Affiliation: | 1. Department of Physics and Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA;2. State Key Laboratory for Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, the People''s Republic of China;3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA;4. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan 30010 |