1. Department of Electrical and Computer Engineering, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto, 860-8555, Japan
2. Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7916, USA
3. ‘Department of Condensed Matter Physics, Royal Institute of Technology, Stockholm, S-10044, Sweden