Introduction

Abstract

The discovery of high transition temperature ($T_c$) superconductivity up to 56 K in iron-based superconductors has attracted numerous interest both experimentally and theoretically. So far, various theories have been proposed for the pairing mechanism, but a consensus for the pairing symmetry in this system is still lacking. Therefore, the first experimental task to this problem is to elucidate the superconducting pairing symmetry, which is intimately related to the pairing interaction. To investigate the superconducting gap structure of iron-based superconductors, we have performed high-precision magnetic penetration depth measurements for several iron-pnictides. In addition, in this study, we have extended our research to explore a possible presence of a quantum critical point beneath the superconducting dome. The zero-temperature magnetic penetration depth measurements revealed the first convincing signature of a second-order quantum phase transition deep inside the dome, which may address a key question on the general phase diagram of unconventional superconductivity in the vicinity of a QCP. In this chapter, we will briefly overview the content of this book.