1,721,173 research outputs found
Anisotropy of the electron g factor in lattice-matched and strained-layer III-V quantum wells
No full textThe influence of quantum confinement and built-in strain on conduction-electron g factors in lattice-matched GaAs/Al0.35Ga0.65As and strained-layer In0.11Ga0.89As/GaAs quantum wells is investigated for well widths between 3 and 20 nm. The magnitude, sign, and anisotropy of the g factors were obtained from quantum beats due to Larmor precession of electron spins in time-resolved, polarization-sensitive, pump-probe reflection at 10 K in magnetic fields applied along and at 45° to the growth axis. Slowly varying shifts of precession frequency, due to buildup of nuclear polarization in the samples over ?1 h and equivalent to up to 0.5 T, occurred for fixed circular pump polarization and oblique applied fields. These Overhauser shifts confirmed the sign of the g factors and were eliminated by modulation of pump polarization to give precise g factors. For both material systems, variation of the g factor with well width follows qualitatively the dependence on energy, determined by quantum confinement, calculated from three-band k?p theory in the bulk well material. For the lattice-matched system there is excellent quantitative agreement with a full three-band k?p calculation including anisotropy effects of the quantum-well potential. For the strained-layer system, detailed quantum-well calculations do not exist but k?p theory for epitaxial layers predicts 10 times greater anisotropy for wide wells than we observe. This discrepancy is also apparent in previous, less complete, investigations of strained-layer systems and highlights the need for further theoretical effort
Nuclear effects in ultrafast quantum-well spin-dynamics
No full textUltrafast optical methods are used to study low temperature nuclear spin polarisation and electron spin precession and relaxation in an oblique magnetic field in undoped GaAs/AlGaAs quantum wells. We find that electron localisation dramatically extends electron spin relaxation and accelerates nuclear spin polarisation which is manifest as a large time-dependent Overhauser shift in electron Larmor precession frequency. Nuclear polarisation is a two-stage process, which we suggest involves contact hyperfine interaction at electron localisation centres and nuclear spin diffusion to the remaining nuclei of the sample. Special samples are used to investigate nuclear spin diffusion through the barriers, which is found to be an order of magnitude slower than within the wells, consistent with extra dipolar and quadrupolar disorder in the alloy
Dataset for 'Overlapped pulsed pumping of tandem pumped fiber amplifiers to increase achievable pulse energy'
No full textDataset for the article:
A. Malinowski, J. H. V. Price and M. N. Zervas, "Overlapped Pulsed Pumping of Tandem Pumped Fiber Amplifiers to Increase Achievable Pulse Energy," in IEEE Journal of Quantum Electronics, vol. 53, no. 2, pp. 1-8, April 2017.
doi: 10.1109/JQE.2017.2657334
It has been reported previously that in the regime appropriate for amplifying femtosecond pulses using the chirped pulse amplification technique in Yb-fiber sources that sub-micro-second pulsed tandem pumping not only provides the thermal benefits of c.w. tandem pumping, but also enables strong suppression of ASE. In that case, the pump pulse preceded the signal pulse train. Here, we propose a tandem pumping scheme in rare-earth-doped fiber amplifiers, where a train of signal pulses is amplified by a pump pulse, which is almost exactly temporally overlapped. Simulations demonstrate that this can be used to create uniform gain across the signal pulse train, even at very high total pulse energies, where there would be significant gain shaping in the previous case. In addition, the pump is absorbed in a much shorter length, which increases the threshold for nonlinear effects and gain of greater than 26 dB is shown to be readily achievable in an amplifier as short as 1.5 m. This results in increased extractable energy before reaching the threshold for limiting nonlinear effects, such as stimulated Raman scattering. These attributes should be attractive for high energy, high average power, ultrashort pulse, coherently combined fiber laser systems.</span
Reciprocity in nonlocal optics
No full textWe derive the analytical condition for eigenwaves in terms of nonlocal optical susceptibilities of media with nonlocal response. A wave picture of the recently observed effect of nonreciprocal polarization plane rotation that is due to gyrotropic linear dichroism is discussed
Dynamic nuclear overhauser shifts in Larmor beats from a quantum well
No full textThe significance of nuclear spin polarisation in time-resolved optical studies of III–V semiconductors is addressed. Electron Larmor beats in pump-probe reflectivity from a GaAs/AlGaAs quantum well show Overhauser shift of 0.7 T due to accumulated nuclear polarisation I/I=0.065. This leads to precision values of electron g-factor, elucidates nuclear spin pumping and diffusion mechanisms in quantum wells and informs discussion of implications for spin-electronics and transport
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
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
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