Progress in nonmagnetic impurity doping studies on Fe-based superconductors

Abstract

We review the progress of nonmagnetic impurity doping study on the Fe-based superconductors.On the theoretical side, two highly potential candidates for the pairing symmetry order parameter, i.e. the multi-gap s++ and s± wave models, have been proposed but continuously debated. The debate arises because of the complex gap structure, exceptive magnetic and metallic behaviors of Fe-based superconductors which may make the influences of nonmagnetic defects varied in chemical potential, impurity disorder, inter-and intra-band scattering strength, and electron localization, and hence difficulty in directly obtaining the most important information for understanding the symmetry order parameter. Experimentally, the nonmagnetic impurity substitution study has been widely carried out, which has provided very useful insights. We review herein the various nonmagnetic impurity doping experiments, including the controlled defects within the superconducting Fe2X2 planes through samples quality improvement, a single impurity effects on the electronic state and local moment, the magnetic response of the Fe2X2 planes both on macroscopic scale as antiferromagnetic state and local scale of moment, as well as the significant effect in modifying the transport properties. The experiments enable us to qualitatively analyze the nonmagnetic impurities effects on the superconducting state for many Fe-based superconductors. We also propose herein some strategies for the nonmagnetic impurity doping study. As an important model for explaining the nonmagnetic impurities doping effects, the pair-breaking model is compared with various theoretical approaches via analyzing the pair-breaking rates of various Fe-superconductors.

Clean to dirty limit andTcsuppression in NdFeAsO0.7F0.3studied byHc2analysis

“…The s symmetry associated to interband interactions between hole and electron pockets mediated by spin fluctuations, which lead to a superconducting order parameter that changes sign over the Fermi surface sheets, is most often used to describe iron pnictides. The theoretical consequences are found to be consistent with most experiments that investigate the role of pair-breaking effect by impurities 46 . Indeed, despite earlier experiments 78,81,82,83 claimed that the rate of Tc suppression by disorder was much slower than what predicted by the s scenario 84 , later theoretical works dealing with multiband s superconductivity considered different types of impurity scattering potential (finite-ranged potential, different ratios of inter-band to intra-band contributions …) and demonstrated that results for the s± state are not inconsistent with experimental data.…”

“…Clearly, the curves are spread over a wide range, as also evidenced in the review of ref. 46 . This spread may be explained by the different compounds, different types of irradiations, different doping levels, different pristine Tc0's, magnetic or non magnetic scattering.…”

Anomalous transverse resistance in 122-type iron-based superconductors

“…With substitution of atomic impurities into the superconducting Fe 2 As 2 layers, the impurities are found to enhance the ATR phenomenon as introduced above. Substituting magnetic Co and nonmagnetic Zn onto the Fe-site in the Fe 2 As 2 layers, the impurity ions can suppress the superconductivity within a region of a coherence length 22,24,25,35 , and then disorganize the superconducting distribution. Particularly, the impurity scattering centers are aligned with a certain direction, for instance the antiferromagnetic a -axis, resulting in highly anisotropic impurity states 36 .…”

Impurity effects on spin dynamics in magnetic and superconducting iron pnictides and chalcogenides

“…On the other hand, the isotropic s ‐wave superconductors are sensitive to magnetic and resistant against nonmagnetic impurities. Such distinction in the impact of impurities or controlled disorder, when observed experimentally on a new type of superconductor, gives a tentative hint on the underlying pairing symmetry and superconducting mechanism .…”