1,720,982 research outputs found
Variational Rashba Effect in GaAlAs/GaAs Heterojunctions
No full textThe Rashba alpha parameter at the Fermi energy of 2DEGs in AlGaAs/GaAs heterojunctions is calculated with a new variational solution to the multi-band envelope-function effective Schroedinger equation based on the 8-band kp Kane model for the bulk. Modified Fang-Howard trial wave-functions that depend on spin and satisfy spin-dependent boundary conditions, are introduced; and the spin splitting is obtained by minimizing the total energy of the 2DEG. The results are compared with simpler model calculations and shown, in particular, to be quite sensitive to the barrier penetration of the first subband wave-function in these heterojunctions
Variational analysis of the Rashba splitting in III-V semiconductor inversion layers
No full textA spin-dependent variational theory is used to analyze the Rashba spin-orbit splitting in two-dimensional electron gases formed in III-V semiconductor inversion layers. The spin split conduction subbands in CdTe/InSb, insulator/InAs, InP/InGaAs, InAlAs/InGaAs, and AlGaAs/GaAs heterojunctions are calculated. The theory, presented here in detail, is based on the 8 x 8 k . p Kane model and on the introduction of simple and convenient spin-dependent Fang-Howard trial functions, and leads to analytical expressions for the split subbands, as well as allows for a detailed knowledge of the Rashba spin-orbit coupling, including its explicit dependence on structure parameters and its decomposition into separate contributions. The Rashba coupling parameter and the population difference in the spin-split subbands, as experimentally determined from the beating pattern of the Shubnikov-de Haas (SdH) oscillations, are obtained as a function of the electron density (n(s)). The separate contributions to the particularly large Rashba splitting in CdTe/InSb heterojunctions are also computed and discussed. It is shown, for example, that due to the spin-dependent boundary conditions, the direct Rashba spin-orbit coupling term in the effective Hamiltonian dominates the splitting only for n(s) > 10(10) cm(-2) while it is the barrier penetration kinetic energy term that gives the largest contribution to the Rashba effect at lower densities
Variational Rashba splitting in two-dimensional electron gases in III-V semiconductor heterojunctions
No full textControl of the Rashba spin-orbit coupling in semiconductor two-dimensional electron gases (2DEGs) is of fundamental interest to the rapidly evolving semiconductor spintronics and depends on the detailed knowledge of the controversial interface and barrier penetration effects. Based on the 8x8 k center dot p Kane model for the bulk, we propose a spin-dependent variational solution for the conduction subbands of III-V heterojuctions, which reveals analytically the different contributions to the Rashba splitting and its dependency on heterostructure and band parameters as the band offset and effective masses. Perturbation expansions are used to derive renormalized parameters for an effective, simple, and yet accurate one band model. Spin-dependent modified Fang-Howard trial functions, which satisfy the spin-dependent boundary conditions, are then introduced. The subband splitting is given as a function of the variational parameter which is obtained minimizing the total energy of the 2DEG. Our calculations applied to InAlAs/InGaAs heterojunctions, where a near 20% increase in the splitting is observed due to the barrier penetration, are in good agreement with both experiment and exact numerical calculations. Well-known expressions in the limit of a perfect insulating barrier are exactly reproduced
Spin-orbit interaction strength and anisotropy in III-V semiconductor heterojunctions
No full textThe spin-orbit interaction strength for electrons in III-V semiconductor heterojunctions and the corresponding in-plane anisotropy are theoretically studied, considering Rashba and Dresselhaus contributions. Starting from a variational solution of Kane's effective Hamiltonian for the Rashba-split subbands, the total spin-orbit splitting at the Fermi level of the two-dimensional electron gas in III-V heterojunctions is calculated analytically, as a function of the electron density and wave-vector direction, by adding the Dresselhaus contribution within quasidegenerate first-order perturbation theory. Available GaAs and InGaAs experimental data are discussed. Effects of the barrier penetration are identified, and the spin-orbit anisotropy is shown to be determined by more than one parameter, even in the small-k limit, contrary to the commonly used alpha/beta (where alpha is the Rashba and beta the Dresselhaus interaction) single-parameter picture. DOI: 10.1103/PhysRevB.87.08130
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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