Superconducting states of a J1-J2-K model for iron pnictides
  We study the symmetry and strength of the superconducting pairing in a two-orbital t-J1-J2-K model for iron pnictides using the salve boson mean-field theory. We show that the nearest-neighbor biquadratic interaction -K (S_i * S_j)^2 inflences the superconducting pairing phase diagram by promoting the d_{x2-y2} B_{1g} and the s_{x^2+y^2} A_{1g} channels. The resulting phase diagram consists of several competing pairing channels, including an isotropic s_{\pm} A_{1g} channel, an anisotropic d_{x2-y2} B_{1g} channel, and two s+id pairing channels. We have investigated the evolution of superconducting states with electron doping, and find that with the biquadratic interaction various pairing channels may dominate at different doping concentrations. We show that this is crucial in understanding the doping evolution of superconducting gap anisotropy observed in some angle resolved photoemission spectrum measurements. 
Rong Yu, Andriy H. Nevidomskyy
cond-mat.supr-con cond-mat.str-el