1,720,960 research outputs found
Multidimensional nonlinear THz spectroscopies in superconductors
Full text linkThis thesis presents an in-depth analysis of the nonlinear optical response in super- conductors exposed to intense THz pulses, using a quasi-equilibrium approach rooted in many-body perturbation theory. By establishing connections between nonlinear response functions and experimentally measurable quantities, we investigate how superconducting systems respond to single-pulse and two-pulse (multidimensional) protocols. Special focus is given to understanding how diverse light-matter coupling mechanisms and electronic excitations shape the observable outcomes across different experimental setups. We examine, for instance, the role of phase fluctuations in single-layer and bilayer cuprates, which show marked differences in third harmonic generation (THG) along the CuO2 stacking direction due to the combined action of their momentum dispersion and light polarization. Our work then moves to two- dimensional (2D) THz spectroscopy, which offers unique capabilities to distinguish various nonlinear pathways in quantum materials. We begin with a simplified semi- conductor model where we can explicitly compute different light-matter interaction processes and analyze two applications in superconducting systems, applying insights from the THG analysis. We then examine 2D THz spectroscopy of cuprates, where internal screening effects present challenges in disentangling distinct contributions, and subsequently turn to NbN thin films. In the latter case, we demonstrate how specific nonlinear pathways can be uniquely sensitive to amplitude fluctuations, revealing a fresh perspective on the analysis of superconducting dynamics
Contribution of Collective Excitations to Third Harmonic Generation in Multiband Superconductors: the case of MgB
Full text linkMultiband superconductors can host collective excitations with marked
differences with respect to their single-band counterpart. We first study the
spectrum of collective amplitude fluctuations in a clean two-bands
superconductor, showing that the spectral weight of the Higgs mode rapidly
deviates from the naive extension of the single band case as the interband
coupling is turned on. These results are then used to critically analyze the
non-linear optical response in MgB, providing an explanation for the
apparently contradictory results of recent experiments, pointing towards a
selective relevance either of the Leggett mode or of the amplitude fluctuations
at twice the lower gap. By using exact numerical simulations and realistic
estimate of disorder we compute the relative contribution of the quasiparticle,
amplitude and phase fluctuations to the non-linear optical response. We show
that at low pumping frequency only the resonance at twice the smaller gap
emerges, as due to the BCS response, while the Leggett mode dominates only in a
narrow range of higher pumping frequencies matching its low-temperature value.
Our findings provide a fresh perspective on the potential of non-linear THz
spectroscopy to detect collective modes in other multiband systems, as e.g.
iron-based superconductors
Optical absorption in tilted geometries as an indirect measurement of longitudinal plasma waves in layered cuprates
Full text linkElectromagnetic waves propagating in a layered superconductor with arbitrary momentum, with respect to the main crystallographic directions, exhibit an unavoidable mixing between longitudinal and transverse degrees of freedom. Here we show that this basic physical mechanism explains the emergence of a well-defined absorption peak in the in-plane optical conductivity when light propagates at small tilting angles relative to the stacking direction in layered cuprates. More specifically, we show that this peak, often interpreted as a spurious leakage of the c-axis Josephson plasmon, is instead a signature of the true longitudinal plasma mode occurring at larger momenta. By combining a classical approach based on Maxwell’s equations with a full quantum derivation of the plasma modes based on modeling the superconducting phase degrees of freedom, we provide an analytical expression for the absorption peak as a function of the tilting angle and light polarization. We suggest that an all-optical measurement in tilted geometry can be used as an alternative way to access plasma-wave dispersion, usually measured by means of large-momenta scattering techniques like resonant inelastic X-ray scattering (RIXS) or electron energy loss spectroscopy (EELS)
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
Manipulating Plasma Excitations with Terahertz Light Pulses in Superconducting Cuprates
Full text linkLayered cuprates offer a preferential playground for optical non-linearity
thanks to the emergence, below Tc, of soft out-of-plane Josephson plasmons. The
hallmark of such a non-linearity is the observation of Third Harmonic
Generation, that has been theoretically understood as a sum-frequency process
involving a two-plasmon excitation. However, recent experiments in cuprates
with two planes per unit cell challenge this interpretation, due to the lack of
resonant response at the temperature where the driving frequency matches the
plasma energy scale, as observed instead in single-layer cuprates. Here we show
that such an apparent discrepancy in bilayer systems can be resolved by taking
into account the combined effect of light polarization and Josephson-coupling
anisotropy on setting the energy range where three-dimensional layered plasma
modes can be resonantly excited. Our results offer a novel perspective on the
possibility to tune on demand high-harmonic generation by artificially
designing Josephson heterostructures
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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