Superconductivity and electronic fluctuations in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ studied by Raman scattering

Using polarization-resolved electronic Raman scattering we study under-doped, optimally-doped and over-doped Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ samples in the normal and superconducting states. We show that low-energy nematic fluctuations are universal for all studied doping range. In the superconducting state, we observe two distinct superconducting pair breaking peaks corresponding to one large and one small superconducting gaps. In addition, we detect a collective mode below the superconducting transition in the B$_{2g}$ channel and determine the evolution of its binding energy with doping. Possible scenarios are proposed to explain the origin of the in-gap collective mode. In the superconducting state of the under-doped regime, we detect a re-entrance transition below which the spectral background changes and the collective mode vanishes.