1. School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
2. Center for Spintronics, Korea Institute of Science and Technology, Seoul 02529, South Korea
3. Department of Electrical and Computer Engineering, University of Miami, Coral Gables, Florida 33199, USA
4. EECS, UC Berkeley, Berkeley, California 94720, USA