A suite-level review of the neutron powder diffraction instruments at Oak Ridge National Laboratory.

The suite of neutron powder diffractometers at Oak Ridge National Laboratory (ORNL) utilizes the distinct characteristics of the Spallation Neutron Source and High Flux Isotope Reactor to enable the measurements of powder samples over an unparalleled regime at a single laboratory. Full refinements over large Q ranges, total scattering methods, fast measurements under changing conditions, and a wide array of sample environments are available. This article provides a brief overview of each powder instrument at ORNL and details the complementarity across the suite. Future directions for the powder suite, including upgrades and new instruments, are also discussed.

Structure and paramagnetism in weakly correlated Y8Co5.

We report the basic physical properties of monoclinic Y8Co5 determined by means of magnetic susceptibility, electrical resistivity, and specific heat measurements. The crystal structure of Y8Co5 is monoclinic (P2(1)/c) with lattice parameters a = 7.0582(6) Å, b = 7.2894(6) Å, c = 24.2234(19) Å, and ß = 102.112(6)° as refined by using synchrotron powder x-ray diffraction data. The compound shows temperature independent paramagnetism with χ0 = 2.1 × 10(-3) emu mol(-1) and Sommerfeld parameter γ = 63 mJ mol(-1) K(-2). The calculated Wilson ratio for Y8Co5, R(W) = 1.4, is close to that expected for a free electron gas R(W) = 1. Low temperature resistivity under high pressure does not reveal superconductivity in this compound down to 1.2 K, up to hydrostatic pressures of 5.56 GPa. Band structure calculations (full-potential linearized augmented plane wave, FP-LAPW) derive the Stoner exchange interaction parameter S = 0.24, excluding magnetic behavior for Y8Co5.

From (pi,0) magnetic order to superconductivity with (pi,pi) magnetic resonance in Fe(1.02)Te(1-x)Se(x).

The iron chalcogenide Fe(1+y)(Te(1-x)Se(x)) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe(1+y)Te exhibits antiferromagnetic order with an in-plane magnetic wave vector (pi,0) (ref. 6). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (pi,pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (pi,pi) (refs 9, 10, 11). A central question in this burgeoning field is therefore how (pi,pi) superconductivity can emerge from a (pi,0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi,0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs as magnetic correlations at (pi,0) are suppressed and the mode at (pi, pi) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.