Andrew Boothroyd

Abstract:

The influence of electrostatic interactions on the ordering of sodium ions in NaxCoO2 is studied theoretically through Monte-Carlo simulations. For large x small di- or tri-vacancy clusters are stable with respect to isolated Na vacancies. At commensurate fillings these small clusters order in triangular superstructures. These results agree with recent electron diffraction data at x = 1/2 and 3/4. We have performed neutron Laue diffraction experiments at higher x, which confirm the predictions of this simple model. The consequences on the properties of the electronic charges in the Co layers are discussed.

Abstract:

We report polarized- and unpolarized-neutron inelastic-scattering measurements of the magnetic excitation spectrum in the spin-charge ordered phase of La3/2 Sr1/2 Ni O4. Up to energies of ∼30 meV we observe broad magnetic modes characteristic of a near checkerboard ordering. A linear spin-wave model for an ideal checkerboard ordering with a single antiferromagnetic exchange interaction J′ =5.8±0.5 meV between next-nearest-neighbor spins on Ni2+ sites, together with a small XY -like single-ion anisotropy, provides a reasonable description of the measured dispersion. Above 30 meV the excitations are not fully consistent with the linear spin-wave model, with modes near the two-dimensional reciprocal space wave vector (0.5, 0.5) having an anomalously large intensity. Furthermore, two additional dispersive modes not predicted by spin-wave theory were observed, both of which are probably magnetic. One disperses away from (0.5, 0.5) in the energy range between 50-56 meV, and the other appears around (h,k) type positions (h,k=integer) in the energy range 31-39 meV. We propose a model in which these anomalous features are explained by the existence of discommensurations in the checkerboard ordering. At low energies there is additional diffuse scattering centred on the magnetic ordering wave vector. We associate this diffuse scattering with dynamic antiferromagnetic correlations between spins attached to the doped holes. © 2005 The American Physical Society.

Abstract:

Soft X-ray resonant diffraction is a new technique and there exist several examples of large resonant enhancements of charge and magnetic scattering that can be obtained at the Ledges of 3d transition metal oxides. Here we have employed resonant soft X-ray scattering at the manganese L edges which provide a direct measurement of the orbital ordering. We have studied the layered manganite La0 5Sr1 5MnO4 that displays charge. spin, and orbital ordering. Energy scans at constant wavevector show that there are two separate contributions to the observed scattering, direct Goodenough orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions. Finally we will show how that the charge, spin and orbital degrees of freedom are strongly correlated in these materials.