682 research outputs found

    Stereospecific and chemoselective copper-catalyzed deaminative silylation of benzylic ammonium triflates

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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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