No abstract

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.