"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 )

Resistivity in Co-doped Ba-122: comparison of thin films and single crystals

The temperature dependence of the resistivity of epitaxial Ba(Fe_(1-x)Co_x)2As2 thin films (with nominal doping x = 0.08, 0.10 and 0.15) has been analyzed and compared with analogous measurements on single crystals taken from literature. The rho(T) of thin films looks different from that of single crystals, even when the cobalt content is the same. All rho(T) curves can be fitted by considering an effective two-band model (with holes and electrons bands) in which the electrons are more strongly coupled with the bosons (spin fluctuations) than holes, while the effect of impurities is mainly concentrated in the hole band. Within this model the mediating boson has the same characteristic energy in single crystals and thin films, but the shape of the transport spectral function at low energy has to be very different, leading to a "hardening" of the electron-boson spectral function in thin films, associated with the strain induced by the substrate.Comment: 13 pages, 4 figure

Advanced surface characterization of Ba(Fe_(0.92)Co_(0.08))_2As_2 epitaxial thin films

We 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

The Order-Parameter Symmetry and Fermi Surface Topology of 122 Fe-Based Superconductors: A Point-Contact Andreev-Reflection Study

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_{1−x}Co_{x} )_{2}As_{2} single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1−x}Co_{x})_{2}As_{2} single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the Density-Functional Theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})_{2}As_{2} unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})_{2}As_{2}, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of "hot spots" in the gaps of 122 compound