1. Advanced Science and Technology Center for Cooperative Research, Kyushu University, Kasuga, Fukuoka 816‐8580, Japan
2. Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816‐8580, Japan
3. Department of Electrical and Electronic Engineering, Faculty of Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514‐8507, Japan
4. NGK INSULATORS, LTD., Suda‐cho, Mizuho‐ku, Nagoya 466‐8555, Japan