1. Department of Physics, The University of Texas, Austin, Texas 78712, USA
2. Department of Physics and EHSRC, University of Ulsan, Ulsan 680-749, South Korea
3. Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
4. ICQD, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China
5. Korea Research Institute of Standards and Science, Daejeon 305-340, South Korea