1. Department of Materials Science and Engineering, KJIST, Kwangju, 500-712, Korea
2. Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan
3. Department of Physics, National Chung-Hsin University, Tai-Chung 402, Taiwan
4. Synchrotron Radiation Research Center, Hsinchu 300, Taiwan
5. Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan