1,720,964 research outputs found
Nuclear-electron spin interaction in low-dimensional semiconductors
Full text linkThe nuclear spins in semiconductors can serve as a quantum mechanical system with minutes long spin relaxation times, enhanced by the decoupling from the surrounding matter in strained nanostructures and the absence of interaction with light. In order to manipulate the nuclear spin system optically, resident electrons confined in the nanostructure are introduced as a mediator. The electron localization leads to an efficient coupling of the electron spin to the nuclear spins via Fermi contact hyperfine interaction. This allows one to polarize the nuclear spins and to detect their dynamics. In this work, the nuclear spin dynamics in a cadmium telluride (CdTe) quantum well is characterized, assisted by nuclear magnetic resonance radio frequency pulses. The independent dynamics of the isotope spins turns out to be harmonized. In the enhanced localization in negatively doped indium gallium arsenide (InGaAs) quantum dots, two nuclear spin polarization protocols are assessed. Firstly, the developed extended pump-probe setup allows to investigate the established nuclei-induced frequency focusing by tracing the electron spin precession with improved spectral resolution. Secondly, a novel technique based on the pulsed excitation with 1 GHz pulse repetition frequency is introduced which allows to establish a substantial nuclear spin polarization in a transverse field while also reducing the fluctuations of the unordered nuclear spins.Die Kernspins in Halbleitern bilden durch die fehlende Wechselwirkung mit Licht ein quantenmechanisches System mit langen Spin-Relaxationszeiten, die durch die Entkopplung vom umgebenden Festkörper in verspannten Nanostrukturen noch verlängert werden. Residente Elektronen in der Nanostruktur erlauben es, die Polarisation des Lichts auf das Kernspinsystem zu übertragen und optisch zugänglich zu machen, weil die Elektronenlokalisierung für eine effiziente Kopplung des Elektronenspins mit den Kernspins über die Fermikontakt-Hyperfeinwechselwirkung sorgt. In dieser Arbeit wird zum einen die isotopenaufgelöste Kernspindynamik in einem Cadmiumtellurid-Quantentopf mit Hilfe von Kernspinresonanzpulsen charakterisiert und eine Angleichung der Dynamik der eigentlich unabhängigen Isotopenspins beobachtet. Für die stärkere Lokalisierung in negativ dotierten Indiumgalliumarsenid-Quantenpunkten werden zum anderen zwei Kernspinpolarisationsprotokolle untersucht. Erstens erlaubt die verbesserte Präzessionsfrequenzauflösung des im Umfeld der Arbeit entwickelten, erweiterten Pump-Probe-Aufbaus die detaillierte Untersuchung der bekannten kerninduzierten Frequenzfokussierung. Zweitens wird die optische Anregung mit einer Pulsfrequenz von 1 GHz beschrieben. Dadurch wird eine signifikante Kernspinpolarisation in einem transversalen Magnetfeld induziert und gleichzeitig die Fluktuation der ungeordneten Kernspins reduziert
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Spin noise spectroscopy from the perspective of scattered light and noise formation principles in variety of systems
No full textThis work is dedicated to a tool to get information about the spin system ground state with the help of optics: spin noise spectroscopy. Its primary goal is to characterize the virtually undisturbed spin dynamics by obtaining parameters of spin fluctuations in thermal equilibrium.
The homodyne detection scheme with phase stabilization is utilized to improve the responsiveness of the polarization analysis in spin noise spectroscopy. By providing power to the local oscillator, it is possible to overcome the electronic noise and effectively increase the acquired signal - without additional disturbance in the probed spin system while the beam bypasses the sample. This possibility allows the work with lower probing power densities, approaching the desired nonperturbing regime. An improvement larger than half an order of magnitude is present for a bulk n-doped GaAs for small probe intensities. Phase manipulation and stabilization make it achievable to choose the desired parameter - Faraday rotation, ellipticity or a mixture - from the experiment. It avoids otherwise necessary modifications in the arrangement of the optical components.
This improved technique examine further the fundamental characteristics of the spin noise signal construction by obtaining various angular dependencies of the scattered light. The distributed Bragg reflector forming a microcavity amplifies the light-matter interplay, making it possible to detect spin noise from an ensemble of -doped (In, Ga)As/GaAs quantum dots reliably and extract weak effects. As a proof of principle, we performed an observation of pure scattered field (outside of the transmitted light aperture), as well as extraction of the primary electron spin properties, g-factor and spin dephasing time. We also studied the impact of the microcavity on the spatial and spectral dispersion of the scattered light intensity. Additionally, the interplay of two beam resonance excitation was considered for the potential signal enlargement capabilities.
From that point, the interest in the work is shifted in the direction of probing new systems and their characterization. The fundamentals of the spin noise spectroscopy is transcription of the spin-system magnetization on the angle of the Faraday rotation, which should be sufficient to be measurable. The presence of such a sufficiency can not be extracted from linear magneto-optical effects, which is especially crucial in inhomogeneously broadened systems, exhibiting the spin noise gain effect. In this part, the connection between the spin noise gain effect and the behavior of the nonlinear resonant Faraday effect is established, allowing us to predict the applicability of the spin noise spectroscopy to this type of paramagnet. The experimental evidence is based on intraconfigurational (4f-4f) transitions of the trivalent rare-earth ions of neodymium and ytterbium in fluorite-based crystals, approving the theoretical estimations.
At last, spin noise spectroscopy is applied to materials conventionally studied by means of the electronic paramagnetic resonance spectroscopy -- dielectrics with paramagnetic impurities, which were thought of as inapplicable for the spin noise spectroscopy before. Such belief was founded on their low specific Faraday rotation for strong optical transitions. This work demonstrates that for forbidden intraconfigurational transitions, one can see the spin noise spectroscopy due to the spin noise gain effect, which is proportional to a relation of inhomogeneous linewidth to the homogeneous one and can be as high as 10^8 in the mentioned system. The requirements for the optical setup to unlock such measurements are discussed, along with the potential applications.
Finally, a method is present to simplify the identification of obtained spectra components. Due to the discovered fact that the sum of squares of the magnetic resonance frequencies stays the same for any direction of the magnetic field in cubic crystals with anisotropic impurity centers, if the magnitude of the field is constant, one can match the peaks to corresponding types of centers. The relation between the invariant and the g-tensor components were derived for various kinds of centers and proofed experimentally with the spin noise spectroscopy on a cubic CaF2-Nd3+ crystal
Coherent spin dynamics and carrier-nuclear interaction in lead halide perovskite crystals
Full text linkIn the present work, the charge carrier spin dynamics in lead halide perovskites APbX3,
with X = {I,Br,Cl} are studied. The lead halide perovskites are distinguished with respect
to the groups of purely inorganic perovskites with A = Cs and hybrid organic inorganic
ones with A = {MA,FA} (methylammonium and formamidinium), as well as their size, i.
e. macroscopic single crystals or nano crystals. The spin dynamics of the charge carriers,
electrons and holes, are mainly investigated using picosecond resolution Kerr and Faraday
spectroscopy. In addition, the observation of the spin dynamics is complemented by exciton
spectroscopy (polarization-resolved photoluminescence, reflection, and transmission) and
time-resolved differential reflectometry. Nanosecond spin dynamics are observed and the
underlying interaction mechanisms are revealed by the experimental techniques, with a focus
on interaction of charge carrier and nuclear spins.In der vorliegenden Arbeit wird die Ladungsträger-Spindynamik in Blei-Halogen-Perowskiten
APbX3, mit X = {I,Br,Cl} untersucht. Die Blei-Halogen-Perowskite werden in rein inorganische
Perowskite mit A =Cs und hybride organisch-inorganische mit A = {MA,FA}
(Methylammonium and Formamidinium) sowie in ihrer Größe als makroskopische Einkristalle
und Nanokristalle unterschieden. Die Spindynamik der Ladungsträger, Elektronen und Löcher,
wird hauptsächlich anhand von Kerr- und Faraday-Spektroskopie mit einer Pikosekunden
Auflösung untersucht. Zusätzlich wird die Spindynamik durch Exzitonen-spektroskopie (polarisationsaufgelöste
Photolumineszenz, Reflektion und Transmission) sowie durch zeitaufgelöste
differentielle Reflektrometrie untersucht. Es werden nanosekundenlange Spindynamiken
beobachtet, deren zugrundeliegende Wechselwirkungsmechanismen durch die experimentellen
Techniken aufgedeckt werden. Ein Schwerpunkt liegt hierbei auf der Untersuchung der
Ladungsträger-Kern-Spinwechselwirkung
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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