Andrew Boothroyd

Abstract:

The effect of uniaxial pressure on resistivity along the ab plane (ρab) and c axis (ρc) in single crystals of bilayer manganite La1.25 Sr1.75 Mn2 O7 has been investigated. A prominent finding is the asymmetric role of pressure on charge transport phenomenon. With increasing pressure along the c axis, ρab decreases and the metal-insulator (MI) transition temperature shifts towards higher temperature at the rate of 46 K GPa. In contrary to this, ρc increases and the MI transition temperature decreases with increasing pressure along the ab plane. The resistivity ratio ρc ρab is quite large, increases with pressure, and shows a peak at around the MI transition. We believe that the unusual behavior of transport properties is related to the Mn-O-Mn linkage between MnO2 layers and the spin reorientation from the c axis to the basal plane and vice versa with pressure. © 2006 The American Physical Society.

Abstract:

We have studied the magnetization of a series of spin-charge-ordered La2-x Srx Ni O4+δ single crystals with 0≤x≤0.5. For fields applied parallel to the ab plane there is a large irreversibility below a temperature TF1 ∼50 K and a smaller irreversibility that persists up to near the charge-ordering temperature. We observed memory effects in the thermoremnant magnetization across the entire doping range. We found that these materials retain a memory of the temperature at which an external field was removed and that there is a pronounced increase in the thermoremnant magnetization when the system is warmed through a spin reorientation transition. © 2006 The American Physical Society.

Abstract:

The structure of bilayer manganite LaSr2Mn2O7 has been investigated under high pressure using synchrotron and laboratory X-rays in the angle-dispersive X-ray diffraction geometry up to 35 GPa at ambient temperature. The Rietveld refinement results of the powder diffraction data show a crossover between the axial and equatorial Mn–O bond length values and a significant suppression of Jahn–Teller (JT) distortion around 16 GPa. Our results imply that a re-entrant behavior in LaSr2Mn2O7, which shows equality in all the three Mn–O bond length values around 16 GPa similar to ambient conditions, may dramatically alter the orbital ordering. Further, the P-V data for the bilayer compounds indicate that the additional substitution of La3+ for Sr2+ lowers the bulk modulus and enhances lattice softening.

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.