1. Institute of Microelectronics, Tsinghua University, Beijing 100084, China
2. Tsinghua National Laboratory for Information Science and Technology, Tsinghua University, Beijing 100084, China
3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, China
4. State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China