Superconductivity-induced reentrance of orthorhombic distortion in   Ba1-xKxFe2As2
  Detailed knowledge of the phase diagram and the nature of the competing magnetic and superconducting phases is imperative for an understanding of the physics of iron-based superconductivity. Here, we show using thermodynamic probes that the phase diagram of the first discovered, and highest Tc, 122-type material, Ba1-xKxFe2As2 is in fact much richer than previously reported. Inside the usual stripe-type magnetic order with C2-symmetry, we find a small pocket of a tetragonal, C4-symmetric phase, which surprisingly reverts back to the C2-phase at or slightly below the superconducting transition. This re-entrance to a low-temperature orthorhombic state induced by superconductivity is discussed in terms of competition of the two magnetic phases with superconductivity and is illustrated by the measured changes in the electronic entropy of the system. Using our thermodynamic data, we make predictions about how the phase diagram of these competing orders will change under pressure. 
A. E. B\"ohmer, F. Hardy, L. Wang, T. Wolf, P. Schweiss, and C.   Meingast
cond-mat.supr-con