Abstract:
Pairing symmetry in oxypnictides, a new family of multiband high-$T_c$ superconductors, is partially imposed by the positions of multiple Fermi pockets, which itself can give rise to new order parameters, such as $s_{+,-}$-states or the state of $d_{x^2-y^2}$ symmetry. Other pairing states may appear on small pockets for long range interactions, but they are expected to be sensitive to defects. We identify the competing antiferromagnetic order with the triplet exciton transition in the semi-metallic background and discuss whether its coexistence with superconductivity explains the doping dependence of $T_c$.
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