Summary of research on orbital orders and fluctuations

这个项目通过对从2p、3d到4f和5f电子的系统进行综合研究，揭示了由于轨道自由度而产生的物质的新性质，并以“轨道有序”和“轨道涨落”为关键词。此外，我们还通过控制轨道自由度发现了新的功能材料。特别提到了高温超导材料MgB_2的发现及其基本性质的阐明。这种材料同时具有西格玛轨道和圆周率轨道，这两种轨道具有非常不同的性质。共存对硼化镁的物理性质有很大的影响。在项目期间，还不可能生长出大的单晶，但该项目已经为未来的发展铺平了道路。另一方面，我们报道了一种新的高温铁磁材料CaB2C2，并对CaB6及相关材料的性能进行了详细的测试。然而，我们还没有排除Fe杂质在实现铁磁性中发挥重要作用的可能性。对于f电子系统，我们揭示了CeB6及相关材料的轨道有序性、URSu2Si2的隐序性、Yb4As3的一维磁性等特征。我们每年都组织向公众开放的国内会议和发现MgB2后的专题会议。2002年，我们在仙台组织了一次国际会议，约有150人参加，并进行了高质量的积极讨论。这些会议记录作为日本物理学会的副刊发表。

This project has revealed new properties of matter due to orbital degrees of freedom by performing comprehensive study of systems ranging from 2p, 3d, to 4f and 5f electrons with "orbital order" and "orbital fluctuation" taken as key words. Furthermore, we have found new functionary materials by controlling the orbital degrees of freedom. In particular we mention the discovery of high-temperature superconductivity MgB2 and elucidation of its fundamental properties. The material has both sigma and pi orbitals which have very different properties. The coexistence influences essentially the physical property of MgB2. It has not been possible to grow large single crystals during the period of the project, but the project has paved the way for future development. On the other hand, we have reported a new high-temperature ferromagnet CaB2C2, and checked in detail the property of CaB6 and related materials. However, we have not yet excluded the possibility that Fe impurities may play an important role in realizing the ferromagnetism. For f electron systems, we have revealed characteristics of the orbital ordering in CeB6 and related materials, hidden order in URSu2Si2, and one-dimensional magnetism in Yb4As3, among other things.We organized every year domestic meetings open to general audience and a special meeting after the discovery of MgB2. In 2002 we organized an international conference in Sendai having about 150 participants and active discussions with high quality. The proceedings were published as supplement of the Physical Society of Japan.