Workshop "NMR, µSR, Mössbauer spectroscopies in the study of Fe-based and other unconventional high-Tc superconductors"
What can impurities tell us on the iron based superconductors? Ru, Mn and F: the good, the bad and the ugly
Pietro Carretta, Universitá degli Studi di Pavia, Italy
I shall review on the effect of diamagnetic and paramagnetic impurities on the microscopic properties of optimally doped LnFeAsO1−yFy iron-based superconductors (Ln a lanthanide ion), in the light of recent µSR and NMR studies. Two limiting cases will be considered: the effect of the isovalent diamagnetic substitution of Fe by Ru and the effect of the doping with Mn paramagnetic impurities. In the first case it will be shown that Ru doping causes the appearance of static magnetism, nanoscopically coexisting with superconductivity. The magnetic ordering temperature TN and the order parameter display a dome-like dependence on the Ru content x, peaked around x = 1/4, with higher TN values for those materials characterized by a larger z cell coordinate of As. Remarkably, those are also the ones with the highest superconducting transition temperatures (Tc) in the impurity free compound. The reduction of Tc is found to be significant in the region of the phase diagram where the static magnetism develops. Upon increasing the Ru content superconductivity eventually disappears, but only for an impurity content x ~ 0.6. On the other hand, Mn doping yields a significant reduction of the superconducting transition temperature which is particularly pronounced for Ln = La. Indeed, in LaFeAsO1−yFy Tc drops from its optimal value to zero for a Mn content around 0.2%, more than two orders of magnitude less than the Ru content required to suppress it! Around that doping level 75As NQR 1/T1T show a low-temperature divergence typical of a two-dimensional antiferromagnet approaching a quantum critical point. At larger Mn doping levels electron localization and a magnetic order are recovered. The implication of these results on the possible pairing mechanisms underlying those materials will be discussed. A fruitful comparison with the effect of impurities on the F free magnetic pnictides will also be presented .