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 ∼10keV 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 (∼10keV), demonstrate the very high-correlation length of the stripes (∼2000Å) 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 130keV 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 (∼300Å), 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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Electrical and Electronic Engineering
- Charge ordering
- Charge stripes
- X-ray scattering