Investigation of electronic order using resonant soft x-ray diffraction

The study of ordering of electronic degrees of freedom: charge, orbital and magnetic order, is important for the understanding of many of the often spectacular properties of transition-metal oxides (TMO), including high-temperature superconductivity and metal-to-insulator transitions. While magnetic structure of a bulk systems can be investigated directly using neutron diffraction, an appropriate technique for the study of orbital and charge order was lacking for a long time. Neutrons are not sensitive to charge and therefore cannot detect charge or orbital order directly; they can only pick-up lattice distortions that such an order might induce on the crystal lattice. The suitable technique has to be directly charge sensitive, but beyond this in particular sensitive to the outer-shell electrons, since exactly these determine the electronic properties of a solid. The conventional x-ray diffraction technique rules therefore out, since the main intensity in the detected signal comes here from the inner-shell electrons. Resonant soft x-ray diffraction is the appropriate technique to investigate electronic order in transition metal oxides [1]. Since soft x-rays have energies in the range 200 - 2000~eV, the photon energy can be tuned such, that the diffraction process will involve one of the following virtual excitations: oxygen 1s -> 2p (K-edge), transition metal 2p -> 3d (L2,3-edge) and lanthanide 3d ->4p (M4,5-edge). It is known from x-ray absorption (XAS) data that variation of the photon scattering cross section across the TM L2,3- edge is extremely sensitive to the electronic state of TMO [2]. Whereas XAS delivers only average information about the electronic state of the system, resonant soft x-ray diffraction is structure-selective, so that only the coherent signal belonging to a long-range order with a certain periodicity is detected. This allows for spectroscopically-resolved structure study or, using an appropriate microscopic model, for structurally-resolved spectroscopy study. Resonant soft x-ray diffraction has therefore a high potential for investigation of mixed-valent transition-metal oxide systems, where an electronic phase-separation is expected. In the last years the electronic order of a few TMO-systems has been investigated applying the resonant soft x-ray diffraction technique [3-16]. Nevertheless, a full spectral analysis of the data in terms of a realistic microscopic theory - as it is state of the art in the closely related x-ray absorption spectroscopy (XAS) - is very rare, showing that the technique is still in its infant state. Up to now most of the studies use a qualitative analysis of the spectral shape for the interpretation of their data. The aim of this PhD work was the application of resonant soft x-ray diffraction technique for the investigation of electronic order in transition metal oxides at the TM L2,3-edge, trying to obtain a quantitative understanding of the data. The method was first systematically explored through application to a model system in order to test the feasibility of the technique and to understand of how x-ray optical effects have to be taken into account. Two more complex systems were investigated; stripe order in La1.8Sr0.2NiO4 and charge and orbital order in Fe3O4. The main focus of the work was on the spectroscopic potential of the technique, trying to obtain a level of quantitative description of the data. For x-ray absorption spectroscopy (XAS) from transition metal oxides, cluster configuration interaction calculation provides a powerful and realistic microscopic theory. In the frame work of this thesis cluster theory, considering explicit hybridization effects between the TM-ion and the surrounding oxygen ligands, has been applied for the first time to describe resonant diffraction data; previous publications confined themselves to a pure ionic picture [1, 5, 6]. [1] C. M. W. Castleton and M. Altarelli, Phys. Rev. B 62, 1033 (2000). [2] F. M. F. de Groot, J. Electron Spectrosc. Relat. Phenom. 67, 529 (1994). [3] S. B. Wilkins, et al., Phys. Rev. Lett. 90, 187201 (2003). [4] S. B. Wilkins, et al., Phys. Rev. Lett. 91, 167205 (2003). [5] S. S. Dhesi, et al., Phys. Rev. Lett. 92, 056403 (2004). [6] K. J. Thomas, et al., Phys. Rev. Lett. 92, 237204 (2004). [7] U. Staub, et al., Phys. Rev. B 71, 214421 (2005). [8] S. B. Wilkins, et al., Phys. Rev. B 71, 245102 (2005). [9] S. B. Wilkins, et al., Phys. Rev. B 74, 049902 (2006). [10] P. Abbamonte, et al., Nature 431, 1078 (2004). [11] P. Abbamonte, et al., Nature Physics 1, 155 (2005). [12] A. Rusydi, et al., Phys. Rev. Lett. 97, 016403 (2006). [13] C. Schuessler-Langeheine, et al., Phys. Rev. Lett. 95, 156402 (2005). [14] V. Scagnoli, et al., Phys. Rev. B 73, 100409 (2006). [15] I. Zegkinoglou, et al., Phys Rev. Lett. 95, 136401 (2005). [16] D. J. Huang, et al., Phys. Rev. Lett. 96, 096401 (2006)

Self-doping processes between planes and chains in the metal-to-superconductor transition of YBa2Cu3O6.9

The interplay between the quasi 1-dimensional CuO-chains and the 2-dimensional CuO2 planes of YBa2Cu3O6+x (YBCO) has been in focus for a long time. Although the CuO-chains are known to be important as charge reservoirs that enable superconductivity for a range of oxygen doping levels in YBCO, the understanding of the dynamics of its temperature-driven metal-superconductor transition (MST) remains a challenge. We present a combined study using x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) revealing how a reconstruction of the apical O(4)-derived interplanar orbitals during the MST of optimally doped YBCO leads to substantial hole-transfer from the chains into the planes, i.e. self-doping. Our ionic model calculations show that localized divalent charge-transfer configurations are expected to be abundant in the chains of YBCO. While these indeed appear in the RIXS spectra from YBCO in the normal, metallic, state, they are largely suppressed in the superconducting state and, instead, signatures of Cu trivalent charge-transfer configurations in the planes become enhanced. In the quest for understanding the fundamental mechanism for high-Tc-superconductivity (HTSC) in perovskite cuprate materials, the observation of such an interplanar self-doping process in YBCO opens a unique novel channel for studying the dynamics of HTSC.Comment: 9 pages, 4 Figure

The interplay of local electron correlations and ultrafast spin dynamics in fcc Ni

The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. By combining high energy and temporal resolution in femtosecond time-resolved X-ray absorption spectroscopy with ab initio time-dependent density functional theory we analyze the electronic structure in fcc Ni on the time scale of these interactions in a pump-probe experiment. We distinguish transient broadening and energy shifts in the absorption spectra, which we demonstrate to be captured by electron repopulation respectively correlation-induced modifications of the electronic structure, requiring to take the local Coulomb interaction into account