1. Samsung Advanced Institute of Technology, Samsung Electronics Co., Ltd. San 14-1, Yongin-si, Gyeonggi-Do, 446-712, Korea, Department of Physics, Ewha Womans University, Seoul 120-750, Korea, Division of Quantum Phases & Devices, School of Physics, Konkuk University, Seoul 143-701, Korea, and Department of Materials Science and Engineering, Stanford University, Palo Alto, California 94305