Conclusions

Abstract

In order to elucidate the detailed superconducting gap structure of iron-based superconductors, we have performed high precision measurements of the magnetic penetration depth for several iron-pnictide superconductors. We have revealed convincing evidence that the superconducting gap structure of iron-based superconductors is not universal depending on the doping materials and its doping levels. In addition, we have performed three different absolute penetration depth measurements for the $\text{ BaFe}_2({\text{ As}}_{1-x}{\text{ P}}_x)_2$ series over a wide range of $x$ to explore the possibility of a quantum phase transition beneath the superconducting dome. The significant influence of quantum critical fluctuations associated with the QCP on the condensed electron pairs in the superconducting state is evident from the present study. In this chapter, we will summarize the results of this study.