  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.
