Lewis acid catalyzed transfer hydromethallylation for the construction of quaternary carbon centers

The design and gram‐scale synthesis of a cyclohexa‐1,4‐diene‐based surrogate of isobutene gas is reported. Using the highly electron‐deficient Lewis acid B(C6F5)3, application of this surrogate in the hydromethallylation of electron‐rich styrene derivatives provided sterically congested quaternary carbon centers. The reaction proceeds by C(sp3)−C(sp3) bond formation at a tertiary carbenium ion that is generated by alkene protonation. The possibility of two concurrent mechanisms is proposed on the basis of mechanistic experiments using a deuterated surrogate.TU Berlin, Open-Access-Mittel - 201

Lande g-tensor in semiconductor nanostructures

Understanding the electronic structure of semiconductor nanostructures is not complete without a detailed description of their corresponding spin-related properties. Here we explore the response of the shell structure of InAs self-assembled quantum dots to magnetic fields oriented in several directions, allowing the mapping of the g-tensor modulus for the s and p shells. We found that the g-tensors for the s and p shells show a very different behavior. The s-state in being more localized allows the probing of the confining potential details by sweeping the magnetic field orientation from the growth direction towards the in-plane direction. As for the p-state, we found that the g-tensor modulus is closer to that of the surrounding GaAs, consistent with a larger delocalization. These results reveal further details of the confining potentials of self-assembled quantum dots that have not yet been probed, in addition to the assessment of the g-tensor, which is of fundamental importance for the implementation of spin related applications.Comment: 4 pages, 4 figure

Geostatistical analysis of mesoscale spatial variability and error in SeaWiFS and MODIS/Aqua global ocean color data

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 22–39, doi:10.1002/2017JC013023.Mesoscale (10–300 km, weeks to months) physical variability strongly modulates the structure and dynamics of planktonic marine ecosystems via both turbulent advection and environmental impacts upon biological rates. Using structure function analysis (geostatistics), we quantify the mesoscale biological signals within global 13 year SeaWiFS (1998–2010) and 8 year MODIS/Aqua (2003–2010) chlorophyll a ocean color data (Level-3, 9 km resolution). We present geographical distributions, seasonality, and interannual variability of key geostatistical parameters: unresolved variability or noise, resolved variability, and spatial range. Resolved variability is nearly identical for both instruments, indicating that geostatistical techniques isolate a robust measure of biophysical mesoscale variability largely independent of measurement platform. In contrast, unresolved variability in MODIS/Aqua is substantially lower than in SeaWiFS, especially in oligotrophic waters where previous analysis identified a problem for the SeaWiFS instrument likely due to sensor noise characteristics. Both records exhibit a statistically significant relationship between resolved mesoscale variability and the low-pass filtered chlorophyll field horizontal gradient magnitude, consistent with physical stirring acting on large-scale gradient as an important factor supporting observed mesoscale variability. Comparable horizontal length scales for variability are found from tracer-based scaling arguments and geostatistical decorrelation. Regional variations between these length scales may reflect scale dependence of biological mechanisms that also create variability directly at the mesoscale, for example, enhanced net phytoplankton growth in coastal and frontal upwelling and convective mixing regions. Global estimates of mesoscale biophysical variability provide an improved basis for evaluating higher resolution, coupled ecosystem-ocean general circulation models, and data assimilation.NASA's Ocean Biology and Biogeochemistry Grant Numbers: NNG05GG30G, NNG05GR34G, NNX14AM36G, NNX14AL86G, NNX15AE65G; Ocean Biology Processing Group (OBPG) at NASA's Goddard Space Flight Cente

Lewis Säure‐katalysierte Transferhydromethallylierung für den Aufbau quartärer Kohlenstoffzentren

Das Design und die Synthese im Grammmaßstab eines cyclohexa‐1,4‐dienbasierten Surrogats für Isobutengas wird beschrieben. Unter Verwendung der stark elektronenarmen Lewis‐Säure B(C6F5)3 wurde die Hydromethallylierung elektronenreicher Styrolderivate zum Aufbau sterisch überfrachteter quartärer Kohlenstoffzentren erreicht. Die Reaktion verläuft unter C(sp3)‐C(sp3)‐Bindungsknüpfung an einem tertiären Carbeniumion, das selbst durch eine Alkenprotonierung erzeugt wird. Die Möglichkeit zweier gleichzeitig ablaufender Mechanismen wird auf der Grundlage mechanistischer Experimente mit einem deuterierten Surrogat vorgeschlagen.TU Berlin, Open-Access-Mittel - 201

Spin filtering and magnetoresistance in ballistic tunnel junctions

We theoretically investigate magnetoresistance (MR) effects in connection with spin filtering in quantum-coherent transport through tunnel junctions based on non-magnetic/semimagnetic heterostructures. We find that spin filtering in conjunction with the suppression/enhancement of the spin-dependent Fermi seas in semimagnetic contacts gives rise to (i) spin-split kinks in the MR of single barriers and (ii) a robust beating pattern in the MR of double barriers with a semimagnetic well. We believe these are unique signatures for quantum filtering.Comment: Added references + corrected typo

Catalytic dehydrogenative Si-N coupling of pyrroles, indoles, carbazoles as well as anilines with hydrosilanes without added base

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.A base-free, catalytic protocol for the dehydrogenative Si–N coupling of weakly nucleophilic N–H groups of heteroarenes or aryl-substituted amines with equimolar amounts of hydrosilanes is reported. Cooperative Si–H bond activation at a Ru–S bond generates a silicon electrophile that forms a Si–N bond prior to the N–H deprotonation by an intermediate Ru–H complex, only releasing H2.DFG, GRK 1143, Komplexe chemische Systeme: Design, Entwicklung und Anwendunge

Integrating Entrepreneurial Mindset in a Multidisciplinary Course on Engineering Design and Technical Communication

The engineering curriculum at XXXX University includes a sophomore level two-course sequence (required for engineering students in all disciplines) in which the primary learning outcomes are engineering design and technical communication. These courses are team-taught by faculty from Engineering and from Communications, specifically, Writing Arts in the fall and Public Speaking in the spring. Historically, the fall course has featured three major course deliverables: (1) a “research sequence” consisting of a rhetorical analysis, and annotated bibliography and a literature review, (2) a humanities assignment in which students explore the impact of technology on societal needs, and (3) laboratory and design reports stemming from hands-on engineering projects completed in lab. During the summer of 2019, the faculty team re-designed each of these three major course deliverables, with the goal of fostering an Entrepreneurial Mindset in students and leveraging synergies between the Entrepreneurial Mindset and the existing goals of the course (engineering design and technical communication). In particular, the faculty team created a new linkage between the research sequence and the humanities assignment. The research sequence is built around the U.N.’s Sustainable Development Goals; each student chooses one of the goals to explore through their individual rhetorical analysis, annotated bibliography, and literature review. The humanities assignment is a team project in which students explore solutions to sustainability problems on the campus of XXXX University. Different sections of the course will use different engineering projects, but the faculty team has crafted a set of guidelines for the projects to ensure some uniformity of experience and expectations across the sections. The faculty team also developed rubrics that will be used to evaluate student performance on these re-designed assignments. The new assignments are being integrated into the Fall 2019 offering of Sophomore Engineering Clinic. This paper will give a detailed description of each of the assignments and how they are designed to align with the goal of fostering an Entrepreneurial Mindset. The paper will also present assessment data that will be collected throughout the Fall 2019 semester

Modelling of Optical Detection of Spin-Polarized Carrier Injection into Light-Emitting Devices

We investigate the emission of multimodal polarized light from Light Emitting Devices due to spin-aligned carriers injection. The results are derived through operator Langevin equations, which include thermal and carrier-injection fluctuations, as well as non-radiative recombination and electronic g-factor temperature dependence. We study the dynamics of the optoelectronic processes and show how the temperature-dependent g-factor and magnetic field affect the polarization degree of the emitted light. In addition, at high temperatures, thermal fluctuation reduces the efficiency of the optoelectronic detection method for measuring spin-polarization degree of carrier injection into non-magnetic semicondutors.Comment: 15 pages, 7 figures, replaced by revised version. To appear in Phys. Rev.

Non-equilibrium spin noise spectroscopy of a single quantum dot operating at fiber telecommunication wavelengths

We report on the spin and occupation noise of a single, positively charged (InGa)As quantum dot emitting photons in the telecommunication C-band. The spin noise spectroscopy measurements are carried out at a temperature of 4.2 K in dependence on intensity and detuning in the regime beyond thermal equilibrium. The spin noise spectra yield in combination with an elaborate theoretical model the hole-spin relaxation time of the positively charged quantum dot and the Auger recombination and the electron-spin relaxation time of the trion state. The extracted Auger recombination time of this quantum dot emitting at 1.55 μm is comparable to the typical Auger recombination times on the order of a few μs measured in traditionally grown InAs/GaAs quantum dots emitting at around 900 nm

Semiclassical theory of spin-orbit interactions using spin coherent states

We formulate a semiclassical theory for systems with spin-orbit interactions. Using spin coherent states, we start from the path integral in an extended phase space, formulate the classical dynamics of the coupled orbital and spin degrees of freedom, and calculate the ingredients of Gutzwiller's trace formula for the density of states. For a two-dimensional quantum dot with a spin-orbit interaction of Rashba type, we obtain satisfactory agreement with fully quantum-mechanical calculations. The mode-conversion problem, which arose in an earlier semiclassical approach, has hereby been overcome.Comment: LaTeX (RevTeX), 4 pages, 2 figures, accepted for Physical Review Letters; final version (v2) for publication with minor editorial change