Andrew Boothroyd

Abstract:

Lithium iron arsenide phases with compositions close to LiFeAs exhibit superconductivity at temperatures at least as high as 16 K, demonstrating that superconducting [FeAs](-) anionic layers with the anti-PbO structure type occur in at least three different structure types and with a wide range of As-Fe-As bond angles.

Abstract:

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 ∼100 meV 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 0.17 eV. The large characteristic spin-fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state. © 2008 The American Physical Society.

Abstract:

The magnetism of charge-stripe-ordered La1.55Sr 0.45NiO4 was studied by a combination of neutron diffraction, muon-spin relaxation (μSR), and bulk susceptibility. Long-range magnetic ordering was observed at a lower temperature by μSR than by neutron diffraction, consistent with a glassy transition to the ordered phase. A second magnetic transition is detected by all techniques and is consistent with a spin reorientation. On cooling below TSR=42 K the spins reorientate from lying 32.9±0.6° away from the stripe direction at 70 K to 56.8±0.4° at 10 K. The magnetic order was observed by neutron diffraction to have both three-dimensional and two-dimensional (without any correlation along the c axis) characters. μSR measurements confirmed this and are consistent with a single magnetically ordered spin-stripe phase. The effects of checkerboard charge order on the ordered phase and the characteristics of the phase diagram of the spin reorientation in charge-ordered La2-xSrxNiO4 are commented on. © 2008 The American Physical Society.

Abstract:

Low-temperature transmission electron microscopy and x-ray diffraction (XRD) studies have been carried out on pellet and powder samples of Pr0.5 Ca0.5 MnO3. These studies have revealed appearance of a different type of charge-orbital ordered (COO) phase, resulting due to flipping of eg orbitals from d3 x2 - r2 / d3 y2 - r2 to d3 x2 - r2 / d3 z2 - r2 configuration. This orbital flip results in a changeover of the COO superlattice-ordering vector from (1/2,0,0) to (1/4,1/2,1/4) in the Pnma phase. This COO phase coexists with the conventional COO phase. Low-temperature XRD studies show that the COO phase appears only in pellet sample and not in the corresponding powder sample. The powder sample shows only conventional COO phase. Volume fractions of conventional and the other type COO phases in pellet sample of Pr0.5 Ca0.5 MnO3 is estimated to be ∼55% and 45%, respectively. The occurrence of orbital flip has been attributed to local strain building up in the pellet sample. The strain builds up during cooling because manganite has anisotropic thermal expansion coefficients. © 2008 The American Physical Society.