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

Dynamic Versus Static Oxidation of Nb/Al-AlOx_x/Nb Trilayer

High quality Nb-based superconductor-insulator-superconductor (SIS) junctions with Al oxide (AlO$_x$) tunnel barriers grown from Al overlayers are widely reported in the literature. However, the thin barriers required for high critical current density (J$_c$) junctions exhibit defects that result in significant subgap leakage current that is detrimental for many applications. High quality, high-J$_c$ junctions can be realized with AlN$_x$ barriers, but control of J$_c$ is more difficult than with AlO$_x$. It is therefore of interest to study the growth of thin AlO$_x$ barriers with the ultimate goal of achieving high quality, high-J$_c$ AlO$_x$ junctions. In this work, 100\%\ O$_2$ and 2\%\ O$_2$ in Ar gas mixtures are used both statically and dynamically to grow AlO$_x$ tunnel barriers over a large range of oxygen exposures. In situ ellipsometry is used for the first time to extensively measure AlO$_x$ tunnel barrier growth in real time, revealing a number of unexpected patterns. Finally, a set of test junction wafers was fabricated that exhibited the well-known dependence of J$_c$ on oxygen exposure (E) in order to further validate the experimental setup

The Source Regions of Whistlers.

We present a new method for identifying the source regions of lightning‐generated whistlers observed at a fixed location. In addition to the spatial distribution of causative lightning discharges, we calculate the ratio of lightning discharges transmitted into ground detectable whistlers as a function of location. Our method relies on the time of the whistlers and the time and source location of spherics from a global lightning database. We apply this method to whistlers recorded at 15 ground‐based stations in the Automatic Whistler Detector and Analyzer Network operating between 2007 and 2018 and to located lightning strokes from the World Wide Lightning Location Network database. We present the obtained maps of causative lightning and transmission rates. Our results show that the source region of whistlers corresponding to each ground station is around the magnetic conjugate point of the respective station. The size of the source region is typically less than 2,000 km in radius with a small fraction of sources extending to up to 3,500 km. The transmission ratio is maximal at the conjugate point and decreases with increasing distance from it. This conforms to the theory that whistlers detected on the ground propagated in a ducted mode through the plasmasphere, and thus, the lightning strokes of their causative spherics must cluster around the footprint of the ducts in the other hemisphere. Our method applied resolves the whistler excitation region mystery that resulted from correlation‐based analysis methods, concerning the source region of whistlers detected in Dunedin, New Zealand

Genomic epidemiology reveals multiple introductions of Zika virus into the United States

Zika virus (ZIKV) is causing an unprecedented epidemic linked to severe congenital abnormalities. In July 2016, mosquito-borne ZIKV transmission was reported in the continental United States; since then, hundreds of locally acquired infections have been reported in Florida. To gain insights into the timing, source, and likely route(s) of ZIKV introduction, we tracked the virus from its first detection in Florida by sequencing ZIKV genomes from infected patients and Aedes aegypti mosquitoes. We show that at least 4 introductions, but potentially as many as 40, contributed to the outbreak in Florida and that local transmission is likely to have started in the spring of 2016-several months before its initial detection. By analysing surveillance and genetic data, we show that ZIKV moved among transmission zones in Miami. Our analyses show that most introductions were linked to the Caribbean, a finding corroborated by the high incidence rates and traffic volumes from the region into the Miami area. Our study provides an understanding of how ZIKV initiates transmission in new regions

Synthesis and characterization of organically modified aluminum and yttrium alkoxides

Zsfassung in dt. SpracheDie Kontrolle der Reaktivität in Hydrolyse- und Kondensationsreaktionen durch die Modifizierung von Metall Alkoxiden mit organischen Liganden ermöglicht ihre Verwendung als Vorstufen für Sol-Gel Materialien. Gleichzeitig wird auch die Struktur des resultierenden Materials beeinflusst und der Verbleib von organischen Liganden im Material eröffnet eine Strategie zur Synthese anorganisch-organischer Hybridmaterialien. Ein Verstehen der Einflüsse der organischen Liganden auf die Struktur und Reaktivität der Vorstufen ist eine Voraussetzung für ein vorausschauendes Design neuer Sol-Gel (Hybrid-)Materialien.Aluminium Alkoxiden wurden mit [beta]-diketonischen Verbindungen ([beta]-Ketoester, Dialkylmalonate, [beta]-Ketoamide) modifiziert.Verbindungen des Typs Al([beta]-Ketoesterat)3 wurde durch Reaktion von [Al(OR)3]n (OR =OtBu, OiPr) mit 3 Äquivalenten [beta]-Ketoester erhalten. Alle Komplexe weisen eine mononukleare oktaedrische Struktur auf. Verbindungen des Typs Al2(my-OiPr)2(OiPr)([beta]-Ketoesterat)2 - welche eine asymmetrisch substituierte zweikernige Struktur aufweisen - konnten nur durch Reaktion bei erhöhten Temperaturen oder durch Deoligomerisation von [Al(OiPr)3]4 vor der Substitutionsreaktion erhalten werden. Der Einfluss von Substituenten in der 4 bzw. 2 Position wurde studiert und geänderte Reaktionsbedingungen und Nebenreaktion festgestellt. Analoge Verbindungen wurden auch für die Modifikation von Al(OiPr)3 mit N,N-Diethylacetoacetamid erhalten. Es wurde kein Einfluss durch Veränderung der Ester-Funktionalität auf die Produkte beobachtet, ausgenommen tert.-Butylacetoacetat, wo Umesterung auftrat.Verbindungen des Typs Al(Malonat)3, Al2(my-OiPr)2(OiPr)2(Malonat)2, [Al(my-OiPr)(Diisopropylmalonat)2]2 und Al3(my-OH)(my-OEt)3(Diethylmalonat)5 wurde für die Modifikation von Aluminium Alkoxiden mit Dialkylmalonaten erhalten. Für Reaktionen von Al(OiPr)3 mit Dimethyl- und Diethylmalonat wurde Umesterung beobachtet.Die Modifikation von Y5O(OiPr)13 mit Isopropylacetoacetat (iprac) lieferte drei neue Yttrium Cluster. Y9O(OH)9(OiPr)8(iprac)8 wurde durch Reaktion mit einem Äquivalent Ligand per Yttrium-Atom erhalten, während die Reaktion mit drei Äquivalenten [Y2(OH)(iprac)5]2 als Produkt ergab.Kristallisation von [Y2(OH)(iprac)5]2 aus Chloroform gab Y9O(OH)9(iprac)16, strukturell eng verwandt mit Y9O(OH)9(OiPr)8(iprac)8.The control of reactivity towards hydrolysis and condensation of metal alkoxides by modification with organic ligands enables their use as precursors for sol-gel materials. Concomitantly the structure of the final material is influenced and retaining of the organic ligands in the final material opens a route to inorganic-organic hybrid materials.Understanding the influence of the organic ligand on the structure and reactivity of the precursors is a prerequisite for a deliberate design of new sol-gel derived (hybrid) materials.Aluminum alkoxides were modified with [beta]-diketonic compounds ([beta]-ketoesters, dialkylmalonates, [beta]-ketoamides). Compounds Al([beta]-ketoesterate)3 were obtained upon reaction of [Al(OR)3]n (OR =OtBu, OiPr) with 3 equivalents of [beta]-ketoester, all exhibiting a mononuclear octahedral structure. Complexes Al2(my-OiPr)2(OiPr)([beta]-ketoesterate)2 - which show a asymmetrically substituted dinuclear structure - could only be obtained upon reaction at elevated temperatures or by de-oligomerization of [Al(OiPr)3]4 prior to the substitution reaction. The influence of substituents in 4 and 2 position was studied, leading to changed reaction conditions and side reactions. Analogous compounds were also obtained for the modification of Al(OiPr)3 with N,N-diethyl acetoacetamide. No influence upon variation of the ester functionality on the products was observed, except for tert.-butyl acetoacetate, where transesterification was observed.The compounds Al(malonate)3, Al2(my-OiPr)2(OiPr)2(malonate)2, [Al(my-OiPr)(diisoproylmalonate)2]2, and Al3(my-OH)(my-OEt)3(diethylmalonate)5 were obtained upon modification of aluminum alkoxides with dialyklmalonates. For reactions of Al(OiPr)3 with dimethyl and diethyl malonate transesterification was observed.The modification of Y5O(OiPr)13 with isopropylacetoacetate (iprac) yielded three new yttrium clusters. Y9O(OH)9(OiPr)8(iprac)8 was obtained upon reaction with one equivalent of ligand per yttrium atom, whereas [Y2(OH)(iprac)5]2 was obtained for the reaction with three equivalents.Crystallization of [Y2(OH)(iprac)5]2 from chloroform yielded Y9O(OH)9(iprac)16, structurally closely related to Y9O(OH)9(OiPr)8(iprac)8.13

Synthesis and reactivity of novel molybdenum amd tungsten PNP pincer complexes

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