1. Department of Physics, Division of Quantum Phases and Devices, Konkuk University, Seoul 143-701, Republic of Korea
2. State Key Lab of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 6150054, People's Republic of China
3. Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA