Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron scattering experiments. There is no indication for static magnetic ordering but inelastic correlations appear at the incommensurate wave vector (0.5 ± δ, 0.5 ∓ δ, 0) with δ ∼0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.PACS numbers: 74.25. Ha,74.25.Jb,78.70.Nx,75.10.Lp Superconductivity in the FeAs-based materials [1] appears to be closely related to magnetism as the superconducting state emerges out of an antiferromagnetic phase by doping [1][2][3][4] or by application of pressure [5]. The only FeAs-based exception to this behavior has been found in LiFeAs, which is an ambient-pressure superconductor with a high T C of ∼17 K without any doping [6][7][8]. LiFeAs exhibits the same FeAs layers as the other materials but FeAs 4 tetrahedrons are quite distorted [8] suggesting a different occupation of orbital bands. Indeed ARPES studies on LiFeAs find an electronic band structure different from that in LaOFeAs or BaFe 2 As 2 type compounds [9]. The Fermi-surface nesting, which is proposed to drive the spin-density wave (SDW) order in the other FeAs parent compounds, is absent in LiFeAs [9] suggesting that this magnetic instability is less relevant. The main cause for the suppression of the nesting consists in the hole pocket around the zone center which is shallow in LiFeAs [10]. In consequence, there is more density of states near the Fermi level which might favor a ferromagnetic instability. Using a three-band model Brydon et al.[10] find this ferromagnetic instability to dominate and discuss the implication for the superconducting order parameter proposing LiFeAs to be a spin-triplet superconductor with odd symmetry. However, other theoretical analyzes of the electronic band-structure still find an antiferromagnetic instability which more closely resembles those observed in the other FeAs-based materials [11].Inelastic neutron scattering (INS) experiments revealed magnetic order and magnetic excitations in many FeAs-based families [2,[12][13][14]. Strong magnetic correlations persist far beyond the ordered state, and, most importantly, the opening of the superconducting gap results in a pronounced redistribution of spectral weight [13][14][15], which is frequently interpreted in terms of a resonance mode. Recently a powder INS experiment on superconducting LiFeAs reported magnetic excitations to be rather similar to those observed in the previously studied materials [16] but with a spin gap even in the normal-conducting phase. Magnetic excitations observed in a recent single-crystal INS study on nonsuperconducting Li deficient Li 1−x FeAs (x∼0.06) were described by spin-waves associated with commensurate antiferromagnetism, again with a large temperature independent spin gap of 13 meV [17]. We have performed INS experiments on superconducting sing...

High-resolution and high-flux neutron as well as x-ray powder-diffraction experiments were performed on the oxypnictide series LaO 1−x F x FeAs with 0 Յ x Յ 0.15 in order to study the crystal and magnetic structure. The magnetic symmetry of the undoped compound corresponds to those reported for REOFeAs ͑with RE a rare earth͒ and for AFe 2 As 2 ͑A =Ba,Sr͒ materials. We find an ordered magnetic moment of 0.63͑1͒ B at 2 K in LaOFeAs, which is significantly larger than the values previously reported for this compound. A sizable ordered magnetic moment is observed up to a F doping of 4.5% whereas there is no magnetic order for a sample with a F concentration of x = 0.06. In the undoped sample, several interatomic distances and FeAs 4 tetrahedra angles exhibit pronounced anomalies connected with the broad structural transition and with the onset of magnetism supporting the idea of strong magnetoelastic coupling in this material.

The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism, which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors.

An hour-glass-shaped magnetic excitation spectrum appears to be a universal characteristic of the high-temperature superconducting cuprates. Fluctuating charge stripes or alternative band structure approaches are able to explain the origin of these spectra. Recently, an hourglass spectrum has been observed in an insulating cobaltate, thus favouring the charge stripe scenario. Here we show that neither charge stripes nor band structure effects are responsible for the hour-glass dispersion in a cobaltate within the checkerboard charge-ordered regime of La 2 À x Sr x CoO 4 . The search for charge stripe ordering reflections yields no evidence for charge stripes in La 1.6 Sr 0.4 CoO 4 , which is supported by our phonon studies. With the observation of an hour-glass-shaped excitation spectrum in this stripeless insulating cobaltate, we provide experimental evidence that the hour-glass spectrum is neither necessarily connected to charge stripes nor to band structure effects, but instead, probably intimately coupled to frustration and arising chiral or non-collinear magnetic correlations.

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