Charge and Spin Ordering in Layered Perovskite-type Oxide Studied by Synchrotron X-ray Diffraction

近年来，人们认识到电荷、自旋和轨道自由度对过渡金属氧化物的电磁性质起着重要的作用。特别是在钙钛矿型锰氧化物中，各种磁场感应现象的发现，如巨磁电阻和磁结构转变，刺激了这一领域的许多活动。用中子和电子衍射等技术可以直接观察到自旋和电荷的有序化。然而，观察轨道有序性的实验技术一直受到限制。在这里，我们发展了一种直接检测轨道有序的共振x射线散射技术，我们成功地利用共振x射线散射观察到了钙钛矿氧化物和f电子系统中多种类型的电荷、自旋和轨道有序态。最近的重大成就是在人工晶格中观察到了电荷和轨道的短程有序和轨道-超晶格。通过对短程有序的观察，阐明了电荷有序化和轨道有序化的机理。同时，通过对轨道超晶格的观察，有望找到一种利用人造晶格制作电子器件的方法。

Recently, it has been recognized that the charge, spin, and orbital degrees of freedom play important roles in the electric and magnetic properties of the transition-metal oxides. In perovskite-type manganited, especially, the discover of a wide variety of magnetic-field-induced phenomena, such as a colossal magnetoresistance and a magnetostructural transition, has stimulated much activity this field. The spin and charge ordering can be directly observed by neutron and electron diffraction, and some other techniques. The experimental technique to observe the orbital ordering, however, has been limited. Here, we have developed a resonant x-ray scattering technique to detect the orbital ordering directly.We have succeeded to observe many types of charge, spin, and orbital ordering states in perovskite oxides as well as a f-electron systems by using the resonant x-ray scattering. Recent great achievements are an observation of short-range ordering of charge and orbital and a observation of orbital-superlattice in an artificial lattice. The mechanism of the charge and orbital ordering has been elucidated by the observation of the short-range order. Meanwhile, it is expected that we can find a way of making electric devices with an artificial lattice by the observation of the orbital-superlattice.