Abstract.The recently discovered Fe pnictide and chalcogenide superconductors display low temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within family as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely "sign-changing s-wave" (s ± ) symmetry. We review theoretical aspects, materials properties, and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.

Based on the effective four-band model we analyze the spin response in the normal and superconducting states of the 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 AF M for the s± (extended s-wave) superconducting order parameter and it disappears rapidly for q < QAF M . The resonance feature is extremely weak for the d x 2 −y 2 -wave order parameter due to specific Fermi surface topology of these compounds. The essential difference between s±-wave and d x 2 −y 2 -wave symmetries for the magnetic excitations can be used for experimental determination of the superconducting wave function symmetry.PACS numbers: 74.20.Mn, 74.20.Rp, 74.25.Ha, 74.25.Jb The relation between unconventional superconductivity and magnetism is one of the most interesting topics in the condensed matter physics. In contrast to the usual electron-phonon mediated superconductors where the paramagnetic spin excitations are suppressed below superconducting transition temperature due to the formation of the Cooper pairs with total spin S = 0, in unconventional superconductors, such as layered cuprates or heavy fermion superconductors, a bound state (spin resonance) with a high intensity forms below T c 1,2,3 . The fact that the superconducting gap is changing sign at a different parts of the Fermi surface together with a presence of the strong electronic correlations yields such an enhancement of the spin response 4 . Most interestingly, an observation of the resonance peak indicates not only that Cooper-pairing is unconventional but also that the magnetic fluctuations are most relevant for superconductivity 5 .Since the discovery of superconductivity in the quaternary oxypnictides LaFePO 6 and LaNiPO 7 , a new class of high-T c materials with Fe-based layered structure is emerging 8,9,10,11,12,13,14 . Although the microscopic nature of superconductivity in these compounds remains unclear at present, certain aspects have been already discussed 15,16,17,18,19,20,21,22,23,24,25,26,27 . In particular, ab initio band structure calculations 15,16,17,18,19,20 have shown that the conductivity and superconductivity in these systems are associated with the Fe-pnictide layer, and the electronic density of states (DOS) near the Fermi level shows maximum contribution from the Fe-3d orbitals. The resulting Fermi surface consists of two hole (h) and two electron (e) pockets. The normal state magnetic spin susceptibility determined from these bands 22 exhibits both small q ∼ 0 fluctuations and antiferromagnetic commensurate spin density wave (SDW) peaks.In this Rapid Communication, using the four-band tight-binding model we study theoretically the spin response in the normal and superconducting states of Febas...

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1−xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57 Fe Mössbauer spectroscopy and muon spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5) µB at the iron site below TN = 138 K, well separated from a structural phase transition at TS = 156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below TN are reproduced. PACS numbers: 76.75.+i, 76.80.+y, 75.30.Fv, The recently discovered Fe-based superconductors LaFeAsO 1−x F x [1] and the related materials in which La is substituted by Sm, Ce, Nd, Pr, and Gd, respectively [2,3,4,5,6,7] has triggered an intense research in the oxypnictides. Besides the high critical temperature above 50 K there are further striking similarities to the properties of the high-T C cuprates. The oxypnictides have a layered crystal structure with alternating FeAs and LaO sheets, where the Fe atoms are arranged on a simple square lattice [1]. Theoretical studies reveal a two-dimensional electronic structure [8] and it is believed that conductivity takes place mainly in the FeAs layers while the LaO layers provide the charge reservoir when doped with F ions. Again similar as in the cuprates, superconductivity emerges when doping a magnetic mother compound with electrons or holes and thereby supressing the magnetic order [9]. This suggests an interesting interplay between magnetism and superconductivity and, indeed, a recent theoretical work suggests that magnetic fluctuations associated with quantum critical point are essential for superconductivity in the electron doped LaFeAsO 1−x F x superconductors [10].However, in contrast to the cuprates, the magnetic mother compound is not a Mott-Hubbard insulator but a poor metal. A large covalency in the FeAs layers was found [8,11], which in the case of tetragonal LaFePO, i.e. the compound where As is replaced by P, leads to a non-magnetic ground state [12,13]. In contrast, in LaFeAsO there is an additional structural distortion at elevated temperatures [14,15] and a long range spin density wave (SDW) antiferromagnetic order has been observed in neutron scattering experiments on powder samples below ∼150 K. [15]. First principle calculations yield antiferromagnetic order with Fe magnetic moments ranging from 1.5 µ B to 2.3 µ B [10,16,17,18], while the neutron scattering experiments indicate a much smaller value. Assuming that the full sample volume is contributing to the magnetic scattering an ordered moment of ∼ 0.35 µ B [15] is inferred from the weak superlattice reflections in powder neutron diffraction. A local probe measurement, which could verify...