1. College of Mechanical and Electrical EngineeringYangtze Normal UniversityChongqing 408100 China
2. Division of Materials ScienceNanyang Technological UniversitySingapore 639798 Singapore
3. Department of Physics and ElectronicsOsaka Prefecture University1‐1 Gakuen‐cho, Naka‐ku, Sakai Osaka 599‐8531 Japan
4. Division of Frontier Materials ScienceOsaka University1‐3 Machikaneyama‐cho, Toyonaka Osaka 560‐8531 Japan