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1 We argue that linear T dependence of the spin susceptibility χ(T) observed in Fe pnictides can be explained within the itinerant Fermi liquid model of hole and electron bands. The spin susceptibility is linear in T in a generic Fermi liquid in 2D. We show that for pnictides, the prefactor for the T term comes chiefly from intra-band scattering and is strongly enhanced compared to an ordinary Fermi liquid as it contains precisely the same interaction that gives rise to spin-density-wave ordering. We compare theoretical slope with the data. 2 The recently synthesized, high-Tc superconducting ferropnictides may be the most enigmatic superconductors discovered so far. One of the most interesting question is the relation between magnetic excitations and superconductivity in these quasi-2D compounds. We analyze the spin response in the normal and superconducting states and show that while the normal state spin excitations are dominated by the continuum of the interorbital antiferromagnetic SDW fluctuations, the unconventional superconductivity yields different feedback: the resonance peak in form of the well-defined spin exciton at the AFM momentum occurs only for the extended s-wave (spm) superconducting order parameter. This observation was later confirmed in neutron experiments. Furthermore, we show that (i) the lack of a coherence peak in NMR is fully consistent with the extended s-wave gap (whether in the clean or dirty limit) and (ii) the low temperature power law behavior of NMR 1/T1 can be also explained in the framework of the same model, but requires going beyond the Born model. Another confirmation of the spm symmetry may come from Raman experiments. We predict that A1g Raman intensity has a true resonance peak below 2Δ for extended s-wave superconducting gap contrary to the d-wave. |
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