1. WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980 8577, Japan
2. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China
3. Max Planck Institute for Solid State Research, Stuttgart 70569, Germany
4. Nanostructures Research Laboratory, Japan Fine Ceramic Centre, Nagoya, 456-8587, Japan
5. Institute of Engineering Innovation, the University of Tokyo, Tokyo, 113-8656, Japan