Consistent picture of the octet-nodal gap and its evolution with doping   in heavily overdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$
  We investigate the pairing symmetry in heavily overdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ based on the spin-fluctuation mechanism. The exotic octet nodes of the superconducting gap and the unusual evolution of the gap with doping observed by the recent experiments are well explained in a unified manner. We demonstrate that the scatterings of electrons on the Fermi patches is mainly responsible for the incommensurate spin fluctuations and consequently the Fermi-surface-dependent multi-gap structure, since the Fermi level is close to the flat band. In addition, we find that a $d$-wave pairing state will prevail over the s-wave pairing state around the Lifshitz transition point. 
Shun-Li Yu, Zi-Jian Yao, Jian-Xin Li
cond-mat.supr-con cond-mat.str-el