1. Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan
2. The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. College of Science, China Jiliang University, Hangzhou 310018, China
4. Department of Physics, Northwest University, Xian 710127, China