1. National Key Laboratory of Crystal Materials and School of Physics, Shandong University, Jinan, Shandong 250100, People's Republic of China
2. School of Physics and Engineering, Qufu Normal University, Qufu, Shandong 273165, People's Republic of China
3. Department of Mechanical and Materials Engineering, Portland State University, P.O. Box 751, Portland, Oregon 97207-0751, USA