Dr Dharmalingam Prabhakaran

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.

Abstract:

We report resonant x-ray scattering data of the orbital and magnetic ordering at low temperatures at the Mn L2,3 edges in La0.5Sr1.5MnO4. The orderings display complex energy features close to the Mn absorption edges. Systematic modeling with atomic multiplet crystal field calculations was used to extract meaningful information regarding the interplay of spin, orbital, and Jahn-Teller order. These calculations provide a good general agreement with the observed energy dependence of the scattered intensity for a dominant orbital ordering of the dx2-z2 dy2-z2 type. In addition, the origins of various spectral features are identified. The temperature dependence of the orbital and magnetic ordering was measured and suggests a strong interplay between the magnetic and orbital order parameters. © 2005 The American Physical Society.