Direct Observation of Oxygen Superstructures in Manganites

We report the observation of superstructures associated with the oxygen $2p$ states in two prototypical manganites using x-ray diffraction at the oxygen $K$ edge. In the stripe order system ${\mathrm{Bi}}_{0.31}{\mathrm{Ca}}_{0.69}{\mathrm{MnO}}_3$, hole-doped O states are orbitally ordered, at the same propagation vector as the Mn orbital ordering, but no oxygen charge stripes are found at this periodicity. In ${\mathrm{La}}_{7/8}{\mathrm{Sr}}_{1/8}{\mathrm{MnO}}_3$, we observe a $2p$ charge ordering described by alternating hole-poor and hole-rich MnO planes that is consistent with some of the recent predictions.

Figure 1

Simplified orderings in BCMO and LSMO. Closed circles represents polaronic ${\mathrm{Mn}}^{3+}$ atoms, open circles are ${\mathrm{Mn}}^{4+}$, and small circles are the O sites; the orbital ordering in BCMO is also shown. The structure in the $a\mathrm{\text{−}}b$ plane of BCMO repeats in adjacent planes, while LSMO planes alternate between being hole rich ($z=0$ and 1) and hole poor ($z=1/2$ and $3/2$) [11]; for clarity the oxygens are represented in the first plane only. The scattering geometry is indicated, $\psi$ is the azimuthal angle, and $\sigma$ and $\pi$ are the two polarization components.

Figure 2

(a) The $(00.310)$ superstructure reflection, corrected from fluorescence but not for self-absorption; nevertheless, the intensity is zero off the O $K$ edge. (b) Temperature dependence of the peak position at the phase transition. (c) Azimuthal dependence displaying a twofold symmetry with a maximum with the incident polarization along $\mathbf{a}$, and zero along $\mathbf{c}$, taken at 528.75 eV, $T=200\mathrm{K}$.

Figure 3

(a) RXD spectra of the $(00\frac{1}{2})$ reflection at the O $K$ edge of ${\mathrm{La}}_{7/8}{\mathrm{Sr}}_{1/8}{\mathrm{MnO}}_3$, with two incident polarizations, $T=30\mathrm{K}$, $\psi \approx 90°$. (b) Temperature dependence of the reflection. (c) Azimuthal dependence at 529.2 eV, $\sigma \to \sigma +\pi$ channels, compared to a ${cos}^2+\mathrm{const}$ function (solid line), 0° is $\sigma$ incident polarization along $\mathbf{b}$, 90° along $\mathbf{a}$.

Figure 4

RXD spectrum of the $(00\frac{1}{2})$ reflection of ${\mathrm{La}}_{7/8}{\mathrm{Sr}}_{1/8}{\mathrm{MnO}}_3$, $\sigma \to \sigma +\pi$ channels, $T=30\mathrm{K}$. The O near edge ($\approx 530\mathrm{eV}$) and the Mn $L_{\mathrm{III}/\mathrm{II}}$ edges ($\approx 640$ and $\approx 652\mathrm{eV}$) probe the active states near ${\epsilon }_f$, while the region between 535 and 560 eV correspond to states that are delocalized (high energy from ${\epsilon }_f$). Inset shows the diffraction peak at 576 eV, emphasizing the existence of the reflection far above the edge.