Optimization of Al/AlOx/AlAl/AlO_x/Al-Layer Systems for Josephson Junctions from a Microstructure Point of View

$Al/AlO_x/Al$-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 $Al/AlO_x/Al$-layer systems deposited on Si(111) substrates with a particular focus on the thickness homogeneity of the $AlO_x$-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 $Al/AlO_x/Al$-layer systems to determine the thickness homogeneity of the $AlO_x$ layer, grain size distribution in the Al layers, Al-grain boundary types and the morphology of the $Al/AlO_x$ interface. We show that the structural properties of the lower Al layer are decisive for the structural quality of the whole $Al/AlO_x/Al$-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 $\mu m$ and large lateral grain sizes up to 1 $\mu m$. Thickness fluctuations of the $AlO_x$-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$_{x}$)/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$_{x}$ 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$_{x}$-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$_{x}$/Al-layers systems with improved properties.Comment: 28 pages, 4 figure

Quantum correlations in the temporal CHSH scenario

We consider a temporal version of the CHSH scenario using projective measurements on a single quantum system. It is known that quantum correlations in this scenario are fundamentally more general than correlations obtainable with the assumptions of macroscopic realism and non-invasive measurements. In this work, we also educe some fundamental limitations of these quantum correlations. One result is that a set of correlators can appear in the temporal CHSH scenario if and only if it can appear in the usual spatial CHSH scenario. In particular, we derive the validity of the Tsirelson bound and the impossibility of PR-box behavior. The strength of possible signaling also turns out to be surprisingly limited, giving a maximal communication capacity of approximately 0.32 bits. We also find a temporal version of Hardy's nonlocality paradox with a maximal quantum value of 1/4.Comment: corrected versio

The power of monitoring stellar orbits

The center of the Milky Way hosts a massive black hole. The observational evidence for its existence is overwhelming. The compact radio source Sgr A* has been associated with a black hole since its discovery. In the last decade, high-resolution, near-infrared measurements of individual stellar orbits in the innermost region of the Galactic Center have shown that at the position of Sgr A* a highly concentrated mass of 4 x 10^6 M_sun is located. Assuming that general relativity is correct, the conclusion that Sgr A* is a massive black hole is inevitable. Without doubt this is the most important application of stellar orbits in the Galactic Center. Here, we discuss the possibilities going beyond the mass measurement offered by monitoring these orbits. They are an extremely useful tool for many scientific questions, such as a geometric distance estimate to the Galactic Center or the puzzle, how these stars reached their current orbits. Future improvements in the instrumentation will open up the route to testing relativistic effects in the gravitational potential of the black hole, allowing to take full advantage of this unique laboratory for celestial mechanics.Comment: Proceedings of the Galactic Center Workshop 2009, Shangha

Massive binaries in the vicinity of Sgr A*

A long-term spectroscopic and photometric survey of the most luminous and massive stars in the vicinity of the super-massive black hole Sgr A* revealed two new binaries; a long-period Ofpe/WN9 binary, GCIRS 16NE, with a modest eccentricity of 0.3 and a period of 224 days and an eclipsing Wolf-Rayet binary with a period of 2.3 days. Together with the already identified binary GCIRS 16SW, there are now three confirmed OB/WR binaries in the inner 0.2\,pc of the Galactic Center. Using radial velocity change upper limits, we were able to constrain the spectroscopic binary fraction in the Galactic Center to $F_{\rm SB}=0.27^{+0.29}_{-0.19}$ at a confidence level of 95%, a massive binary fraction similar to that observed in dense clusters. The fraction of eclipsing binaries with photometric amplitudes $\Delta m>0.4$ is $F^{\rm GC}_{\rm EB}=3\pm2$, which is consistent with local OB star clusters ($F_{\rm EB}=1$). Overall the Galactic Center binary fraction seems to be close to the binary fraction in comparable young clusters.Comment: 5 figures, submitted to Ap

Shock recovery experiments confirm the possibility of transferring viable microorganisms from Mars to Earth

Extract from introduction: With regard to the impact and ejection phase we tested the case for the transfer of microorganisms from Mars to Earth. Using a high explosive set-up thin layers of bacterial endospores of Bacillus subtilis, of the lichen Xanthoria elegans and of the cyanobacterium Chroococcidiopsis sp. embedded between two plates of gabbro were subjected to 10, 20, 30, 40 and 50 GPa which is the pressure range observed in Martian meteorites [1]