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Register a new userStudy of the Electronic Structure in Oxides Using Absorption and Resonant X-Ray Scattering
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Yves Joly
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Resonant X-ray scattering (RXS) is a spectroscopy where both the power of site selective diffraction and the power of local absorption spectroscopy regarding atomic species are combined. By virtue of the dependence on the core level state energy and the three dimensional electronic structure of the intermediate state, this technique is specially suited to study charge, orbital or spin orderings and associated crystal distortions. In the case of charge ordering, we exploit the fact that atoms with closely related site symmetries but with small charge differences exhibit resonances at slightly different energies. The sensitivity of this effect allows for quantitative estimations of the charge disproportion. Opposite to fluorescence or absorption measurements, the power of diffraction relies on the capability of detecting differences that are smaller than the inverse lifetime of the core hole level. To account for the uncertainty of the crystallographic structure and the fact that the charge ordering must be disentangled from the associated atomic displacements, a complete methodology is proposed and applied to the low temperature phase of magnetite. Relative sensitivity on spin, toroidal and orbital ordering is also shown and compared in different transition metal oxide compounds, like V2O3 and GaFeO3.
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Motivated by the recent synthesis of Ba$_2$CuO$_{3+delta}$ (BCO), a high temperature superconducting cuprate with putative $d_{3z^2-r^2}$ ground state symmetry, we investigated its electronic structure by means of Cu $L_3$ x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) at the Cu $L_3$ edge on a polycrystalline sample. We show that the XAS profile of BCO is characterised by two peaks associated to inequivalent Cu sites, and that its RIXS response features a single, sharp peak associated to crystal-field excitations. We argue that these observations are only partially compatible with the previously proposed crystal structure of BCO. Based on our spectroscopic results and on previously published powder diffraction measurements, we propose a crystalline structure characterized by two inequivalent Cu sites located at alternated planes along the $c$ axis: nominally trivalent Cu(1) belonging to very short Cu-O chains, and divalent Cu(2) in the oxygen deficient CuO$_ {1.5}$ planes. We also analyze the low-energy region of the RIXS spectra to estimate the magnitude of the magnetic interactions in BCO and find that in-plane nearest neighbor superexchange exceeds 120~meV, similarly to that of other layered cuprates. Although these results do not support the pure $d_{3z^2-r^2}$ ground state scenario, they hint at a significant departure from the common quasi-2D electronic structure of superconducting cuprates of pure $d_{x^2-y^2}$ symmetry.
A new method is presented for describing vibrational effects in x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) using a combination of the classical Franck-Condon (FC) approximation and classical trajectories run on the core-excited state. The formulation of RIXS is an extension of the semiclassical Kramers-Heisenberg (SCKH) formalism of Ref. {Ljungberg_2010} to the resonant case, retaining approximately the same computational cost. To overcome difficulties with connecting the absorption and emission processes in RIXS the classical FC approximation is used for the absorption, which is seen to work well provided that a zero-point-energy correction is included. In the case of core-excited states with dissociative character the method is capable of closely reproducing the main features for one-dimensional test systems, compared to the quantum mechanical formulation. Due to the good accuracy combined with the relatively low computational cost, the method has large potential of being used for complex systems with many degrees of freedom, such as liquids and surface adsorbates.
We investigate mixed-valence oxide Co$_3$O$_4$ using Co $2p3d$ resonant inelastic X-ray scattering (RIXS). By setting resonant edges at Co$^{2+}$ and Co$^{3+}$ ions, the $dd$ excitations on the two Co sites are probed selectively, providing detailed information on the local electronic structure of Co$_3$O$_4$. The $2p3d$ RIXS result reveals the $^4$T$_{2}$ excited state of tetrahedral Co$^{2+}$ site at 0.5 eV beyond the discriminative power of optical absorption spectroscopies. Additionally, the $^3$T$_{2g}$ excited stated at 1.3 eV is uniquely identified for the octahedral Co$^{3+}$ site. Guided by cluster multiplet simulations, the ground-state character of the Co$^{2+}$ and Co$^{3+}$ site is determined to be high-spin $^4$A$_{2}$(T$_d$) and low-spin $^1$A$_{1g}$(O$_h$), respectively. This indicates that only the Co$^{2+}$ site is magnetically active site at low-temperatures in Co$_3$O$_4$. The ligand-to-metal charge transfer analysis suggests a formation of a strong covalent bonding between Co and O ions at the Co$^{3+}$ site, while Co$^{2+}$ is rather ionic.
Resonant x-ray scattering is performed near the Mn K-absorption edge for an epitaxial thin film of BiMnO3. The azimuthal angle dependence of the resonant (003) peak (in monoclinic indices) is measured with different photon polarizations; for the $sigmatopi$ channel a 3-fold symmetric oscillation is observed in the intensity variation, while the $sigmatosigma$ scattering intensity remains constant. These features are accounted for in terms of the peculiar ordering of the manganese 3d orbitals in BiMnO3. It is demonstrated that the resonant peak persists up to 770 K with an anomaly around 440 K; these high and low temperatures coincide with the structural transition temperatures, seen in bulk, with and without a symmetry change, respectively. A possible relationship of the orbital order with the ferroelectricity of the system is discussed.
Chemical interaction and changes in local electronic structure of Cr, Fe, Co, Ni and Cu transition metals (TMs) upon formation of an $Al_{8}Co_{17}Cr_{17}Cu_{8}Fe_{17}Ni_{33}$ compositionally complex alloy (CCA) have been studied by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. It was found that upon CCA formation, occupancy of the Cr, Co and Ni 3d states changes and the maximum of the occupied and empty Ni 3d states density shifts away from Fermi level ($E_f$) by 0.5 and 0.6 eV, respectively, whereas the Cr 3d empty states maximum shifts towards $E_f$ by 0.3 eV, compared to the corresponding pure metals. The absence of significant charge transfer between the elements was established, pointing to the balancing of the 3d states occupancy change by involvement of delocalized 4s and 4p states into the charge redistribution. Despite the expected formation of strong Al-TMs covalent bonds, the Al role in the transformation of the TMs 3d electronic states is negligible. The work demonstrates a decisive role of Cr in the Ni local electronic structure transformation and suggests formation of directional Ni-Cr bonds with covalent character. These findings can be helpful for tuning deformation properties and phase stability of the CCA.
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