1. Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya 456-8587, Japan
2. Department of Materials Science and Engineering, Kyushu University, Fukuoka 819-0395, Japan
3. Research Center for Functional Materials, National Institute for Materials Science, Tsukuba 305-0047, Japan
4. Department of Applied Physics, National Defense Academy, Yokosuka 239-8686, Japan
5. Research Institute for Advanced Electronics and Photonics, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan