Andrew Boothroyd

Abstract:

We report the growth of single crystals of La1-xSrxMnO3 (x = 0.0-0.3) by the floating-zone technique in an image furnace under high argon pressure (6-8 atm). Ail excess of Mn in the polycrystalline starting material was used to compensate for the evaporation during growth. single crystals of diameter 8 mm and length 70 mm have been obtained. The phase purity, composition and quality of the crystals were analysed by X-ray diffraction and electron probe microanalysis. We discuss the effect on the crystal growth of varying the amount of Mn excess and of the composition of the growth atmosphere. We also present magnetization and resistivity data for a few of the samples. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

We report on an investigation into the growth of large, high-quality single crystals of La2-xSrxNiO4+ (x = 0-0.5) by the floating zone technique. The results of attempts to grow crystals in different atmospheres are discussed. The oxygen non-stoichiometry (delta) of the as-grown crystals is found to decrease with the Sr substitution level. Magnetic and electrical transport measurements on the crystals are reported. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

The La2-xSrxNiO4 system is isostructural with the high TC superconducting cuprate La2-xSrxCuO4 and is a prototypical system for the understanding of strongly correlated electron-phonon coupling, and the resultant effects on material properties. X-ray scattering studies have been performed on La5/3Sr1/3NiO4 that demonstrate the two-dimensional nature of these charge stripes. Such studies, demonstrate the very high correlation length of the stripes (~ 2000 Å) at low temperatures. We have undertaken a series of experiments measuring the wavevector and charge stripe correlation length on a variety of crystals with the compositions La2-xSrxNiO4 (x=0.20, 0.25, 0.275, 0.30 and 0.33) using ~10 keV X-rays. The results demonstrate that for x=0.275, and above, the charge stripes are highly correlated in a well-ordered crystalline lattice. Measurements of the incommensurability, ε, as a function of temperature for the series revealed that it is commensurate and temperature independent for the x=0.33 sample. For other compositions it is incommensurate and also temperature dependent. However for the x=0.20 and 0.25 crystals a much reduced correlation length was observed suggestive of a charge stripe glass. However, such experiments are sensitive to such charge ordering only in the near (top few micron) surface region. High energy X-rays however can probe the charge stripe ordering within the bulk of the single crystal by utilising the dramatic increase in penetration depth. We have used 130 keV X-rays and demonstrate that in La5/3Sr1/3NiO4 the charge stripes are far less correlated in the bulk than in the near surface region. This reduced correlation length (~300 Å), consistent with neutron scattering measurements, is indicative of a charge stripe glass, reminiscent of that observed below x=0.25, in the near surface region.

Abstract:

The La2−xSrxNiO4 system is isostructural with the high TC superconducting cuprate La2−xSrxCuO4 and is a prototypical system for the understanding of strongly correlated electron–phonon coupling, and the resultant effects on material properties. At low temperatures La2−xSrxNiO4 undergoes a transition into a charge ordered regime whereby the dopant holes migrate to form hole rich regions, or stripes, behaving as anti-phase domain boundaries surrounded by hole deficient antiferromagnetic regions. X-ray scattering studies have been performed on La5/3Sr1/3NiO4 that demonstrate the two-dimensional nature of these charge stripes. Critical exponents governing the temperature variation of the intensity below TC, and the inverse correlation length above TC, have been measured that demonstrate this reduced dimensionality. We have undertaken a series of experiments measuring the wave vector and charge stripe correlation length on a variety of crystals with the compositions La2−xSrxNiO4 (x=0.20, 0.25, 0.275, 0.30 and 0.33) using not, vert, similar10 keV X-rays. The results demonstrate that for x=0.275, and above, the charge stripes are highly correlated in a well-ordered crystalline lattice. However, for the x=0.20 and 0.25 crystals, a much reduced correlation length was observed suggesting a charge stripe glass. Such studies, performed with traditional X-ray energies (not, vert, similar10 keV), demonstrate the very high-correlation length of the stripes (not, vert, similar2000 Å) at low temperatures. However, such experiments are sensitive to such charge ordering only in the near (top few μm) surface region. High energy X-rays, however, can probe the charge stripe ordering within the bulk of the single crystal by utilising the dramatic increase in penetration depth. We have used 130 keV X-rays and demonstrate that in La5/3Sr1/3NiO4 the charge stripes are far less correlated in the bulk than in the near surface region. This reduced correlation length (not, vert, similar300 Å), consistent with neutron scattering measurements, is indicative of a charge stripe glass, reminiscent of that observed below x=0.25 in the near surface region.

Abstract:

Lovesey and Staub have argued [S.W. Lovesey and U. Staub, Phys. Rev. B 61, 9130 (2000)] that experimental data on the temperature dependence of the paramagnetic relaxation of lanthanide ions doped into YBa2Cu3O6+x are in agreement with the predictions of a model that describes the relaxation as due to the scattering of phonons via a magnetoelastic interaction. By generalizing their model I show that the level of agreement is strongly dependent on the number of intermediate lanthanide energy levels included in the calculation, and that inclusion of a more complete set of levels leads to very different results that do not necessarily support the phonon damping picture.