1. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
2. Physic Division, National Center of Theoretical Sciences, Taipei 10617, Taiwan
3. Department of Physics, University of California, Berkeley, California 94720-7300, United States
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
5. Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06520, United States
6. Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States