Superconductivity on the verge of electronic topological transition in   Fe based superconductors
  A comprehensive first principles study on the electronic topological transition in a number of 122 family of Fe based superconductors is presented. Doping as well as temperature driven Lifshitz transitions are found from first principles simulations in a variety of Fe based superconductors that are consistent with experimental findings. In all the studied compounds the Lifshitz transitions are consistently found to occur at a doping concentration where superconductivity is highest and magnetism disappears. Systematically, the Lifshitz transition occurs in the electron Fermi surfaces for hole doping, whereas in hole Fermi surfaces for electron doping as well as iso-electronic doping. Temperature driven Lifshitz transition is found to occur in the iso-electronic Ru-doped BaFe$_2$As$_2$ compounds. Fermi surface areas are found to carry sensitivity of topological modifications more acutely than the band structures and can be used as a better experimental probe to identify electronic topological transition. 
Haranath Ghosh, Smritijit Sen
cond-mat.supr-con