1. Division of Quantum Phases and Devices, Department of Physics, Konkuk University, Seoul 143-701, Korea.
2. Department of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, Korea.
3. Department of Physics, Sogang University, Seoul 121-742, Korea.
4. Korea Institute for Advanced Study, Seoul 130-722, Korea.
5. Graduate School of Energy, Environment, Water, and Sustainability, NanoCentury KI, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea.
6. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.