1. State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, P. R. China
2. Department of Physics, University of Nebraska at Omaha, Omaha, Nebraska 68182-0266, United States
3. Department of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience, Theodore Jorgensen Hall, 855 North 16th Street, University of Nebraska—Lincoln, P.O. Box 880299, Lincoln, Nebraska 68588-0299, United States
4. Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan