAs through a modular and hardware independent GUI interface, and allows remote operation and monitoring. We present measurements of receiver noise, beam quality, efficiency and stability in addition to astronomical observations obtained during engineering runs at the HHT
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