1. Department of Electrical and Electronic EngineeringGraduate School of EngineeringKobe University 1‐1 Rokkodai, Nada Kobe Hyogo 657‐8501 Japan
2. Frontier Research LaboratoryNational Institute of Information and Communications Technology 4‐2‐1 Nukui‐Kitamachi Koganei Tokyo 184‐8795 Japan
3. World Premier International Research Center‐Advanced Institute for Materials Research (WPI‐AIMR)Tohoku University 2‐1‐1 Katahira Aoba‐ku Sendai 980‐8577 Japan