1,721,008 research outputs found
Strange-metal behavior of cuprates driven by dissipative charge collective modes
Full text linkThis work focuses on the study of high-temperature superconducting copper oxides, commonly known in the literature as cuprates. Beyond high-temperature supercon- ductivity, these systems are characterized by a complex phenomenology which shows considerable deviations from the standard theory of metals and whose comprehen- sive understanding is still an open problem in condensed matter theory. Within our discussion, we will analyze some of the most remarkable physical phenomena observed in the so-called strange-metal phase of cuprates, for which there is still no unanimously shared theory that can explain them exhaustively. In the description that we will propose, a central role is played by appropriate short-ranged charge collective modes, recently observed throughout the strange-metal phase thanks to the RIXS experiments. Under suitable hypotheses regarding the Landau damping to which these collective modes are subject, we will show that they are capable of inducing a strange-metal behavior in the systems in which they are present. A discussion on the strongly anisotropic properties observed in the metallic phase of some cuprate thin films is also included
Filamentarity and inhomogeneities in low dimensional superconductors
Full text linkIn this Thesis, I investigate the problem of the interplay between disorder and low dimensionality in superconductors.
From the microscopic point of view, I show that the presence of impurities in the superconducting condensate can produce a pairbreaking effect at the Lifshitz transition in multibands superconductors, avoiding or at least circumventing Anderson’s Theorem. This is consistent with the observed suppression of the superconducting critical temperature Tc in SrTiO3 -based heterostructures as a function of the gate potential Vg . This study allows us to disentangle microscopic from mesoscopic disorder in SrTiO3 -based interfaces: microscopic impurities are in fact necessary to explain the suppression of Tc observed when multiband superconductivity is involved; the global behavior is instead well captured only if the strongly inhomogeneous nature of such compounds is considered.
Indeed, disorder can also appear on a mesoscopic length scale. The electronic condensate can in fact segregate into regions large enough to define a local phase but small compared with the sample. The reasons behind this inhomogeneity of the superconducting order parameter can be several, depending on the system in exam: in oxides heterostructures it may be connected to the thermodynamic instability caused by the electrostatic potential confining the electron gas at the interface, in transition metal dichalcogenides the instability may be introduced by the gating ionic liquid, whereas sometimes the phase separation can be caused by the presence of microscopic impurities in addition with the competition of superconductivity with another order parameter, e.g., the charge ordering in the case of cuprates. Such a phase separation may appear as a filamentary structure.
Concerning mesoscopic inhomogeneities, I will focus on two different materials.
In SrTiO3 -based eterostructures, where filamentary superconductivity is embedded in a metallic matrix, I study the consequences of filamentarity in transport properties, assuming a priori a fractal-like organization of the electronic condensate and calculating the complex conductance with a Random Impedance Network model.
The geometry of the filamentary structure plays a crucial role, especially in the behavior of the superfluid stiffness as a function of the temperature. Although the motivation of this work is connected to resonant microwave experiments performed on a LaAlO3 /SrTiO3 interface, the results are quite general and can in principle be
applied to other materials displaying filamentary superconductivity.
In cuprates, I focus on the phase competition as the most likely reason of filamentarity. The charge ordering-superconducting (CO-SC) competition is studied by means of Monte Carlo simulations within an anisotropic Heisenberg model accounting for the basic physical symmetries involved, the out-of-plane pseudospin component
mapping two possible charge density waves (CDW) variants while the in-plane component standing for the SC order parameter. The anisotropy term α is taken as the control parameter, tuning the transition from Berezinskii-Kosterlitz-Thouless (BKT) to the charge ordered state. The phase diagram T c vs α is studied both in the clean case and in a random field, the presence of microscopic impurities being necessary to stabilize the clustering of charge
ordered domains and the appearance of filamentary superconductivity
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
d-wave superconductivity near charge instabilities
No full textWe investigate the symmetry of the superconducting order parameter in the proximity of a phase separation or of an incommensurate charge-density-wave instability. The attractive effective interaction at small or intermediate transferred momenta is singular near the instability. This strongly q-dependent interaction, together with a residual local repulsion between the quasiparticles and an enhanced density of states for band structures appropriate for the high-temperature superconducting oxides, strongly favors the formation of d-wave superconductivity. The relative stability with respect to superconductivity in the s-wave channel is discussed in detail, finding this latter hardly realized in the above conditions. The superconducting temperature is mostly determined by the closeness to the quantum critical point associated with the charge instability and displays a stronger dependence on doping with respect to a simple proximity to a van Hove singularity. The relevance of this scenario and the generic agreement of the resulting phase diagram with the properties displayed by high-temperature superconducting oxides is discussed
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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