1. Department of Physics and TcSUH, University of Houston, Houston, Texas 77204, USA
2. Center for Advanced Materials and Energy, Xihua University, Chengdu, Sichuan 610039, China
3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
4. Robert Bosch LLC, Research and Technology Center, Cambridge, Massachusetts 02139, USA