The effect of nonmagnetic Zn impurity on superconductivity in iron-based superconductors is reviewed. Zn impurity can severely suppress the antiferromagnetic (AFM) order of Fe ions in the parent compound LaFeAsO . In the 1111 type F-doped LaFeAs ( O , F ) system, the superconducting transition temperature (Tc) increases in the underdoped regime, remains unchanged in the optimally-doped regime, and is severely suppressed in the overdoped regime in the presence of Zn impurity. The results suggest a switch of the symmetry of the superconducting order parameters from a s-wave to s±- or d-wave states as the charge carrier doping increases in this system. To our surprise, Zn impurity strongly suppresses Tc in the Co -doped LaFeAsO and BaFe 2 As 2 systems despite of the Co doping level. The absence of universal Zn impurity effect implies that the pairing symmetry of the iron-based superconductors may be dependent on the details of the electronic structure.
Impurity effect as a probe for the pairing symmetry of graphene-based superconductors
