454 research outputs found
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
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