Spectroscopy of Stripe Order in ${\mathrm{La}}_{1.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_4$ Using Resonant Soft X-Ray Diffraction

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in ${\mathrm{La}}_{1.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_4$ are observed when x-ray energies in the vicinity of the Ni $L_{2,3}$ absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the ${\mathrm{NiO}}_2$ layers; the holes are mainly located on in-plane oxygens surrounding a ${\mathrm{Ni}}^{2+}$ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.

Figure 1

Intensities of the (a) $(1-ϵ,0,0)$ spin-order and (b) $(2ϵ,0,1)$ charge-order superstructure peaks as a function of photon energy at 60 K for $\pi$-polarized (solid line) and $\sigma$-polarized (dashed line) light. The insets show scans through the respective superstructure peaks on a linear scale recorded with $\pi$-polarized light along the $[100]$ ($H$) and $[001]$ ($L$) directions; (c) x-ray absorption signal.

Figure 2

Comparison between measured data (symbols) and the results of the cluster simulation. The simulated diffraction spectra have been scaled by one common factor to match the experiment. The La $M_4$ peaks at 849.2 eV, marked by arrows, are not included in the Ni $L_{2,3}$ simulation.

Figure 3

Sensitivity of the simulated spectra to the model as seen at the $L_3$ resonance of the $(2ϵ,0,1)$ charge-order peak: the oxygen holes are (a) confined in-plane and antiferromagnetically coupled to the Ni, (b) not confined in-plane [$(pd\sigma {)}_1=(pd\sigma {)}_2=-1.88\mathrm{eV}$], (c) ferromagnetically coupled to the Ni ($U_{dd}=2\mathrm{eV}$). The circles are the experimental data.