Simon Cassidy

Abstract:

The development of a technique for following in situ the reactions of solids with alkali metal/ammonia solutions, using time-resolved X-ray diffraction methods, reveals high-temperature superconducting ammonia-rich intercalates of iron selenide which reversibly absorb and desorb ammonia around ambient temperatures.

Abstract:

The effect of the isovalent substitution of Sr2+ by Ca 2+ on the structure and superconducting properties of Sr 2-xCaxVO3FeAs is described in the compositional range 0 ≤ x ≤ 0.5. SQUID magnetometry measurements reveal that after an initial increase in Tc, which is maximised at 29.5 K in Sr 1.95Ca0.05VO3FeAs, a rapid suppression of superconductivity is observed with increasing x. XANES spectra of Sr 2-xCaxVO3FeAs collected on the Fe and V absorption K-edges show that the position of both edges are invariant with composition within the experimental uncertainty. A combination of synchrotron X-ray powder diffraction and neutron powder diffraction techniques is used to rationalise the observed changes in Tc with x, in terms of changes to the structure of the FeAs layer upon partial Ca substitution. These findings demonstrate that superconductivity in the Fe-based superconductors is extremely sensitive to the crystal structure with Tc maximised in samples with regular FeAs4-tetrahedra. © 2014 Elsevier Inc.

Abstract:

The effect of the isovalent substitution of Sr by Ca on the structure and superconducting properties of Sr Ca VOFeAs is described in the compositional range 0≤x≤0.5. SQUID magnetometry measurements reveal that after an initial increase in T, which is maximised at 29.5 K in SrCaVOFeAs, a rapid suppression of superconductivity is observed with increasing x. XANES spectra of Sr Ca VOFeAs collected on the Fe and V absorption K-edges show that the position of both edges are invariant with composition within the experimental uncertainty. A combination of synchrotron X-ray powder diffraction and neutron powder diffraction techniques is used to rationalise the observed changes in T with x, in terms of changes to the structure of the FeAs layer upon partial Ca substitution. These findings demonstrate that superconductivity in the Fe-based superconductors is extremely sensitive to the crystal structure with T maximised in samples with regular FeAs-tetrahedra. © 2014 Elsevier Inc. All rights reserved.

Abstract:

Magnetic fluctuations in the molecular-intercalated FeSe superconductor Lix(ND2)y(ND3)1-yFe2Se2 (Tc=43K) have been measured by inelastic neutron scattering from a powder sample. The strongest magnetic scattering is observed at a wave vector Q≈1.4 Å-1, which is not consistent with the (π,0) nesting wave vector that characterizes magnetic fluctuations in several other iron-based superconductors but is close to the (π,π/2) position found for AxFe2-ySe2 systems. At the energies probed (∼5kBTc), the magnetic scattering increases in intensity with decreasing temperature below Tc, consistent with the superconductivity-induced magnetic resonance found in other iron-based superconductors. © 2013 American Physical Society.