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

Microstructural changes in the reward system are associated with post-stroke depression

Background: Studies of lesion location have been unsuccessful in identifying mappings between single brain regions and post-stroke depression (PSD). Based on studies implicating the reward system in major depressive disorder without stroke, we investigated structural correlates within this system and their associations with PSD. Methods: The study enrolled 16 healthy controls, 12 stroke patients with PSD and 34 stroke patients free of PSD. Participants underwent 3T structural and diffusion MRI. Graph theoretical measures were used to examine global topology and whole-brain connectome analyses were employed to assess differences in the interregional connectivity matrix between groups. Structural correlates specific to the reward system were examined from grey matter volumes and by reconstructing its main white matter pathways, namely the medial forebrain bundle and cingulum connections, using deterministic tractography. Fractional anisotropy (FA) was derived as a measure of microstructural organization, and extracellular free-water (FW) as a possible proxy of neuroinflammation. Results: Subnetworks of decreased FA-weighted and increased FW-weighted connectivity were observed in patients with PSD relative to healthy controls. These networks subsumed the majority of regions constituting the reward system. Within the reward system, FA and FW of major connection pathways and grey matter volume were collectively predictive of PSD, explaining 37.8% of the variance in depression severity. Conclusions: PSD is associated with grey matter volume loss, reduced FA and increased extracellular FW in the reward system, similar to features observed in major depression without stroke. Structural characterization of the reward system is a promising biomarker of vulnerability to depression after stroke

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

Electron g-Factor Anisotropy in Symmetric (110)-oriented GaAs Quantum Wells

We demonstrate by spin quantum beat spectroscopy that in undoped symmetric (110)-oriented GaAs/AlGaAs single quantum wells even a symmetric spatial envelope wavefunction gives rise to an asymmetric in-plane electron Land\'e-g-factor. The anisotropy is neither a direct consequence of the asymmetric in-plane Dresselhaus splitting nor of the asymmetric Zeeman splitting of the hole bands but is a pure higher order effect that exists as well for diamond type lattices. The measurements for various well widths are very well described within 14 x 14 band k.p theory and illustrate that the electron spin is an excellent meter variable to map out the internal -otherwise hidden- symmetries in two dimensional systems. Fourth order perturbation theory yields an analytical expression for the strength of the g-factor anisotropy, providing a qualitative understanding of the observed effects

Highly anisotropic g-factor of two-dimensional hole systems

Coupling the spin degree of freedom to the anisotropic orbital motion of two-dimensional (2D) hole systems gives rise to a highly anisotropic Zeeman splitting with respect to different orientations of an in-plane magnetic field B relative to the crystal axes. This mechanism has no analogue in the bulk band structure. We obtain good, qualitative agreement between theory and experimental data, taken in GaAs 2D hole systems grown on (113) substrates, showing the anisotropic depopulation of the upper spin subband as a function of in-plane B.Comment: 4 pages, 3 figure

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

The impacting cantilever: modal non-convergence and the importance of stiffness matching.

The problem of an Euler-Bernoulli cantilever beam whose free end impacts with a point constraint is revisited from the point of view of modal analysis. It is shown that there is non-uniqueness of consistent impact laws for a given modal truncation. Moreover, taking an N-mode compliant, bilinear formulation and passing to the rigid limit leads to a sequence of impact models that does not converge as N--> ∞. The dynamics of such truncated models are studied numerically and found to give rise to quite different dynamics depending on the number of degrees of freedom taken. The simulations are compared with results from simple experiments that show a propensity for multiple-tap dynamics, in which higher-order modes lead to rapidly cycling intermittent contact. The conclusion reached is that, to derive an accurate model, one needs to avoid the impact limit altogether, and take sufficiently many modes in the formulation to match the actual stiffness of the constraining stop

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

Spin noise spectroscopy under resonant optical probing conditions: coherent and non-linear effects

High sensitivity Faraday rotation spectroscopy is used to measure the fluctuating magnetization noise of non-interacting rubidium atoms under resonant and non-resonant optical probing conditions. The spin noise frequency spectra in dependence on the probe light detuning with respect to the D2-transition reveals clear signatures of a coherent coupling of the participating electronic levels. The results are explained by extended Bloch equations including homogeneous and inhomogeneous broadening mechanisms. Our measurements further indicate that spin noise originating from excited states are governed at high intensities by collective effects