Direct Observation of Electron Density Distribution of the Molecular Orbital Detected by Synchrotron Radiation X-Pay Diffraction

同步辐射(SR)X射线在材料科学研究中的应用越来越多。在本项目中，我们利用自旋共振X射线衍射法测定了新型分子晶体中的电子态。由于与电子之间的库仑排斥相比，稀有电子气体系在高温下具有较大的动能，因此是均匀的。正如上世纪30年代初S在理论上所预言的那样，在极端低温下，星系将倾向于凝聚并形成不均匀的结块。由于这些结块带有负电荷，它们形成了一种结构，在空间位置上它们试图避开彼此。我们称这种现象为维格纳结晶。分子导体(DI-DCNQI)_2Ag即使在室温下也是绝缘的。在220K以下，Hiraki和Kanoda的核磁共振研究表明，DCNQI分子上的4K_F Wigner晶型为电荷有序(CO)。SEO等人从理论上预测了这种CO基态。艾尔电子晶体在真实固体中是否(或如何)稳定是非常有趣的。在Pf，KEK用SR进行了单晶X射线衍射测量。根据结构分析的结果，基态的两重结构不仅包括电荷有序柱，还包括由于DCNQI柱之间的几何挫折而导致的单调电荷二聚柱。尽管有这些复杂的有序参数，这种电荷有序结构被认为是由柱间库仑排斥引起的Wigner晶体。这是对3D电子晶体的首次直接观测。此外，我们还研究了项目期间几种分子材料中的电子态。

The Synchrotron Radiation (SR) X-ray is finding increasing use in the study of material science. In this project, we utilized the SR X-ray diffraction methods for the determination of the electronic state in the novel molecular crystals.Rarefied electron gas system at high temperature is homogeneous due to the large kinetic energy compared to the Coulomb repulsion among electrons. At extreme low temperatures, the system will tend to condense and make uneven lumps, as theoretically predicted in the early 1930's. Since these lumps carry a negative charge, they form a structure where they try to avoid each other in the spatial positioning. We call this phenomenon the Wigner crystallization. A molecular conductor (DI-DCNQI)_2Ag is insulating even at room temperature. Below 220 K, Hiraki and Kanoda suggested that the 4k_F Wigner crystal type of charge order (CO) on DCNQI molecules by NMR technique. This CO ground state was theoretically predicted by Seo, et. al. It is very interesting whether (or how) the electron crystal is stable in the real solid. Single crystal x-ray diffraction measurements using SR were performed at PF, KEK. According to the result of the structural analysis, two-fold structure in ground state consists of not only charge ordering columns but also monotonic charge dimerized columns caused by geometrical frustration among DCNQI columns. In spite of these complicated order parameters, this charge ordered structure is regarded as a Wigner crystal caused by inter-column Coulomb repulsion. This is a first direct observation of the 3D electron crystal. Furthermore, we investigated the electronic state in several molecular materials during the term of project.