Andrew Boothroyd

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.

Abstract:

The dispersion of the low-energy magnetic excitations of the Pr sublattice in PrBa(2)Cu(3)O(6.2) is determined by inelastic neutron scattering measurements on a single crystal. The dispersion, which shows the effect of interactions with the Cu spin waves, is well described by a model of the coupled Cu-Pr magnetic system. This model enables values for the principal exchange constants to be determined. The results suggest that both Pr-Pr and Cu-Pr interactions are important in producing the anomalously high ordering temperature of the Pr sublattice. Measurements of the Cu optic spin wave mode show that the interlayer Cu-Cu exchange is significantly lower than in YBa(2)Cu(3)O(6.2).