1. a Department of Physics, National Taiwan Normal University, Taiwan 116, R.O.C.
2. b Department of Materials Science and Engineering, National Tsing-Hua University, Taiwan 300, R.O.C.
3. c Department of Materials Science and Engineering, National Tsing-Hua University, Taiwan 300, R.O.C.
4. d Department of Physics Tamkang University Tamsui 251, Taiwan, R.O.C.