Short-range symmetry breaking induced structural collapse and Tc   enhancement in 122-type iron pnictide superconductors
  In this paper, we investigate analytically the experimental observations of structural collapse and Tc enhancement in the rare earth-doped 122-type iron-based pnictide superconductors [S. R. Saha et.al. Phys. Rev. B 85, 024525 (2012), arXiv:1105.4798 (2011); B. Lv et al. PNAS 108, 15705 (2011).]. Based on the real-space effective c-axis lattice constant theory of superconductivity [X. Q. Huang, arXiv:1001.5067], it is shown that the abrupt c-axis reduction of the superconductors is due to the structural phase transition (an ultra-short-range symmetry breaking) of the charge stripe lattice. The existence of this phase transition corresponds to the change from the full-doped superconducting planes to the half-doped superconducting planes in the studied superconductors. It is estimated that this phase transition may help to promote the superconducting transition temperature of the 122 family up to as high as 80 K. 
X. Q. Huang
cond-mat.supr-con cond-mat.str-el