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After a long ISIS shutdown on target station 1 during 2016, it was found that the neutron flux on the MAPS spectrometer was substantially compromised, with an energy-dependent reduction of between two and five measured. To determine and remedy the cause of this problem, extensive investigations were performed, both on the instrument and involving neutronic and ray-tracing simulations. The key measurement that provided the most insight into the nature of the problem was to use a pinhole camera setup to obtain a spatially-resolved image of the neutron source as viewed from the sample. This revealed a highly inhomogeneous distribution of flux from the source, in contrast to a similar measurement performed on the adjacent VESUVIO beamline which views the same moderator face but showed no such inhomogeneity. The problem was eventually solved by replacing the MAPS beamline shutter, an expensive and difficult task.

©2015 American Physical Society. An inelastic neutron scattering study of the spin waves corresponding to the stripe antiferromagnetic order in insulating Rb0.8Fe1.5S2 throughout the Brillouin zone is reported. The spin wave spectra are well described by a Heisenberg Hamiltonian with anisotropic in-plane exchange interactions. Integrating the ordered moment and the spin fluctuations results in a total moment squared of 27.6±4.2μB2/Fe, consistent with S≈2. Unlike XFe2As2 (X=Ca, Sr, and Ba), where the itinerant electrons have a significant contribution, our data suggest that this stripe antiferromagnetically ordered phase in Rb0.8Fe1.5S2 is a Mott-like insulator with fully localized 3d electrons and a high-spin ground state configuration. Nevertheless, the anisotropic exchange couplings appear to be universal in the stripe phase of Fe pnictides and chalcogenides.

We have performed inelastic neutron-scattering measurements on a powder sample of the superconductor lithium iron selenide hydroxide Li1−xFexODFe1−ySe (x ≈ 0.16, y ≈ 0.02, Tc = 41 K). The spectrum shows an enhanced intensity below Tc over an energy range of 0.64 × 2Δ < E < 2Δ, where Δ is the superconducting gap, and has maxima at the wave vectors Q1 ≈ 1.46 and Q2 ≈ 1.97 °A−1. The behavior of this feature is consistent with the spin-resonance mode found in other unconventional superconductors, and in particular strongly resembles the spin resonance observed in the spectrum of another molecular-intercalated iron selenide Li0.6(ND2)0.2(ND3)0.8Fe2Se2. The signal can be described with a characteristic two-dimensional wave-vector (π,0.67π) in the Brillouin zone of the iron square lattice, consistent with the nesting vector between electron Fermi sheets.

We report neutron inelastic scattering measurements on polycrystalline LaFePO and Sr2ScO3FeP, two members of the iron phosphide families of superconductors. No evidence is found for any magnetic fluctuations in the spectrum of either material in the energy and wavevector ranges probed. Special attention is paid to the wavevector at which spin-density-wave-like fluctuations are seen in other iron-based superconductors. We estimate that the magnetic signal, if present, is at least a factor of four (Sr2ScO3FeP) or seven (LaFePO) smaller than in the related iron arsenide and chalcogenide superconductors. These results suggest that magnetic fluctuations are not as influential on the electronic properties of the iron phosphide systems as they are in other iron-based superconductors.

We report neutron scattering measurements of cooperative spin excitations in antiferromagnetically ordered BaFe2As2, the parent phase of an iron pnictide superconductor. The data extend up to ~100meV and show that the spin excitation spectrum is sharp and highly dispersive. By fitting the spectrum to a linear spin-wave model we estimate the magnon bandwidth to be in the region of 0.17eV. The large characteristic spin fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state., 4 pages, 4 figures; v2: minor changes to the text and abstract and more general expressions for the linear spin-wave cross section included

A technique for measuring the difference between the splay and bend flexoelectric coefficients in a nematic liquid crystal is demonstrated. The method uses the flexoelectric-optic effect, but instead of a uniform lying helix structure, a TN cell geometry with an in-plane electric field is used. This has the advantage of avoiding difficulties associated with aligning the helix and can be used to measure achiral materials. The effects due to ionic screening are also taken into account.

Author(s): Wang, M; Valdivia, P; Yi, M; Chen, JX; Zhang, WL; Ewings, RA; Perring, TG; Zhao, Y; Harriger, LW; Lynn, JW; Bourret-Courchesne, E; Dai, P; Lee, DH; Yao, DX; Birgeneau, RJ