1,720,962 research outputs found
High-Pressure Optical Properties and Chemical Stability of Picene
Full text linkPicene is a polycyclic aromatic hydrocarbon belonging to the class of phenacenes which have been recently found to behave as high-temperature superconductors upon alkali metal doping. The electronic properties of organic crystals can be finely and largely modified by the density changes obtained by the application of an external pressure. In this work, the role of pressure in tuning the optical properties of crystalline picene has been investigated from room conditions up to 15 GPa through the measurement of UV–visible absorption spectra, two-photon excitation profiles, and one- and two-photon excited fluorescence spectra in a diamond anvil cell. The pressure dependence of the optical band gap was determined, and the frequencies of several vibronic bands belonging to electronic transitions from the ground state (S0) to the four lowest-energy excited singlet states (S1 to S4) were determined as a function of pressure. We evidence a very different density dependence of the transition energy of S0 → S1, which undergoes a remarkable red shift of ∼400 cm–1/GPa, and of the transitions from S0 to the higher excited states, which remain constant in the whole investigated range. This is consistent with a S1 state of 1La character in solid picene. The high-pressure chemical stability of solid picene was investigated through visible absorption and Fourier transform infrared spectroscopy (FTIR). A chemical transformation involving the bulk picene crystal occurs above ∼23 GPa, giving rise to a disordered material similar to the amorphous hydrogenated carbon obtained in the pressure-induced reactivity of benzene. The combination of electronic and vibrational data allows us to identify the presence of reaction intermediates at ∼10 GPa, preferentially forming at crystal defects
Local structural investigation of SmFeAsO(1)-(x)F(x) high temperature superconductors
No full textA strong revitalization of the field of high temperature superconductivity (HTSC) has been
induced recently by the discovery of TC around 26 K in F-doped LaFeAsO iron pnictides.
Starting from this discovery, a huge amount of experimental data have been accumulated. This
important corpus of results will allow the development of suitable theoretical models aimed at
describing the basic electronic structure properties and nature of superconducting states in these
fascinating new systems. A close correlation between structural features and physical properties
of the normal and superconducting states has already been demonstrated in the current
literature. Advanced theoretical models are also based on the close correlation with structural
properties and in particular with the Fe–As tetrahedral array. As for other complex materials, a
deeper understanding of their structure–properties correlation requires a full knowledge of the
atomic arrangement within the structure. Here we report an investigation of the local structure
in the SmFeAsO1−xFx system carried out by means of x-ray total scattering measurements and
pair distribution function analysis. The results presented indicate that the local structure of these
HTSC significantly differs from the average structure determined by means of traditional
diffraction techniques, in particular the distribution of Fe–As bond lengths. In addition, a model
for describing the observed discrepancies is presented
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
HOMO-LUMO transitions in solvated and crystalline picene
No full textThe optical properties of picene at ambient conditions have been investigated through the measurement of UV/Vis absorption and fluorescence spectra and of excitation profiles, using one-and two-photon excitation, in solution and in the crystal phase. For solvated picene an assignment of the vibronic structure of the transitions to the four lowest-energy excited singlet states (S-1-S-4) has been obtained from the absorption data, and the vibronic structure of the fluorescence spectra has been assigned. The absorption and fluorescence spectra of the solid phase can be interpreted according to the single molecule analysis. Nevertheless, the strong increase of the optical density in the spectral region of the lowest HOMO-LUMO transitions and the frequency shift of absorption and fluorescence bands may be explained by a mixing of the states of adjacent molecules in the crystal. Moreover, peculiar emission features depending on the crystal dimensions (10(-1) to 10(2) mu m) are observed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770265
Combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C_{22}H_{14}
Full text linkWe present high-quality optical data and density functional perturbation theory calculations for the vibrational spectrum of solid picene (C22H14) under pressure up to 8 GPa. First-principles calculations reproduce with a remarkable accuracy the pressure effects on both frequency and intensities of the observed phonon peaks. We use the projection on molecular eigenmodes to unambiguously fit the experimental spectra, resolving complicated spectral structures, in a system with hundreds of phonon modes. With these projections, we can also quantify the loss of molecular character under pressure. Our results indicate that picene, despite a ∼20% compression of the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon modes displaying a smooth and uniform hardening with pressure, without any evidence of structural phase transitions. The Grüneisen parameter of the 1380 cm−1 a1 Raman peak (γp=0.1) is much lower than the effective value (γd=0.8) due to K doping. Therefore, doping and pressure have very different effects and it can be argued that softening of the 1380 cm−1 mode is probably due to coupling with electronic states in K-doped solid picene
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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