The strontium doped La2NiO4 system is iso structural with the high TC superconducting cuprate LacCuO4 and is a prototypical system for the understanding of strong electron-phonon coupling, and the resultant effects on material properties. Whereas the doped cuprates show superconductivity above a certain doping level, doped La2NiO4 shows no such effect over the whole phase diagram. However La2NiO4 undergoes a transition into a charge-ordered regime upon cooling whereby the dopant holes migrate to form hole rich regions or 'stripes' behaving as anti-phase domain boundaries surrounded by hole deficient antiferromagnetic regions [1]. X-ray diffraction provides an ideal tool for the study of such charge modulations, but at traditional energies it is only sensitive to such charge ordering in the near surface region. We present results taken on the high-energy X-ray scattering beamline ID15A. In using an incident X-ray energy of 130 keV there is a dramatic increase in penetration depth, even for such highly-absorbing samples, which increases the scattering volume and allows for bulk sensitive measurements.

Measurements were made on a single crystal sample of La1.667Sr1.333NiO4 grown at Bell Labs., measuring approximately 2 mm x 2 mm x 1 mm, and mounted on the cold finger of a closed-cycle refrigeration on the 4-circle triple-axis diffractometer at ID15A. Charge-Ordered (CO) satellite reflections were studied at previously determined locations [2] with a modulation wavevector GCO = (2, 0, 1) with = 0.333. The integrated intensity and inverse correlation length of the CO satellite (4.667, 0, 5) are shown in Figure 73

Figure 73
Fig. 73: Temperature dependence of the integrated intensity and inverse correlation length of the (4.667 0 5) CO peak.

In our previous measurements at 8 keV, the CO satellites were very sharp with a long-range correlation length of ~ 2400 Å. However with 130 keV X-rays, the CO satellites are broad and hence only short-range ordered with a correlation length of ~ 300 Å (see Figure 74). In addition, the integrated intensity of the CO satellites was considerably less than found by us in previous 8 keV studies suggesting that the number of stripes, or the charge amplitude may be sample dependent. The shorter correlation lenghts are consistent with previous neutron measurements [2]. We postulate that in such a high-quality crystal, charge stripes are semi-disordered and hence only correlated over a short distance. However in the near surface region the charge stripes are "pinned" to the lattice, causing a much higher degree of correlation.

Figure 74
Fig. 74: Comparison of near surface and bulk correlation lengths in La 1.667Sr 1.333NiO 4.

Measurements were also taken on the Colossal Magneto-Resistant (CMR) manganite Nd0.5Sr0.5MnO3 using high-energy diffraction. In contrast to our results on the nickelate, we found that in the bulk the charge ordering is long-range in order, with the correlation length being almost the same value of the Bragg peak correlation lengths. Due to the large sample volume probed, the intensity of the satellites is far greater than that of previous 8 keV measurements, and we have also been able to locate the satellites corresponding to the spin-ordering satellites.

[1] J. M. Tranquada, Nature 375, 561 (1995).
[2] S.-H. Lee and S.-W. Cheong, Phys. Rev. Lett. 79, 2514 (1997).

Principal Publication and Authors
S.B.Wilkins (a), P.D. Hatton (a), K-D. Liss (b), M. Ohler (b), T. Katsufuji (c) and S-W Cheong (c,d), Int. J. Mod. Phys. B 14, 3753 (2000).
(a) University of Durham (UK)
(b) ESRF
(c) Bell Laboratories, Murray Hill (USA)
(d) Rutgers University, Pitscataway (USA)