Andrew Boothroyd

Abstract:

We present high-resolution high-energy and resonant x-ray-diffraction results from La2-2xSr1+2xMn2O7 for x=0.55, 0.575, and 0.60. These compounds show superlattice reflections at wave vectors of (h±δ,k±δ, l) and (h±2δ,k±2δ,l), arising from orbital ordering with associated Jahn-Teller distortions and charge ordering, respectively. We observe a phase transition boundary between the x=0.55 and x=0.575 doping levels. Samples with x=0.55 display structural characteristics similar to those previously reported for x=0.5. Compared to this the long-range order in samples with x=0.55 and x=0.6 have a distinct change in wave-vector and correlation length. We attribute this to a new orbital bi-stripe phase, accompanied by weak, frustrated, charge ordering. The observed azimuthal dependence of the orbital order reflection supports the model proposed for this new phase. © 2005 The American Physical Society.

Abstract:

We present high resolution X-ray measurements characterising the charge stripe order state in the La2-xSrxNiO4 system with x = 0.20, 0.225 and 0.25. We find that in the x = 0.20, 0.225 and 0.25 systems the charge stripe order exists in a charge stripe glass characterised by weak, poorly correlated incommensurate charge stripes in contrast to the strong well correlated charge stripes in the commensurate x = 1/3 system. No stabilisation of the charge order was observed at the next possible commensurate value of ε= 0.25. A comparison with high energy X-ray measurements suggested that the charge order may exist in a charge stripe glass in the bulk in the doping region x = 0.20 - 0.33. Finally at low temperature there was an initial increase in the intensity and correlation not observed with neutron measurements and it appears to be an effect that X-rays are sensitive to but neutrons are not. © EDP Sciences, Società Italiana di Fisica/Springer-Verlag 2005.

Abstract:

The infrared conductivity σ(ω) of NaxCoO2 is studied as a function of doping and temperature for 0.5≤x≤1. A far-infrared peak (FIP) in σ(ω), which coexists with a small Drude contribution, indicates charge localization in the CoO2 layers. Long-range ordering at x=0.5 is confirmed to create a far-infrared gap, in addition to the FIP. At low T and high incommensurate x values, in correspondence with the reported formation of a spin-density wave, the FIP abruptly shifts to higher energy, indicating a deepening of the localizing potential. An analysis of the in-plane E1u phonon lifetime shows that Na+ ions lattice is "frozen in" at any T<295K for commensurate x and at T150K for incommensurate x. A comparison with the behavior of the FIP suggests that the Na+ "freezing" induces carrier localization only for low charge density and high Na+ concentration. © 2005 The American Physical Society.

Abstract:

The use of softer-energy X-rays produced by synchrotron radiation for diffraction is an area of current interest. In this paper, experiments exploiting resonant scattering at the L absorption edges of 3d transition metal elements are reported. Such energies, typically 500 - 1000 eV, are at the extreme limit of soft X-ray diffraction where absorption effects are so severe that the sample and diffractometer must be placed in a windowless high-vacuum vessel. In addition, the Ewald sphere is so small as to likely contain, at most, only a single Bragg reflection. Advantages of using such radiation for the study of weak diffraction effects such as anomalous scattering, charge ordering, magnetic diffraction and orbital ordering are reported.