7,514 research outputs found
Optimization of -Layer Systems for Josephson Junctions from a Microstructure Point of View
-layer systems are frequently used for Josephson junction-based
superconducting devices. Although much work has been devoted to the
optimization of the superconducting properties of these devices, systematic
studies on influence of deposition conditions combined with structural analyses
on the nanoscale are rare up to now. We have focused on the optimization of the
structural properties of -layer systems deposited on Si(111)
substrates with a particular focus on the thickness homogeneity of the
-tunnel barrier. A standard high-vacuum electron-beam deposition system
was used and the effect of substrate pretreatment, different Al-deposition
temperatures and Al-deposition rates was studied. Transmission electron
microscopy was applied to analyze the structural properties of the
-layer systems to determine the thickness homogeneity of the
layer, grain size distribution in the Al layers, Al-grain boundary
types and the morphology of the interface. We show that the
structural properties of the lower Al layer are decisive for the structural
quality of the whole -layer system. Optimum conditions yield an
epitaxial Al(111) layer on a Si(111) substrate with an Al-layer thickness
variation of only 1.6 nm over more than 10 and large lateral grain
sizes up to 1 . Thickness fluctuations of the -tunnel barrier are
minimized on such an Al layer which is essential for the homogeneity of the
tunnel current. Systematic variation of the Al-deposition rate and deposition
temperature allows to develop an understanding of the growth mechanisms
Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits
This work is concerned with Al/Al-oxide(AlO)/Al-layer systems which are
important for Josephson-junction-based superconducting devices such as quantum
bits. The device performance is limited by noise, which has been to a large
degree assigned to the presence and properties of two-level tunneling systems
in the amorphous AlO tunnel barrier. The study is focused on the
correlation of the fabrication conditions, nanostructural and nanochemical
properties and the occurrence of two-level tunneling systems with particular
emphasis on the AlO-layer. Electron-beam evaporation with two different
processes and sputter deposition were used for structure fabrication, and the
effect of illumination by ultraviolet light during Al-oxide formation is
elucidated. Characterization was performed by analytical transmission electron
microscopy and low-temperature dielectric measurements. We show that the
fabrication conditions have a strong impact on the nanostructural and
nanochemical properties of the layer systems and the properties of two-level
tunneling systems. Based on the understanding of the observed structural
characteristics, routes are derived towards the fabrication of
Al/AlO/Al-layers systems with improved properties.Comment: 28 pages, 4 figure
Quantum analogues of Hardy's nonlocality paradox
Hardy's nonlocality is a "nonlocality proof without inequalities": it
exemplifies that quantum correlations can be qualitatively stronger than
classical correlations. This paper introduces variants of Hardy's nonlocality
in the CHSH scenario which are realized by the PR-box, but not by quantum
correlations. Hence this new kind of Hardy-type nonlocality is a proof without
inequalities showing that superquantum correlations can be qualitatively
stronger than quantum correlations.Comment: minor fixe
Interferometric Astrometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: Detection Limits for sub-Stellar Companions
We report on a sub-stellar companion search utilizing interferometric
fringe-tracking astrometry acquired with Fine Guidance Sensor 3 (FGS 3) on the
Hubble Space Telescope. Our targets were Proxima Centauri and Barnard's Star.
We obtain absolute parallax values for Proxima Cen pi_{abs} = 0.7687 arcsecond
and for Barnard's Star pi_{abs} = 0.5454 arcsecond.
Once low-amplitude instrumental systematic errors are identified and removed,
our companion detection sensitivity is less than or equal to one Jupiter mass
for periods longer than 60 days for Proxima Cen. Between the astrometry and the
radial velocity results we exclude all companions with M > 0.8M_{Jup} for the
range of periods 1 < P < 1000 days. For Barnard's Star our companion detection
sensitivity is less than or equal to one Jupiter mass for periods long er than
150 days. Our null results for Barnard's Star are consistent with those of
Gatewood (1995).Comment: 35 pages, 13 figures, to appear in August 1999 A
Photometry of Proxima Centauri and Barnard's Star Using HST Fine Guidance Sensor 3: A Search for Periodic Variations
We have observed Proxima Centauri and Barnard's Star with Hubble Space
Telescope Fine Guidance Sensor 3. Proxima Centauri exhibits small-amplitude,
periodic photometric variations. Once several sources of systematic photometric
error are corrected, we obtain 2 milli-magnitude internal photometric
precision. We identify two distinct behavior modes over the past four years:
higher amplitude, longer period; smaller amplitude, shorter period. Within the
errors one period (P ~ 83d) is twice the other. Barnard's Star shows very weak
evidence for periodicity on a timescale of approximately 130 days. If we
interpret these periodic phenomena as rotational modulation of star spots, we
identify three discrete spots on Proxima Cen and possibly one spot on Barnard's
Star. We find that the disturbances change significantly on time scales as
short as one rotation period.Comment: 39 pages, 17 figure
Contributing to WUDAPT: A Local Climate Zone Classification of Two Cities in Ukraine
Local climate zones (LCZs) divide the urban landscape into homogeneous types based on urban structure (i.e.,morphology of streets and buildings), urban cover (i.e., permeability of surfaces), construction materials, and human activities (i.e., anthropogenic heat). This classification scheme represents a standardized way of capturing the basic urban form of cities and is currently being applied globally as part of the world urban database and portal tools (WUDAPT) initiative. This paper assesses the transferability of the LCZ concept to two Ukrainian cities, i.e., Kyiv and Lviv, which differ in urban form and topography, and considers three ways to validate and verify this classification
scheme. An accuracy of 64% was achieved for Kyiv using an independent validation dataset while a comparison of the LCZ maps with the GlobeLand30 land cover map resulted in a match that was greater than 75% for both cities. There was also good correspondence between the urban classes in the LCZ maps and the urban points of interest in OpenStreetMap (OSM). However, further research is still required to produce a standardized validation protocol that could be used on a regular basis by contributors to WUDAPT to help produce more accurate LCZ maps in the future
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