X-ray scattering studies of charge stripes in La_2-xSr_xNiO₄ (x=0.20-0.33)

The La_2-xSr_xNiO₄ system is isostructural with the high T_C superconducting cuprate La_2-xSr_xCuO₄ and is a prototypical system for the understanding of strongly correlated electron-phonon coupling, and the resultant effects on material properties. At low temperatures La_2-xSr_xNiO₄ 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 La_5/3Sr_1/3NiO₄ that demonstrate the two-dimensional nature of these charge stripes. Critical exponents governing the temperature variation of the intensity below T_C, and the inverse correlation length above T_C, 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 La_2-xSr_xNiO₄ (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 La_5/3Sr_1/3NiO₄ 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.