1,721,072 research outputs found
Optical gap between dangling-bond states of a single-domain diamond C(111)-2 x 1 by reflectance anisotropy spectroscopy
No full textThe optical gap between dangling- bond states has been measured on a single- domain diamond C( 111)- 2 x 1 surface. The experimental result is 1.47 eV. Such a large surface gap ( even larger if excitonic effects are considered) so far has not been obtained by state- of- the- art band calculations and its physical origin is still not elucidated. The experimental result suggests a strong electron- phonon interaction in this surface. The renormalization of the bare electronic transition energy by electron- phonon interaction provides a clue to solve the apparent disagreement between theory and experiment
Infrared reflectance anisotropy spectroscopy of Si(111)-2X1: Surface excitons and polarons
No full textThe experimental data regarding the energy distribution of dangling-bond electronic states at the Si(111)-2x1 surface, with special attention to the gap between filled and empty states, are revisited. Clearly some of the results are not compatible. Theorists almost unanimously conclude in favor of a strong (about 0.25 eV) excitonic energy in the optical transitions between surface states. On the experimental side, on the other hand, conflicting results seem to support either large or negligible exciton energy. In order to reconcile all available data, it has been suggested that in highly doped samples a sizeable gap shrinking should occur. We have performed a reflectance anisotropy spectroscopy experiment in order to verify this hypothesis. The optical peak at 0.45 eV associated to dangling-bond transitions does not shift, in highly doped crystals (N-D=4x10(18) cm(-3)), within the experimental accuracy of 20 meV, thereby ruling out the hypothesis of a shrinking of the gap. Under the assumption that all experimental results are correct, an alternative explanation of this puzzling problem, based on the existence of surface polarons together with surface excitons, is suggested. Although very difficult to detect spectroscopically in a direct way, it is argued that polaronic states could play an important role in highly doped samples. A number of possible experiments to clarify the last point are suggested. Accurate theoretical calculations of the polaron energy would also be helpful
Infrared surface absorption in Si(111)2x1 observed with reflectance anisotropy spectroscopy
No full textOptical transitions between surface states associated with dangling bonds in Si(111)2X1 have been measured by means of reflection anisotropy spectroscopy in the near infrared. The results confirm those previously obtained with other optical techniques (namely surface differential reflectivity and photothermal deflection spectroscopies). The method does not require oxidation of the surface and thus opens the way to studying a number of problems in surface physics, including the temperature dependence of surface transitions in Si(111)2x1 and Ge(111)2x1
Confinement effects in pi -bonded chains at group IV semiconductor (111) surfaces
No full textThe degree of 1D character of surface chains at group IV (111)-2 × 1 reconstructed surfaces is established by surface sensitive optical spectroscopy. Optical experiments on a diamond C(111)-2 × 1 surface show that the absorption peak related to dangling-bond transitions exhibits a marked blueshift upon oxygen exposure of the clean surface. Such behaviour is analogous to that observed on a clean Si(111)-2 × 1 surface. For both surfaces the experimental finding is interpreted in terms of quantum confinement of surface electrons in quasi-one-dimensional π-bonded chains, whose length decreases with oxygen uptake. A different behaviour is observed in Ge(111)-2 × 1, where only a very slight blueshift of the surface-state optical transition is detected upon oxidation. The almost negligible blueshift in Ge(111)-2 × 1 is consistent with a significant coupling between the π-bonded chains resulting in a much less pronounced one-dimensional character of Ge(111)-2 × 1 surface electrons compared to diamond and silicon reconstructed surfaces. © 2013 IOP Publishing Ltd
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
Optical properties of bismuth-terminated GaAs(110) surfaces
No full textWe have investigated the room temperature deposition of ultrathin bismuth layers on GaAs(110) surfaces by using Surface Differential Reflectance (SDR) spectroscopy. In SDR spectra measured at the "as grown" surfaces, optical transitions related to the new electronic structure of the overlayer appear at 0.8 eV and 1.35 eV, below the gap of bulk gallium arsenide. Because of their energy position and dependence upon deposited Bi thickness, we interpret them as related to Bi-overlayer and Bi-substrate bonds, respectively. Subsequent annealings of the overlayer at temperatures in the range 200-350 degrees C have not produced any improvement in the quality of the interface, but a new tweak) transition at about 1.1 eV possibly related to Bi crystallites present at the surface. Annealing at T > 350 degrees C produces desorption of the adsorbed bismuth
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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