1. Department of Physics Tokyo Metropolitan University Tokyo 192‐0397 Japan
2. Department of Applied Physics Nagoya University Nagoya 464‐8603 Japan
3. Institute of Advanced Energy Kyoto University Kyoto 611‐0011 Japan
4. Innovative Functional Materials Research Institute AIST Nagoya 463‐8560 Japan
5. Device Technology Research Institute AIST Tsukuba 305‐8562 Japan