Abstract
In this article, theoretical aspects of the newly discovered iron pnictide superconductors are reviewed. First we present the effective model, which turns out to contain all five 3d orbitals. Then we show how the nesting of the disconnected sheets of the Fermi surface can result in multiple modes of spin fluctuations. When one of the spin fluctuation modes dominate, the superconductivity has an s-wave symmetry where the gap changes sign between different sheets of the Fermi surface. On the other hand, when the spin fluctuation modes compete with each other, d-wave pairing becomes competitive or even dominates over s-wave pairing. We explain how the lattice structure may affect the Fermi surface and thus the Tc as well as the competition between different pairing symmetries.
