1,721,017 research outputs found
Quasi-particles dynamics in underdoped Bi2212 under strong optical perturbation.
Get PDFIn this work an optical pump-probe set-up is used to study the photo-induced non-equilibrium dynamics of a superconducting underdoped Bi2212 single crystal in a strong excitation regime (10<<600 μJ/cm2). The use of a tunable repetition rate 120 fs pulsed laser source allows us to avoid significant average heating of the sample and to optimize the signal-to-noise ratio in the detection of the transient reflectivity variation. A discontinuity of the transient reflectivity is observed at high excitation intensities (~70 μJ/cm2). Numerical simulations of the heat diffusion problem indicate that, in this regime, the local temperature of the sample is lower than TC, confirming the impulsive nature of this phenomenon. The quasi-particles (QP) dynamics in the strongly perturbed superconducting state (10<<70 μJ/cm2) is analysed within the framework of the Rotwarf-Taylor model. The picture emerging from the data is consistent with a dynamics governed by high-frequency phonon (HFP) population, which causes a bottleneck effect in the QP recombinatio
Time resolved core level spectroscopy reveals light-induced structural changes in GdTe3
No full textComplex materials frequently exhibit broken symmetry phases, driven by a delicate balance of electronic, lattice, spin and orbital anisotropies. In this context, new phases of matter may emerge when this equilibrium is disturbed by photoexcitation. Here we investigate the response to ultrafast optical excitation of the GdTe3 charge density wave (CDW) compound, by time-resolved X-ray photoemission spectroscopy (TR-XPS). By measuring the energy separation between two atomic species of Te 4d core levels, we identify a sharp discontinuity occurring precisely at the CDW transition temperature, confirming the sensitivity of this technique to the onset of the CDW order. Optical excitation with a novel Echelon-based pulse-replication scheme reveals the formation of a third low-valence tellurium specie, absent under equilibrium conditions, indicative of a light-induced structural rearrangement. The study provides new information on the rich dynamics in GdTe3 and opens new avenues for research in this field by using TR-XPS at synchrotron sources
Ultrafast broadband optical spectroscopy for quantifying subpicometric coherent atomic displacements in WTe2
Get PDFHere we show how time-resolved broadband optical spectroscopy can be used to quantify, with femtometer resolution, the oscillation amplitudes of coherent phonons through a displacive model without free tuning parameters, except an overall scaling factor determined by comparison between experimental data and density functional theory calculations. WTe2 is used to benchmark this approach. In this semimetal, the response is anisotropic and provides the spectral fingerprints of two A1 optical phonons at ≈8 and ≈80 cm−1. In principle, this methodology can be extended to any material in which an ultrafast excitation triggers coherent lattice modes modulating the high-energy optical properties
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
Ultrafast all-optical manipulation of the charge-density-wave in VTe
Get PDFThe charge-density wave (CDW) phase in the layered transition-metal
dichalcogenide VTe is strongly coupled to the band inversion involving
vanadium and tellurium orbitals. In particular, this coupling leads to a
selective disappearance of the Dirac-type states that characterize the normal
phase, when the CDW phase sets in. Here, by means of broadband time-resolved
optical spectroscopy (TR-OS), we investigate the ultrafast reflectivity changes
caused by collective and single particle excitations in the CDW ground state of
VTe. Remarkably, our measurements show the presence of two collective
(amplitude) modes of the CDW ground state. By applying a double-pulse
excitation scheme, we show the possibility to manipulate these modes,
demonstrating a more efficient way to control and perturb the CDW phase in
VTe
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