Resonant inelastic x-ray scattering study of two lamellar copper oxides.

摘要

This Thesis reports a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in two lamellar copper oxides: the structurally simple high-temperature superconductor HgBa2CuO4+delta (Hg1201) near optimal doping ( Tc = 96.5 K) and the antiferromagentic (T N ∼ 320 K) Mott insulator La2CuO4. The two crystals were grown at Stanford University, and the RIXS experiments were performed at two separate state-of-the-art beam lines: 9IDB at the Advanced Photon Source in the United States; and BL11XU at SPring-8 in Japan. HgBa 2CuO4+delta exhibits the highest value of T c among all single-layer cuprates, and La2CuO4 is the parent compound of the original high-temperature superconductor La 2-xBaxCuO 4. We find that the Cu K-edge RIXS spectra in Hg1201 exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly-dispersive ( 0.5 eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a remnant charge-transfer gap deep in the superconducting phase.; Subsequent measurements in La2CuO4 reveal a very similar multiplet of excitations, which was investigated both for in-plane and out-of-plane polarizations. The close similarity between the multiplet structures for the two polarizations is indicative of the essential role played by the is core hole in inducing charge excitations within the CuO2 planes. On the other hand, we observe a polarization dependence of the spectral weight. A careful analysis reveals two separate features near 2 eV that may be related to different charge-transfer processes, and we suggest that a significant involvement of 4p electron relaxation in the RIXS process can be used to account for the difference in energy positions and spectral weight. A shake-up of valence excitations and third-order perturbation arguments are then applied to describe the likely final-energy resonance in the spectra. As an alternative scenario, we propose characteristic fluorescence due to transitions from the 1s state to a narrow continuum 4 p band.