1. Division of Quantum Phases & Devices, Department of Physics, Konkuk University, Seoul 143-701, Korea
2. Department of Physics, Sogang University, Seoul 121-742, Korea
3. The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
4. Korea Institute for Advanced Study, Seoul 130-722, Korea
5. Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States