1. Department of Electronic Science and Engineering, Kyoto University, 615-8510 Kyoto, Japan
2. Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
3. Physik-Department, Technische Universität München, 85748 Garching, Germany
4. Nanosystems Initiative Munich, 80799 München, Germany