Charge correlations in cobaltates La2–xSrx CoO4

We report on an elastic neutron scattering study of the charge correlations in La_2–xSr_x CoO₄ with x = 1/3, 0.4 and 0.5. We found that the checkerboard charge ordering correlations present in the x = 0.5 sample persist in the x = 0.4 and 1/3 materials. These checkerboard charge ordering correlations are robust and explain the occurrence of nano‐phase separation in layered cobaltates for Sr‐concentrations away from half‐doping. The half‐integer reflections then arise from the nanometer‐sized hole‐rich regions (blue areas in title figure) instead of the undoped ones (red areas in title figure). The appearance of nano‐phase separation is an important ingredient for understanding the formation of hour‐glass shaped magnetic excitation spectra in La_2–xSr_x CoO₄.

Nano‐phase separation in La_2–xSr_x CoO₄ (schematically). Red areas: undoped La₂CoO₄ islands, blue areas: checkerboard charge ordered regions; black, green and blue balls represent nonmagnetic Co³⁺ ions, magnetic Co²⁺ ions and oxygen ions, respectively; green arrows indicate Co²⁺ spins [1, 2].     

Determination of the orbital moment and crystal-field splitting in LaTiO3

Utilizing a sum rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO3 is strongly reduced from its ionic value, both below and above the Ne el temperature. Using Ti L2,3 x-ray absorption spectroscopy as a local probe, we found that the crystal-field splitting in the t2g subshell is about 0.12-0.30 eV. This large splitting does not facilitate the formation of an orbital liquid.     

Spectroscopy of stripe order in La1.8Sr0.2NiO4 using resonant soft x-ray diffraction

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 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 NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.     

Asymmetric orbital-lattice interactions in ultrathin correlated oxide films

Using resonant x-ray spectroscopies combined with density functional calculations, we find an asymmetric biaxial strain-induced d-orbital response in ultrathin films of the correlated metal LaNiO3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials.     

Magnetic field induced orbital polarization in cubic YbInNi4: determining the quartet ground state using x-ray linear dichroism

We have been able to induce a linear dichroic signal in the Yb M(5) x-ray absorption white line of cubic YbInNi(4) by the application of a magnetic field. The nonzero integrated intensity of the magnetic field induced dichroic spectrum indicates a net noncubic 4f orbital polarization. A quantitative analysis of the temperature and field strength dependence establishes that the crystal-field ground state is a Γ(8) quartet. The results demonstrate the potential of magnetic field induced linear dichroism as a new powerful approach for the investigation of the degeneracy and orbital degrees of freedom of cubic heavy-fermion and Kondo systems.     

Fe valence state at the surface of the Fe0.5Cu0.5Cr2S4 spinel

Here we present X‐ray absorption measurements of a vacuum cleaved Fe_0.5Cu_0.5Cr₂S₄ single crystal. Measurements at different positions on the cleaved sample surface clearly reveal a difference between the valence state of the Fe ions in the sur‐ face layers and the valency of the Fe ions present in the bulk. These results confirm the findings of recent measurements outlined previously. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal-field level inversion in lightly Mn-doped Sr3Ru2O7

Sr3(Ru(1-x)Mnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn, is studied by x-ray dichroism and spin-resolved density functional theory. We find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+ acceptors; the extra eg electron occupies the in-plane 3d(x2-y2) orbital instead of the expected out-of-plane 3d(3z2-r2). We propose that the 3d-4d interplay, via the ligand oxygen orbitals, is responsible for this crystal-field level inversion and the material's transition to an antiferromagnetic, possibly orbitally ordered, low-temperature state.     

Direct observation of t2g orbital ordering in magnetite

Using soft-x-ray diffraction at the site-specific resonances in the Fe L2,3 edge, we find clear evidence for orbital and charge ordering in magnetite below the Verwey transition. The spectra show directly that the (001/2) diffraction peak (in cubic notation) is caused by t2g orbital ordering at octahedral Fe2+ sites and the (001) by a spatial modulation of the t2g occupation.