Superconductivity induced by Ni doping in SmFe(1-x)Ni(x)AsO

Magnetism in Fe-based superconductors

In this review, we present a summary of experimental studies of magnetism in Fe-based superconductors. The doping dependent phase diagram shows strong similarities to the generic phase diagram of the cuprates. Parent compounds exhibit magnetic order together with a structural phase transition, both of which are progressively suppressed with doping, allowing superconductivity to emerge. The stripe-like spin arrangement of Fe moments in the magnetically ordered state shows identical in-plane structure for the RFeAsO (R = rare earth) and AFe(2)As(2) (A = Sr, Ca, Ba, Eu and K) parent compounds, notably different than the spin configuration of the cuprates. Interestingly, Fe(1 + y)Te orders with a different spin order despite having very similar Fermi surface topology. Studies of the spin dynamics of the parent compounds show that the interactions are best characterized as anisotropic three-dimensional interactions. Despite the room temperature tetragonal structure, analysis of the low temperature spin waves under the assumption of a Heisenberg Hamiltonian indicates strong in-plane anisotropy with a significant next-nearest-neighbor interaction. For the superconducting state, a resonance, localized in both wavevector and energy, is observed in the spin excitation spectrum as for the cuprates. This resonance is observed at a wavevector compatible with a Fermi surface nesting instability independent of the magnetic ordering of the relevant parent compound. The resonance energy (E(r)) scales with the superconducting transition temperature (T(C)) as E(r) ∼ 4.9k(B)T(C), which is consistent with the canonical value of ∼ 5k(B)T(C) observed for the cuprates. Moreover, the relationship between the resonance energy and the superconducting gap, Δ, is similar to that observed for many unconventional superconductors (E(r)/2Δ ∼ 0.64).

Effects of Co doping on the transport properties and superconductivity in CeFe(1-x)Co(x)AsO

A series of CeFe(1-x)Co(x)AsO oxyarsenide compounds with Co doping on iron sites (x = 0-0.2) have been synthesized by a solid state reaction method. The effects of Co doping on the electrical transport properties and superconductivity were analyzed with a special emphasis on the analysis of thermopower. Undoped CeFeAsO shows an electrical resistivity anomaly at about 150 K, which was ascribed to a spin-density-wave (SDW) instability. This anomaly is suppressed and a superconducting transition occurs at T(c) = 3.2 K in CeFe(0.95)Co(0.05)AsO, the maximum superconducting transition temperature (T(c)) of 12.5 K is observed in CeFe(0.90)Co(0.10)AsO, and the thermopower is increased by the Co doping. As has been previously suggested, the emergence of superconductivity seems to be closely linked to the thermopower, and there is a close correlation between T(c) and the thermopower, both showing a similar dome-like doping dependence.