Pairing symmetry and Antiferromagnetic Exchange Coupling in Fe-Based Superconductors*

I discuss the existence of strikingly identical paradigms applicable to both cuprates and iron-based superconductors in understanding magnetism, superconductivity and the interplay between the two. The magnetic states and transitions in iron- based superconductors are well described by a $J_1-J_2-J_z$ magnetic exchange model where $J_1$, $J_2$ and $J_z$ are nearest neighbour, next nearest neighbour and inter-layer couplings respectively. Differing from the t-J model for cuprates where d-wave pairing symmetry is favored, the magnetic exchange in the iron based supercondunctors predicts an unconventional s-wave $cosk_x cosk_y$ pairing. I will show that the predicted pairing symmetry is supported by many experimental results, and also discuss new predictions associated with the pairing symmetry. \\[4pt] References: \\[0pt] [1]: Chen Fang, Hong Yao, Wei-Feng Tai, Jiangping Hu and S. Kivelson ``Theory of Electron Nematic Order in LaOFeAs''; Phys. Rev. B 77 224509 (2008).\\[0pt] [2]: Kangjun Seo, A. B. Bernevig and JiangPing Hu, ``Pairing Symmetry in a Two-Orbital Exchange Coupling Model of Oxypnictides''; PRL 101, 206404 (2008) ArXiv: 0805.2958.\\[0pt] [3]: Meera M. Parish, Jiangping Hu and B. Andrei Bernevig ``Experimental Consequences of the S-wave cos(kx)cos (ky) Superconductivity in the Iron-Pnictides'', Phys. Rev. B 78, 144514 (2008) ArXiv:0807.4572.\\[0pt] [4]: Chen Fang, B. Andrei Bernevig, Jiangping Hu,``Theory of Magnetic Order in $ Fe_{1+y}Te_{1-x}Se_x$'', arXiv:0811.1294 (2008).