The proximity of superconductivity and antiferromagnetism in the phase diagram of iron arsenides [1, 2], the apparently weak electron-phonon coupling [3] and the "resonance peak" in the superconducting spin excitation spectrum [4, 5, 6, 7] have fostered the hypothesis of magnetically mediated Cooper pairing. However, since most theories of superconductivity are based on a pairing boson of sufficient spectral weight in the normal state, detailed knowledge of the spin excitation spectrum above the superconducting transition temperature T c is required to assess the viability of this hypothesis [8,9]. Using inelastic neutron scattering we have studied the spin excitations in optimally doped BaFe 1.85 Co 0.15 As 2 (T c = 25 K) over a wide range of temperatures and energies. We present the results in absolute units and find that the normal state spectrum carries a weight comparable to underdoped cuprates [10,11]. In contrast to cuprates, however, the spectrum agrees well with predictions of the theory of nearly antiferromagnetic metals [12], without complications arising from a pseudogap [13, 14, 15] or competing incommensurate spin-modulated phases [16]. We also show that the temperature evolution of the resonance energy follows the superconducting energy gap ∆, as expected from conventional Fermi-liquid approaches [17, 18]. Our observations point to a surprisingly simple theoretical description of the spin dynamics in the iron arsenides and provide a solid foundation for models of magnetically mediated superconductivity.

We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe 1.85 Co 0.15 As 2 and BaFe 1.91 Ni 0.09 As 2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the threedimensional (3D) 4 2 /m screw symmetry around the ( 1 2 1 2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal-and SC-state spin dynamics and in the out-ofplane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe-sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe-sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken ("nematic") ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode ω res , as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship ħ hω res ≈ 4.3 k B T c holds for all the studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.