1. Department of Physics, Graduate Institute of Electronic Engineering, Center for Condensed Matter Sciences, Graduate Institute of Photonics and Optoelectronics, Department of Electrical Engineering, National Taiwan University, Taipei 106, Taiwan, Department of Electrical Engineering and Computer Science, Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4037, and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan