1. Joint Research Center for Atom Technology (JRCAT), Advanced Semiconductor Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Central 4, Tsukuba 305-8562, Japan, Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China, and Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, Republic of China