Pairing strength and symmetries of 122 iron selenides in comparison with iron pnictides

High temperature superconductivity with comparable transition temperatures has been observed in the vicinity of an antiferromagnetic phase, in both 122-alkaline iron selenides and 122-iron pnictides. In contrast to iron pnictides, where the parent state is an antiferromagnetic semimetal, the parent state of 122-iron selenides is a large moment, antiferromagnetic insulator. This provides a clear indication of strong electronic correlations. The 122-selenides possess only electron pockets, while the pnictides have both hole and electron pockets. In addition, the observed block spin magnetic order in 122-selenides can not be explained by Fermi surface nesting. At the same time, the comparable $T_c$ suggests a commonality in the underlying mechanism for superconductivity in the two classes of materials. Motivated by these observations and considerations, we present a comparative strong coupling analysis of the pairing strength and symmetries in these two classes of materials [1,2]. The analysis of appropriate five orbital $t-J_1-J_2$ models, reveals a similar pairing phase diagram for both materials, with $A_{1g}$ $s(x^2y^2)$ and $B_{1g}$ $d(x^2-y^2)$ as two dominant pairing channels. The pairing amplitudes in both materials are of comparable strength, making it natural for a comparable maximum $T_c$ . In contrast to the pnictides case, an $A_{1g}$ $s(x^2+y^2)$ state is not competitive, making the dominant pairing channels fully gapped. We also discuss the magnetism of the vacancy-ordered insulating 122 iron selenides [3], showing that the observed block-spin state occurs over a wide parameter range. The predicted magnetic excitation spectrum has been verified by inelastic neutron scattering experiments. Our study also reveals some commonality with the magnetism of the parent iron pnictides [4].\\[4pt] Work was done in collaboration with Rong Yu, Predrag Nikolic, Jian-Xin Zhu, Qimiao Si and Elihu Abrahams. \\[4pt] [1] Rong Yu, Pallab Goswami, Qimiao Si, Predrag Nikolic, and Jian-Xin Zhu, ``Pairing strength and symmetries of 122 iron selenides in comparison with iron pnictides,'' to be published; arXiv:1103.3259. \\[0pt] [2] Pallab Goswami et al, ``Superconductivity in Multi-orbital $t-J_1-J_2$ Model and its Implications for Iron Pnictides,'' Europhys. Lett. {\bf 91}, 37006 (2010).\\[0pt] [3] Rong Yu, Pallab Goswami, and Qimiao Si, ``The magnetic phase diagram of an extended $J_1-J_2$ model on a modulated square lattice and its implications for the antiferromagnetic phase of $\mathrm{K}_y\mathrm{Fe}_x\mathrm{Se}_2$,'' Phys. Rev. B {\bf 84}, 094451 (2011). \\[0pt] [4] Pallab Goswami et al, ``Spin Dynamics of a $J_1-J_2$ Antiferromagnet and its Implications for Iron Pnictides,'' Phys. Rev. B {\bf 84}, 155108 (2011).