1. Solid State Physics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland
2. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
3. Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
4. Quantum Center, ETH Zurich, CH-8093 Zurich, Switzerland