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| Author(s): |
Matheus Radaelli
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | Campinas, SP. |
| Institution: | Universidade Estadual de Campinas (UNICAMP). Instituto de Física Gleb Wataghin |
| Defense date: | 2017-07-07 |
| Examining board members: |
Pascoal José Giglio Pagliuso;
Abner de Siervo;
José Gerivaldo dos Santos Duque
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| Advisor: | Pascoal José Giglio Pagliuso |
| Abstract | |
The Fe-based superconductors (SCs) have become focus of great interest since their discovery due to their remarkable physical properties such as high critical temperatures (Tc), high critical currents, high malleability and unusual interplay between magnetism and superconductivity, with phase diagrams very similar to the Cuprates and Heavy Fermions systems. A deep microscopic understanding of this unusual interplay between magnetism and superconductivity and its relationship with the pairing mechanism of this class of compounds could lead to important breakthroughs in condensed matter physics. Furthermore, searching for physical parameters that can be tuned in order to achive superconductivity in these materials is extremely important and may guide the search for new superconductors. Previous studies performed in our group in the Ba1-x EuxFe2As2 series show that a decrease in the Fe-As distance, dFeAs, is closely linked to the suppression of the SDW magnetic phase and the localization of the Fe 3d electrons in the FeAs plane. The increase in the planar character of the Fe 3d orbitals, that is, favoring the occupation of the xy and x2-y2 orbitals on the Fermi surface appears to be a key ingredient that favors the suppression of the SDW phase and the emergence of superconductivity in this class of compounds. In this work, we report a systematic study of the properties of the intermetallic tetragonal compound SrFe2As2 as a function of Eu substitution in the Sr crystallographic site, where our goal was to extend the understanding obtained through the previous studies on the Ba1-xEuxFe2As2 series to other compounds of the same family. Single crystals of SrFe2As2 and Sr1-xEuxFe2As2 were grown by the alternative In-flux method. The macroscopic characterization of the studied single crystas was made by measurements of electrical resistivity, magnetization, DC magnetic susceptibility, specific heat, powder x-ray diffraction and Elemental analysis (EDS, WDS). The microscopic characterization was performed using the Electronic Spin Resonance (EPR) technique using the Eu2+ paramagnetic ion as the resonanting probe. The Eu2+ ion was used to study the evolution of the spin dynamics of Fe 3d electrons when the system evolves from magnetically ordered to superconducting. Our ESR results evidenced a clear evolution of the spin dynamics of Fe 3d electrons as we suppressed the SDW magnetic order. From the linear temperature dependence of Eu2+ ESR linewidth, known as the Korringa relaxation rate (b), and from mapping of the anomalies revealed in electrical resistivity data, we argue that there is a large separation between the structural phase transition (Ts) and the spin-density-wave transition SDW (TSDW) induced by doping which seems much larger than that observed in the Ba1-xEuxFe2As2 series. In the low Eu-concentration region, we have found a trend which is in contrast to the one observed in the series of BaFe2As2 where the suppression of the SDW phase follows the suppression of the Eu2+ ESR Korringa rate. In this work, for the the Sr1-xEuxFe2As2 series in the low Eu-concentration regime, we have found a decrease of the localization of the 3d orbitals in the FeAs plane as the TSDW descreases as a function of x, whereas for the x > 0.5 regime, a decrease of boht TSDW and the Eu2+ Korringa rate are consistent to a localization of the 3d electrons in the FeAs plane. These results allowed us to suggest that the orbital differentiation of the 3d bands of Fe in these materials seems to be governed by the dependence of Ts with x throughout the series (AU) | |
| FAPESP's process: | 15/09701-7 - Growth and characterization of single crystals of (Sr,Ca)Fe2As2 prepared by in flux method |
| Grantee: | Matheus Radaelli |
| Support Opportunities: | Scholarships in Brazil - Master |