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Experimental study of iron-based superconductors: advanced characterization and fundamental properties
Get PDFIron-based superconductors (SCs) were discovered in 2008 and their specific features immediately captured the great interest of the scientific community. The critical temperature of these compounds was the highest known so far with the exception of cuprates. They show a layered crystal structure and quasi-2D Fermi surface, made up of multiple disconnected sheets, and usually with multiple order parameters. Their parent (undoped) compounds are metals and the proximity to magnetism, clearly visible in the phase diagram of these materials, seems to be of fundamental importance in order to understand the superconductivity in Fe-based SCs. Their observed features have led theoretical physicist to propose an unconventional, spin fluctuation-mediated pairing. Many experimental results obtained so far seem to confirm the theoretical predictions, but the final validation of this model still requires additional steps. The research on Fe-based materials has been recently boosted by the progress in the techniques of film deposition. Films of very high quality are necessary for applications in superconducting electronics, i.e. for the fabrication of Josephson junctions, SQUIDs and so on. However, they can be fruitfully used also to investigate fundamental properties of these compounds (especially for those materials that cannot be grown easily in the form of single crystals). For instance, they are the perfect playground for transport, optical and spectroscopic measurements of various kind; thin films offer an additional way to tune the critical temperature, thanks to strain/stress effects that can be induced by the substrate; finally, they are necessary to realize some proposed phase-sensitive experiments to determine the order parameter symmetry s++ or s± [1]. Many other experimental measurements and theoretical calculations are crucial in order to clarify some open points for example the superconducting order parameter and its symmetry, the amplitude and the number of the energy gaps and their temperature dependence. This dissertation presents the advanced characterization and the fundamental study of Fe-based superconductors (122 and 11 families) mainly in the form of epitaxial thin films by means of different experimental techniques, namely electrical transport measurements, point-contact Andreev-reflection spectroscopy (used both for advanced characterization and fundamental investigation) and other techniques for the morphological and chemical characterization of the surface (AFM, FESEM, Energy-dispersive X-ray spectroscopy). This research was developed within the activities of the Eu-Japan project IRON SEA (establishing the basic science and technology for Iron-based superconducting electronics applications) funded within the Seventh Framework Programme FP7 under grant number 283141. The epitaxial thin films of 122 and 11 superconductors were grown by partners of the consortium that are world leaders in this field, and thus they are high-quality state-ofthe- art samples. In particular, the Co-doped Ba-122 films were deposited by the group of prof. B. Holzapfel at IFW Dresden, while the Fe(Te,Se) films were grown by the group of prof. A. Maeda at Tokyo University. The data collected by means of the aforementioned techniques allowed the systematic characterization and the study of the homogeneity of the superconducting properties and of the chemical composition at the surface, and also the effects of aging and degradation (especially for Ba-122 samples). In the framework of the IRON SEA project, this large amount of information was required in order to assess the possible use of these films for the development of superconducting electronic devices. From the point of view of the fundamental properties, the PCARS study of the Co-doped Ba-122 and Fe(Te,Se) thin films allowed gathering information about the phase diagram of these materials, i.e. the effect of isovalent and aliovalent doping on the critical temperature (Tc) and on the superconducting gaps (i.e. number, amplitude and symmetry) - or, conversely, the determination of the trend of the gaps as a function of doping and critical temperature. Thanks to a theoretical analysis of the results carried out by Dr. G. Ummarino within the multi-band Eliashberg theory, the results of PCARS measurements allowed extracting the characteristic energy of the mediating boson, verifying the spin-fluctuation mechanism, determining the evolution of the coupling constants from the underdoped to the overdoped regime. The activity of the candidate has been focused on the experimental aspects of the research. She spent two months (October - December 2012) at IFW Dresden during which she carried out a part of the PCARS measurements on Co-doped Ba-122 films (otherwise carried out at Politecnico di Torino) and contributed to their characterization (i.e. by DC Resistivity, RHEED and X-ray Spectroscopy) as discussed in chapter 4. The characterization of these films was completed at Politecnico of Torino by AFM, FESEM and EDX measurements, performed by F. Laviano and M. Raimondi and later analysed by the candidate. Similarly for Fe(Te,Se) films, widely characterized by the Japanese partner, the research was mainly focused on PCARS and transport measurements (chapter 5
Doping and critical-temperature dependence of the energy gaps in Ba(Fe_{1−x}Co_{x})_{2}As_{2} thin films
Get PDFThe dependence of the superconducting gaps in epitaxial Ba(Fe_{1−x}Co_{x})_{2}As_{2} thin films on the nominal doping x (0.04 ≤ x ≤ 0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the two-dimensional Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps — although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps ∆_{S} and ∆_{L} show similar monotonic trends as a function of the local critical temperature T_{c}^{A} (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e., to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with T_{c} according to the empirical law Ω_{0} = 4.65k_{B}T_{c}, and with a total electron-boson coupling strength λ_tot = 2.22 for x ≤ 0.10 (i.e., up to optimal doping) that slightly decreases to λ_tot = 1.82 in the overdoped samples (x = 0.15
Point contact spectroscopy in Fe-based multiband superconductors: Recent advancements and future challenges
No full textAdvanced surface characterization of Ba(Fe_(0.92)Co_(0.08))_2As_2 epitaxial thin films
No full textWe report on the systematic characterization of Ba(Fe_(0.92)Co_(0.08))_2As_2 epitaxial thin films on CaF2 substrate in view of their possible use for superconducting electronic applications. By using different and complementary techniques we studied the morphological characteristics of the surface, the structural properties, the magnetic response, and the superconducting properties in terms of critical temperature, critical current, and energy gaps. Particular attention was paid to the homogeneity of the films and to the comparison of their superconducting properties with those of single crystals of the same compound
Point-contact Andreev-reflection spectroscopy in anisotropic superconductors: The importance of directionality (Review Article)
No full textPoint-contact Andreev-reflection spectroscopy (PCARS) has demonstrated to be one of the most effective experimental tools for the investigation of fundamental properties of superconductors such as the superconducting gap and the electron–phonon (or, more generally, electron–boson) coupling. By reviewing relevant examples reported in literature and presenting new results, in this paper we show that when the direction of the interface with respect to the crystallographic axes can be controlled (as in single crystals and epitaxial films) PCARS can provide invaluable information about the anisotropy of the pairing wavefunction or — in the case of multiband superconductors
— on the number, amplitude and symmetry of the energy gaps. Moreover, the analysis of PCARS results within a suitable 3D generalization of the BTK model allows obtaining qualitative information about the topology of the Fermi surface
Point-contact Andreev-reflection spectroscopy in Fe(Te,Se) films: multiband superconductivity and electron-boson coupling
No full textWe report on a study of the superconducting order parameter in Fe(Te_{1−x}Se_{x}) thin films (with different Se contents: x=0.3, 0.4, 0.5) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude Δ_E≃2.75kBTc and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude Δ_H≃1.75k_BTc. The existence of this gap and its amplitude are confirmed by PCARS measurements in Fe(Te_{1−x}Se_{x}) single crystals. The values of the two gaps Δ_E and Δ_H, once plotted as a function of the local critical temperature Tc^A, turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature
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
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