1. Department of Physics, Oakland University, Rochester, Michigan 48309, USA
2. Department of Electronic and Computer Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
3. Department of Physics, University of Arizona, Tucson, Arizona 85721, USA
4. Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China