Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity. The high transition temperatures (high-T(c)) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials. The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at T(c). The most obvious feature in the magnetic excitations of high-T(c) superconductors such as YBa2Cu3O6+x is the so-called 'resonance'. Although the resonance may be strongly coupled to the superconductivity, it is unlikely to be the main cause, because it has not been found in the La2-x(Ba,Sr)(x)CuO4 family and is not universally present in Bi2Sr2CaCu2O8+delta (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa2Cu3O6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at T(c), and have spectral weight far exceeding that of the 'resonance'. The discovery of similar excitations in La2-xBa(x)CuO4 (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.

Polarized and unpolarized neutron triple-axis spectrometry was used to study the dynamical magnetic susceptibility χ ′′ (q, ω) as a function of energy (hω) and wave vector (q) in a wide temperature range for the bilayer superconductor YBa2Cu3O6+x with oxygen concentrations, x, of 0. 45, 0.5, 0.6, 0.7, 0.8, 0.93, and 0.95. The most prominent features in the magnetic spectra include a spin gap in the superconducting state, a pseudogap in the normal state, the much-discussed resonance, and incommensurate spin fluctuations below the resonance. We establish the doping dependence of the spin gap in the superconducting state, the resonance energy, and the incommensurability of the spin fluctuations. The magnitude of the spin gap (Esg) up to the optimal doping is proportional to the superconducting transition temperature Tc with Esg/kBTc = 3.8. The resonance, which exists exclusively below Tc for highly doped YBa2Cu3O6+x with x = 0.93 and 0.95, appears above Tc for underdoped compounds with x ≤ 0.8. The resonance energy (Er) also scales with kBTc, but saturates at Er ≈ 40 meV for x close to 0.93. The incommensurate spin fluctuations at energies below the resonance have structures similar to that of the single-layer superconducting La2−xSrxCuO4. However, there are also important differences. While the incommensurability (δ) of the spin fluctuations in La2−xSrxCuO4 is proportional to Tc for the entire hole-doping range up to the optimal value, the incommensurability in YBa2Cu3O6+x increases with Tc for low oxygen doping and saturates to δ = 0.1 for x ≥ 0.6. In addition, the incommensurability decreases with increasing energy close to the resonance. Finally, the incommensurate spin fluctuations appear above Tc in underdoped compounds with x ≤ 0.6 but for highly doped materials they are only observed below Tc. We discuss in detail the procedure used for separating the magnetic scattering from phonon and other spurious effects. In the comparison of our experimental results with various microscopic theoretical models, particular emphasis was made to address the similarities and differences in the spin fluctuations of the two most studied superconductors. Finally, we briefly mention recent magnetic field dependent studies of the spin fluctuations and discuss their relevance in understanding the microscopic origin of the resonance.

Inelastic neutron scattering was used to study the wave vector- and frequency-dependent magnetic fluctuations in single crystals of superconducting YBa2Cu3O6+x. The spectra contain several important features, including a gap in the superconducting state, a pseudogap in the normal state, and the much-discussed resonance peak. The appearance of the pseudogap determined from transport and nuclear resonance coincides with formation of the resonance in the magnetic excitations. The exchange energy associated with the resonance has the temperature and doping dependences as well as the magnitude to describe approximately the electronic specific heat near the superconducting transition temperature (Tc).

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wavevectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear resonance peak is observed at this wavevector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) kBTC . This is in good agreement with the canonical value of 5 kBTC observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c-axis, indicating that the magnetic fluctuations are two-dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.PACS numbers: 78.70.Nx, 74.20.Mn Understanding the physics of superconductivity in high-T c cuprates and other unconventional superconductors remains a central unresolved problem at the forefront of condensed matter physics. One widespread school of thought maintains that magnetic fluctuations are intimately involved in the pairing mechanism. This view is supported by a growing number of neutron scattering investigations showing the appearance of a magnetic excitation coincident with the onset of superconductivity [1,2,3,4,5,6,7,8]. The spectrum shows a resonance at a wavevector related to the antiferromagnetic order in the non-superconducting parent compounds. The apparent resonance energy scales with T C for different cuprate materials exhibiting a wide range of superconducting transition temperatures [9], providing tantalizing evidence for a common mechanism related to magnetic fluctuations.The discovery of a new family of Fe-based high temperature superconductors with T C as high as 55 K [10,11,12,13,14,15,16] presents an exciting opportunity to examine the relationship of spin excitations to the superconducting condensate in unconventional superconductors. The new materials are composed of Fe containing planes (FeAs or FeSe). Both theory and experiment indicate that simple electron-phonon coupling cannot describe superconductivity in these materials [17,18]. Furthermore, the superconducting state exists in close proximity to magnetism as the parent compounds exhibit spin-density wave order [19,20]. These observations have been put forth as evidence that the superconductivity in the Fe-based materials is unconventional. The presence of the Fe planes suggests quasi-two-dimensionality, as observed in the cuprates. However, neutron scattering investigations of the spin waves in the undoped parent compounds SrFe 2 As 2 [21], BaFe 2 As 2 [22], and CaFe 2 As 2 [23], indicate anisotropic exchange that cannot be classified as two dimensional. Band structure calculations [24,25] indicate that doping should enhance the twodimensionality of the Fermi surface, favoring superconductivity [25]. Directly probing the magnetic fluctuations in superconducting Fe-based systems is crucial for further progress.Recent measurements on a polycrystalline sample of Ba 0.6 K 0.4 Fe 2 As 2 found a spin excitation that appears at the onset...