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.