A study on dynamical charge fluctuation and lattice dynamics of metal oxcides by X-ray inelastic scattering

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The purpose of this project is to perform a systematic study on the interplay between dynamics of charge, spin and lattice in metal oxides with strongly correlated electrons.One experimental uniqueness of this project is a complementary use of both X-ray inelastic scattering and neutron scattering techniques. Another uniqueness is to use both large single crystals and concentration-graded crystals. Particularly, the latter crystals are firstly utilized to the comprehensive spectroscopic studies by X-ray inelastic scattering in this project. Theses experimental uniqueness contributed a lot to obtain the new results which are shown below.(I)In a wide doping region, we succeeded in growing concentration-graded crystals of Sr doped La_<2-x>Sr_xrCuO_4 with x extending from 0.00 to 0.30. By using a scanning SQUID magnetometer we also succeeded in observing a boundary between the semiconducting-superconducting phases in real space as a boundary of vortex states below the transition temperature.(II)By using a synchrotron X-ray beam with high luminosity and by emitting the beam spot onto the different positions on the concentration-graded crystal we could obtain the information of lattice dynamics as a function of doping concentration in a wide doping region for the first time. One of the important discoveries is the anomalous softening of the Cu-O bond stretching mode only seen in the superconducting region where the life-time of phonons is also anomalously short. These anomalies suggest the strong electron-phonon coupling in the superconducting doping region and provide us important information on the role of electron-phonon coupling in this system for the paring mechanism.