1. Department of Physics, Technical University of Denmark, Fysikvej, DK-2800 Kongens Lyngby, Denmark
2. Department of Electrical and Electronic Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan
3. Geballe Laboratory for Advanced Materials, Stanford University, 476 Lomita Mall, Stanford, California 94305, United States