1,720,977 research outputs found
Terahertz and optical response of novel quantum materials
Full text linkQuantum materials are a class of systems that host low energy emergent properties due to the strong correlations between the lattice, charge, spin and orbital degrees of freedom. In recent years, this classification has encompassed the world of strongly correlated materials, like high-temperature superconductors and Mott-Hubbard insulators, and the topological states of matter, like Dirac/Weyl semimetals and topological insulators. This doctoral thesis discusses the study of the optical characterization of novel quantum materials, with a special focus on the terahertz (THz) spectral range, where low energy emergent features arise as a consequence of strong electronic correlations, symmetry breaking and/or topological transitions. The low energy photons associated to THz radiation grant an easy access to the quasiparticles occupying the low energy modes of quantum materials. The recent progress in the generation and detection of THz radiation have enabled the production of ultrashort pulses at the picosecond scale and their exploitation in time-domain spectroscopy measurements, like pump-probe and nonlinear spectroscopy, from which it is possible to study the out-of-equilibrium dynamics of electrons and dipole-coupled low energy states, along with the appearance of nonlinear responses at the presence of high external electric fields. This thesis shows that quantum materials can host a plethora of THz-related responses which can be tuned in terms of their thickness, temperature and external drivings, like the perturbation associated to an ultrashort optical pulse. It addresses the optical and THz study of novel magnetic quantum materials like Co2MnGa, a magnetic nodal line semimetal, MnBi2Te4, the first intrinsic magnetic topological insulator discovered, and CrI3, a layered ferromagnetic insulator with research interests ranging from spintronics to topological Majorana modes. In particular, the interplay between the magnetic, electronic and phononic states is addressed through linear and nonlinear spectroscopy, along with the direct sampling of the topological features by optical means. As a final purpose, this thesis also reports the novel findings for the direct optical sampling of the superconductive gap in the Sr-doped nickelate NdNiO2, a strain-induced superconductor. The results show how the electrodynamic properties of this material are quantitatively different from the ones found in cuprate superconductors, sharing a similar infinite layer structural phase
THz generation from the topological nodal line semimetal Co2MnGa
Full text linkNonlinear optical spectroscopy is a fundamental probe for the investigation of topological effects in quantum materials. In this paper, we report on the terahertz (THz) emission from thin films at various thicknesses of the magnetic topological nodal semimetal Co2MnGa (CMG) when excited by femtosecond optical pulses. Experimental results suggest the presence of multiple THz generation mechanisms, originating from both bulk and surface states of CMG. The former is explained in terms of a photon-drag effect as induced by radiation pressure. The latter emission mechanism instead appears to be related to the photovoltaic effect coming from the topological surface states. This interplay between generation mechanisms indicates that Co2MnGa topological nodal semimetals are a valuable platform for THz emitter devices
Achromatic terahertz quarter-wave Fresnel rhomb retarder
No full textAchromatic terahertz (THz) quarter-wave retarder is widely desired to manipulate the polarization states of broadband THz beams, which are essential for spectroscopic applications, such as circular dichroism spectroscopy and steering THz vortex beams. A retarder based on Fresnel reflection exhibits the potential for designing an achromatic THz quarter-wave retarder. However, special care should be taken to make a Fresnel retarder capable of manipulating the beam ellipticity by simply rotating its fast axis without affecting its propagation path. Hereby, we design a 4-bounce achromatic quarter-wave Fresnel rhomb retarder free of affecting beam propagation, which can easily change the input beam's ellipticity by simply rotating the retarder's fast axis
Simultaneous elliptically and radially polarized THz from one-colour laser-induced plasma filament
Full text linkTHz-based technologies and research applications have seen a rapid increment in recent period together with the development of novel radiation sources based both on relativistic electrons and laser techniques. In this framework, laser-induced plasma filament plays an important role in generating intense and broadband THz radiation. Although many attentions have been paid to THz emission from two-color plasma filaments, one-color plasma emission has been scarcely investigated. In particular, the polarization state of one-color THz emission is still controversial due to the limitations of the existing THz detection techniques, which are incapable of simultaneously detecting elliptically and radially polarized THz radiation. In this manuscript, we develop a novel detection method and unambiguously demonstrate for the first time that one-color laser-induced plasma filament simultaneously emits elliptically and radially polarized THz radiation. These polarization states suggest that the generation mechanism results from electric quadrupole, showing a new route for producing more complex polarization states and THz vortex beams
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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