Unconventional effects in the magnetization and magnetoconductivity of iron based superconductors near Tc as probed by thermal fluctuation spectroscopy

Resumo

The main objective of this thesis was to study unconventional effects on the magnetization and Magnetoconductivity of iron-based superconductors, FeSCs, [1] mainly family '122'. This was done through the so-called superconducting fluctuation spectroscopy above the superconducting critical temperature, Tc, [2]. This technique has been used in a considerable number of work to characterize the properties of superconductors (see for example [3- 5]). Moreover, the nature of the thermal fluctuations is determined, verifying the applicability of the Ginzburg-Landau equations in the Gaussian region. The results presented in this thesis are highly relevant and were published in high level scientific journals. Specifically, the magnetoconductivity and the magnetization above Tc over Ba (Fe1-xNiX) 2As2 crystal with different doping levels, x, in the overdoped regime were analyzed. These measurements allow us to determine some superconducting parameters. The main result was to find an increase of anisotropy factor with x. These results were published in Refs. [6-7]. Additionally in this compound, magnetic penetration length it was studied directly from magnetization measurements. Through this measure the superfluid density was determined and compared with theoretical models. This analysis revealed that this material has a nature multielectrónica (several bands at the Fermi surface with different types of carriers). The results and conclusions were published in Ref. [8]. In the second part, BaFe2(As1-xPx) 2 crystals were analyzed through measurements of magnetization and magnetoconductivity near to Tc. In this study, significant differences between the transverse coherence length determined by measures of electric transport and magnetization measurements were found. In accordance with recent theoretical models, these differences may be attributed to the existence of several bands in these materials. Moreover, analysis of data in the low-field region close to the transition temperature suggests that the phase fluctuations of the order parameter play a significant role in this compound. The measurements and experimental analysis of this study was published in Ref. [9]. Finally, another interesting result in this compound was the determining of the angular dependence of the upper critical field, Hc2, through magnetoconductivity measurements. The results confirmed a slight deviation of the Ginzburg-Landau equations for anisotropic materials whose relative angle between the c-axis (perpendicular to the planes of Fe-As) and the applied magnetic field is close to 90 °. This behavior is associated with the nature of the material. [10] References: [1] For reviews on the properties of Fe-based superconductors see, e.g.: Johnston D C 2010 Adv. Phys. 59 803; Mazin I I 2010 Nature 464 183; Paglione J y Greene R 2010 Nat. Phys. 6 645; Wang F y Lee D-H 2011 Science 332 200; Stewart G R 2011 Rev. Mod. Phys. 83 1589 [2] Vidal F, Torrón C, Viña J y Mosqueira J 2000 Physica C 332 166 [3] Putti M et al 2010 Supercond. Sci. Technol. 23 034003 [4] Mosqueira J, Dancausa J D, Vidal F, Salem-Sugui S, Alvarenga A D, Luo H Q, Wang Z S y Wen H H 2011 Phys. Rev. B 83 094519 [5] Pryo G, Lascialfari A, Rigamonti A, Romanó L, Sanna S, Putti M y Tropeano M 2011 Phys. Rev. B 84 064507 [6] Ramos-Álvarez A, Mosqueira J, Vidal F, Lu X, Luo H 2015 Supercond. Sci. Technol. 28 075004 [7] Rey R I, Ramos-Álvarez A, Carballeira C, Mosqueira J, Vidal F, Salem-Sugui Jr. S, Alvarenga A D, Zhang R y Luo H 2014 Supercond. Sci. Technol. 27 075001 [8] Rey R I, Ramos-Álvarez A, Mosqueira J, Salem-Sugui Jr S, Alvarenga A D, Luo H-Q, Lu X-Y, Zhang R y Vidal F 2014 Supercond. Sci. Technol. 27 055015 [9] Ramos-Álvarez A, Mosqueira J, y Vidal F 2015 Phys. Rev. B 92 094508 [10] Cho K et al 2011 Phys. Rev. B 83 060502