中文摘要：

轨道自由度为凝聚态关联电子材料带来丰富多彩的新量子相的同时也导致了复杂性，对理解强关联电子体系的本质带来了挑战。文章系统地介绍了多轨道关联电子体系的各种物理性质。首先简要地介绍了轨道自由度在过渡金属氧化物和稀土化合物中的重要作用和轨道序的物理图象；其次论述了轨道极化、轨道序及其与晶格畸变的联系；然后讨论了轨道序的理论研究及其在实验上的可能探测；接着介绍了多轨道体系中的金属一绝缘体相变和多轨道超导电性的新特征；最后简短讨论了目前轨道物理研究中面临的问题和挑战。

英文摘要：

Orbital degrees of freedom bring about rich and colorful quantum phases in correlated electron systems of condensed matter, and lead to complexity, hence challenging our understanding of the nature of strongly correlated systems. In this review the various properties of multi-orbital correlated electron systems are introduced： we first briefly describe the important role of the orbital degrees of freedom and the scenario of orbital order; next we discuss the inter-relationship among orbital polarization, orbital ordering and lattice distortion in some classical orbital-ordered compounds, and outline the theories and experiments on detecting orbital order; then we introduce the magnetic modulation of multi-orbital metal-insulator transitions and multi-orbital superconductivity; finally we discuss the present problems and challenges in the research of orbital physics.