685 research outputs found
Stereospecific and chemoselective copper-catalyzed deaminative silylation of benzylic ammonium triflates
A method for the synthesis of benzylsilanes starting from the corresponding ammonium triflates is reported. Silyl boronic esters are employed as silicon pronucleophiles, and the reaction is catalyzed by copper(I) salts. Enantioenriched benzylic ammonium salts react stereospecifically through an SN2‐type displacement of the ammonium group to afford α‐chiral silanes with inversion of the configuration. A cyclopropyl‐substituted substrate does not undergo ring opening, thus suggesting an ionic reaction mechanism with no benzyl radical intermediate.DFG, 388910461, Ionische und radikalische Kreuzkupplungen zur Kohlenstoff‒Silicium-BindungsknüpfungTU Berlin, Open-Access-Mittel - 201
Spin noise spectroscopy in GaAs
We observe the noise spectrum of electron spins in bulk GaAs by Faraday
rotation noise spectroscopy. The experimental technique enables the undisturbed
measurement of the electron spin dynamics in semiconductors. We measure
exemplarily the electron spin relaxation time and the electron Lande g-factor
in n-doped GaAs at low temperatures and find good agreement of the measured
noise spectrum with an unpretentious theory based on Poisson distribution
probability.Comment: 4 pages, 4 figure
Closing the gap between spatial and spin dynamics of electrons at the metal-to-insulator transition
We combine extensive precision measurements of the optically detected spin
dynamics and magneto-transport measurements in a contiguous set of n-doped bulk
GaAs structures in order to unambiguously unravel the intriguing but complex
contributions to the spin relaxation at the metal-to-insulator transition
(MIT). Just below the MIT, the interplay between hopping induced loss of spin
coherence and hyperfine interaction yields a maximum spin lifetime exceeding
800~ns. At slightly higher doping concentrations, however, the spin relaxation
deviates from the expected Dyakonov-Perel mechanism which is consistently
explained by a reduction of the effective motional narrowing with increasing
doping concentration. The reduction is attributed to the change of the dominant
momentum scattering mechanism in the metallic impurity band where scattering by
local conductivity domain boundaries due to the intrinsic random distribution
of donors becomes significant. Here, we fully identify and model all intricate
contributions of the relevant microscopic scattering mechanisms which allows
the complete quantitative modeling of the electron spin relaxation in the
entire regime from weakly interacting up to fully delocalized electrons
Anomalous Spin Dephasing in (110) GaAs Quantum Wells: Anisotropy and Intersubband Effects
A strong anisotropy of electron spin decoherence is observed in GaAs/(AlGa)As
quantum wells grown on (110) oriented substrate. The spin lifetime of spins
perpendicular to the growth direction is about one order of magnitude shorter
compared to spins along (110). The spin lifetimes of both spin orientations
decrease monotonically above a temperature of 80 and 120 K, respectively. The
decrease is very surprising for spins along (110) direction and cannot be
explained by the usual Dyakonov Perel dephasing mechanism. A novel spin
dephasing mechanism is put forward that is based on scattering of electrons
between different quantum well subbands.Comment: 4 pages, 3 postscript figures, corrected typo
Spin noise spectroscopy in GaAs (110) quantum wells: Access to intrinsic spin lifetimes and equilibrium electron dynamics
In this letter, the first spin noise spectroscopy measurements in
semiconductor systems of reduced effective dimensionality are reported. The
non-demolition measurement technique gives access to the otherwise concealed
intrinsic, low temperature electron spin relaxation time of n-doped GaAs (110)
quantum wells and to the corresponding low temperature anisotropic spin
relaxation. The Brownian motion of the electrons within the spin noise probe
laser spot becomes manifest in a modification of the spin noise line width.
Thereby, the spatially resolved observation of the stochastic spin polarization
uniquely allows to study electron dynamics at equilibrium conditions with a
vanishing total momentum of the electron system
GHz Spin Noise Spectroscopy in n-Doped Bulk GaAs
We advance spin noise spectroscopy to an ultrafast tool to resolve high
frequency spin dynamics in semiconductors. The optical non-demolition
experiment reveals the genuine origin of the inhomogeneous spin dephasing in
n-doped GaAs wafers at densities at the metal-to-insulator transition. The
measurements prove in conjunction with depth resolved spin noise measurements
that the broadening of the spin dephasing rate does not result from thermal
fluctuations or spin-phonon interaction, as previously suggested, but from
surface electron depletion
Electron spin relaxation in bulk GaAs for doping densities close to the metal-to-insulator transition
We have measured the electron spin relaxation rate and the integrated spin
noise power in n-doped GaAs for temperatures between 4 K and 80 K and for
doping concentrations ranging from 2.7 x 10^{-15} cm^{-3} to 8.8 x 10^{-16}
cm^{-3} using spin noise spectroscopy. The temperature dependent measurements
show a clear transition from localized to free electrons for the lower doped
samples and confirm mainly free electrons at all temperatures for the highest
doped sample. While the sample at the metal-insulator-transition shows the
longest spin relaxation time at low temperatures, a clear crossing of the spin
relaxation rates is observed at 70 K and the highest doped sample reveals the
longest spin relaxation time above 70 K.Comment: 6 pages, 4 figure
Measurement of heavy-hole spin dephasing in (InGa)As quantum dots
We measure the spin dephasing of holes localized in self-assembled (InGa)As
quantum dots by spin noise spectroscopy. The localized holes show a distinct
hyperfine interaction with the nuclear spin bath despite the p-type symmetry of
the valence band states. The experiments reveal a short spin relaxation time
{\tau}_{fast}^{hh} of 27 ns and a second, long spin relaxation time
{\tau}_{slow}^{hh} which exceeds the latter by more than one order of
magnitude. The two times are attributed to heavy hole spins aligned
perpendicular and parallel to the stochastic nuclear magnetic field. Intensity
dependent measurements and numerical simulations reveal that the long
relaxation time is still obscured by light absorption, despite low laser
intensity and large detuning. Off-resonant light absorption causes a
suppression of the spin noise signal due to the creation of a second hole
entailing a vanishing hole spin polarization.Comment: accepted to be published in AP
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