1. State Key Laboratory of Rare Earth Materials Chemistry and Applications PKU-HKU Joint Laboratory on Rare Earth Materials and Bioinorganic Chemistry College of Chemistry and Molecular Engineering Peking University, Beijing 100871, China Department of Physics The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong, China Department of Physics The Graduate School of the Chinese Academy of Sciences P.O. Box 3908, Beijing 100039, China