1. Department of Physics and Key Laboratory of Atomic and Molecular Nanosciences, Tsinghua University, Beijing 100084, China
2. Institute of Applied Physics and Computational Mathematics, P. O. Box 8009, Beijing 100088, P. R. China
3. Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397, Japan
4. Macedonian Academy of Sciences and Arts, P. O. Box 428, 1000 Skopje, Macedonia