The anisotropy of the magnetic excitations in BaFe2As2 was studied by polarized inelastic neutron scattering which allows one to separate the components of the magnetic response. Despite the inplane orientation of the static ordered moment we find the in-plane polarized magnons to exhibit a larger gap than the out-of-plane polarized ones indicating very strong single-ion anisotropy within the layers. It costs more energy to rotate a spin within the orthorhombic a-b plane than rotating it perpendicular to the FeAs layers.

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.

Magnetic excitations in Ba(Fe0.94Co0.06)2As2: are studied by polarized inelastic neutron scattering above and below the superconducting transition. In the superconducting state, we find clear evidence for two resonancelike excitations. At a higher energy of about 8 meV, there is an isotropic resonance mode with weak dispersion along the c direction. In addition, we find a lower excitation at 4 meV that appears only in the c-polarized channel and whose intensity strongly varies with the l component of the scattering vector. These resonance excitations behave remarkably similar to the gap modes in the antiferromagnetic phase of the parent compound BaFe2As2.

Using the angle-resolved photoemission spectroscopy (ARPES) data accumulated over the whole Brillouin zone (BZ) in LiFeAs we analyze the itinerant component of the dynamic spin susceptibility in this system in the normal and superconducting state. We identify the origin of the incommensurate magnetic inelastic neutron scattering (INS) intensity as scattering between the electron pockets, centered around the (π, π) point of the BZ and the large two-dimensional hole pocket, centered around the Γ-point of the BZ. As the magnitude of the superconducting gap within the large hole pocket is relatively small and angle dependent, we interpret the INS data in the superconducting state as a renormalization of the particle-hole continuum rather than a true spin exciton. Our comparison indicates that the INS data can be reasonably well described by both the sign changing symmetry of the superconducting gap between electron and hole pockets as well as sign preserving gap, depending on the assumptions made for the fermionic damping.The relation between unconventional superconductivity and magnetism is one of the most interesting topics in condensed-matter physics. For example, in most of the iron-based superconductors superconductivity occurs in close vicinity to an antiferromagnetic (AF) state[1-3]. Moreover, superconductivity emerges when antiferromagnetic order in parent compounds is suppressed, either by electron/hole doping or disorder. In addition, shortrange AF spin excitations are still present in the normal state of the doped systems and also become resonant in the superconducting state at energies below twice the superconducting gap magnitude, 2∆ 0 [4]. This resonant enhancement is believed to be a signature of a certain phase structure of the superconducting gap as the paramagnetic spin response of the Bogolyubov quasiparticles at the antiferromagnetic wave vector Q AF is sensitive to the anomalous coherence factor 1 − ∆ k ∆ k+Q AF |∆ k ||∆ k+Q AF | . Once the superconducting gap at parts of the Fermi surface, connected by Q AF , changes sign, the spin response acquires an additional enhancement at Ω ≤ 2∆ 0 , which is a hallmark of unconventional superconductivity. The observation of the spin resonance in many iron-based superconductors provides strong evidence for the so-called s +− -wave symmetry of the superconducting gap, where the gap structure changes sign between electron and hole pockets [5][6][7]. Note that this does not exclude the gap on each pocket to have a strong angular variation and even accidental nodal lines, allowed by A 1g symmetry [3]. The angular variation of the gap, measured in ARPES [8], is inconsistent with idealized lattice version of s +− , but can be modeled by taking into account interaction effects. While the behavior, described above, is observed in the majority of the iron-based superconductors, there are some notable exceptions. Perhaps the most interesting one is the stoichiometric LiFeAs, which superconducts at T c =17 K without any doping [9][10][11]. In addition, LiFeAs shows ne...