Ilya Eremin, MPIPKS-Dresden
Since the discovery of superconductivity in the quaternary oxypnictides LaOFeP, a new class of high-T$_c$ superconductors with Fe-based layered structure is emerging. Although the microscopic nature of superconductivity in these compounds remains unclear at present, certain aspects has been already discussed in the literature. Here, I will review the existing experimental and theoretical results relevant for understanding the physics of oxypnictides. Based on the effective four-band model we analyze the spin response in the normal and superconducting state of the novel Fe-pnictide superconductors. While the normal state spin excitations are dominated by the continuum of the interorbital antiferromagnetic fluctuations and the intraband spin density wave fluctuations, the unconventional superconductivity yields different feedback. The resonance peak in form of the well-defined spin exciton occurs only for the interband scattering at the antiferromagnetic momentum, Q_AFM, for the extended s-wave superconducting order parameter and it disappears rapidly for q smaller than Q_AFM. The resonance feature is extremely weak for the d(x2-y2)-wave order parameter due to specific Fermi surface topology of these compounds. The essential difference between extended s-wave and d(x2-y2)-wave symmetries for the magnetic excitations can be used for experimental determination of the superconducting wave function symmetry.