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

本项目的目的是系统地研究具有强关联电子的金属氧化物中电荷、自旋和晶格动力学之间的相互作用，其实验独特性之一是X射线非弹性散射和中子散射技术的互补使用。另一个独特之处是同时使用大单晶和浓度梯度晶体。特别是，后者晶体是第一次利用X射线非弹性散射的综合光谱研究在这个项目中。这些实验的独特性对获得如下所示的新结果做出了很大贡献。(I)In在较宽的掺杂范围内，我们成功地生长了Sr掺杂La_<2-x>Sr_xrCuO_4（x从0.00到0.30）的浓度梯度晶体。通过使用扫描SQUID磁强计，我们还成功地观察到在真实的空间中的超导-超导相之间的边界作为在转变温度以下的涡旋状态的边界。（2）采用高亮度同步辐射X射线束，通过将束斑照射到浓度梯度晶体的不同位置，首次获得了宽掺杂范围内的晶格动力学信息。其中一个重要的发现是Cu-O键伸缩模的反常软化现象，这种现象只出现在声子寿命也非常短的超导区。这些反常现象表明超导掺杂区存在较强的电子-声子耦合，并为我们研究该体系中电子-声子耦合的作用提供了重要信息。

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.