"Experimental study of iron-based superconductors: advanced characterization and fundamental properties"

Iron-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 ).