Strontium-doped La2NiO4 (LSN) is isostructural with the high-Tc cuprates, La2-xSrxCuO4 (LSC) and La2-xBaxCuO4 (LBC). With substitution of 15% Sr, LSC shows a superconducting transition at TSC = 35 K, but LSN remains insulating at doping levels up to 70%. The substitutions of La with Sr provide hole carriers, and the doped holes tend to order in a stripe-like structure acting as domain walls within the NiO2 planes. The interplay between this ordered state and the spins of Ni dominates the transport behaviour at low temperatures. Despite the demonstrations of charge ordering and spin ordering in related colossal magnetoresistance manganites by neutron scattering, X-ray scattering has the merits of superior spatial resolution and direct probing of the charge density modulation.

Using high-resolution synchrotron X-ray diffraction on BM28 (the UK - CRG XMaS beamline), we have demonstrated the existence of quenched disordered charge stripes in a single crystal of La5/3Sr1/3NiO4 at low temperatures. In accord with the neutron scattering studies, satellite reflections were observed at low temperatures at positions of (h ± 2 0, l), ~ 1/3, h = even and l = odd.

Figure 60 shows a plot of the integrated intensity, along each of the reciprocal space directions, of the charge ordering peak (4.66 0 5) versus temperature. Clearly, the data can be fitted to a power law


l(t) ∝ ( Tco-T/ Tco­­­­­­)2ß
 

with 2ß ~ 0.23 ± 0.02, and Tco ~ 239.2 ± 0.2 K, and shows the transition to be second order in nature. The measured exponent of 2ß clearly demonstrates that below Tco the charge stripes in La5/3Sr1/3NiO4 are in the 2-D universality class.

In Figure 61, we display the evolution of the correlation lengths with temperature. Above the transition temperature Tco ~ 240 K, only very weak scattering is observed at the expected positions in reciprocal space. It is caused by critical scattering, owing to dynamic spatial fluctuations into the charge stripe phase. We believe this to be the first observation of charge stripe critical fluctuations existing above Tco. The size of these clusters varies with temperature and shows a marked divergence close to Tco. Fitting the data in Figure 61 to a power law for the inverse correlation length,


(t)-­1 ∝ (T - Tco­­­­­­ / Tco),
 

allows the extraction of for the exponent of the correlation length of charge ordering. The fitting gave the value of = 1.08 ± 0.2, in excellent agreement with the predicted value of = 1 expected for a 2-dimensional system.

In conclusion, we have shown that 2-dimensional disordered charge stripes are formed below Tco in a single crystal of La5/3Sr1/3NiO4. The correlation length of these charge stripes does not increase as the temperature is lowered significantly below Tco, indicating that the length scale of the disorder is quenched at low temperatures. We have obtained quantitative measurements of the correlation lengths and critical exponents that demonstrate this reduced dimensionality. In addition we have, for the first time, observed critical fluctuations above Tco and up to Tco + 20 K which also demonstrate that within these fluctuations the stripes are still two-dimensional. The doping with Sr provides extra holes to form the striped structure in the NiO planes, and it is likely that the stripes are disordered by the strain energy resulting from the mismatch of the radius of cations of La(rA = 1.26 Å) and Sr (rA = 1.31 Å).

Principal Publication and Authors
C-H. Du (a), M.E. Ghazi (a), Y. Su (a), I. Pape (a), P.D. Hatton (a), S.D. Brown (b,c), S-W. Cheong (d,e), Phys. Rev. Lett., 84(17), 3911-3914 (2000).

(a) Department of Physics, University of Durham (UK)
(b) ESRF
(c) Department of Physics, University of Liverpool (UK)
(d) Department of Physics and Astronomy, Rutgers University, New Jersey (USA)
(e) Bell Laboratories, Lucent Technologies, New Jersey (USA)