1. Nanostructures Research Laboratory, Japan Fine Ceramics Center, Atsuta, Nagoya 456-8587, Japan
2. Advanced Institute for Materials Research, Tohoku University, Aoba, Sendai 980-8577, Japan
3. Institute of Engineering Innovation, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
4. Department of Quantum Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan