Andrew Boothroyd

Abstract:

The sodium reordering in Nax CoO2 in the vicinity of room temperature is rationalized at high x in terms of phase transitions between square and striped phases. A striking hexagon-of-hexagons diffraction pattern observed for x=0.78 can be reproduced using coexisting square and striped phases that are related by simple shear deformations. All compositions exhibit a partial melting transition to a disordered stripe phase just below room temperature, which alters the topology of the electrical conduction pathways. © 2009 The American Physical Society.

Abstract:

We have investigated orbital ordering in the bilayer manganite, Pr (Sr0.1 Ca0.9) 2 Mn2 O7, using synchrotron x-ray diffraction techniques, in particular resonant soft x-ray diffraction. Diffraction signals are observed at (0, 1 2, 0) above 300 K and (1 2, 0,0) below 300 K, corresponding to orbital stripes along the a and b axes, respectively. The previously observed transition at 300 K is evidence of a rotation of the orientation of the 3d orbital stripes. Large resonances of the orbital signals are observed at the MnL edges, with a complex dependence on incident photon energy indicative of a weakly Jahn-Teller distorted system. A structural transition observed at 92 K is found not to involve a further change in the orbital order. © 2009 The American Physical Society.

Abstract:

We present resonant x-ray scattering experimental data from YVO3. By scattering at the vanadium K edge we are able to observe diffraction from the anisotropic tensor of susceptibility at the Bragg forbidden (010). The resonant energy spectra from these reflections are unusually complex, giving an indication of the crystal field distortions around the vanadium site.