1. Nano Characterization Unit, National Institute for Materials Science, Tsukuba 305-0047, Japan
2. Faculty of Physics and Materials Sciences Center, Philipps-Universität Marburg, 35032 Marburg, Germany
3. Department of Physics, University of Florida, Gainesville, Florida 32611, USA
4. Department of Physics and Astronomy and Pittsburgh Quantum Institute, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA