1. Department of Advanced Materials and Nanotechnology, Peking University, Beijing 100871, China
2. Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284
3. Center for Applied Physics and Technology, Peking University, Beijing 100871, China
4. School of Physical Science and Technology, Southwest University, Chongqing 40071, China
5. Institute for Materials Research, Tohoku University, Sendai 980-77, Japan