Mechanism for a Pairing State with Time-Reversal Symmetry Breaking in   Iron-Based Superconductors
  The multipocket Fermi surfaces of iron-based superconductors promote pairing states with both s_{+-}-wave and d_{x^2-y^2}-wave symmetry. We argue that the competition between these two order parameters could lead to a time-reversal-symmetry breaking state with s+id-pairing symmetry in the iron-based superconductors, and propose serveral scenarios in which this phase may be found. To understand the emergence of such a pairing state on a more rigorous footing, we start from a microscopic 5-orbital description representative for the pnictides. Using a combined approach of functional renormalization group and mean-field analysis, we identify the microscopic parameters of the s+id-pairing state. There, we find the most promising region for s+id-pairing in the electron doped regime with an enhanced pnictogen height. 
Christian Platt, Ronny Thomale, Carsten Honerkamp, Shou-Cheng Zhang,   Werner Hanke
cond-mat.supr-con cond-mat.str-el