X-ray scattering studies of charge stripes in La2-xSrxNiO4 (x=0.20-0.33)

The La2-xSrxNiO4 system is isostructural with the high T-C 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 (similar to 2000 Angstrom) at low temperatures. We have undertaken a series of experiments measuring the wavevector and charge stripe correlation length an a variety of crystals with the compositions La2-xSrxNiO4 (x = 0.20, 0.25, 0.275, 0.30 and 0.33) using similar to 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, c, 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 (similar to 300 Angstrom), 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.