1. Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA
2. Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210, USA
3. Nuclear Reactor Laboratory, The Ohio State University, Columbus, Ohio 43210, USA
4. National Institute of Information Communications Technology, Koganei, Tokyo 184-8795, Japan
5. Universität Leipzig, Felix-Bloch-Institut für Festkörperphysik, D-04103 Leipzig, Germany
6. Semiconductor Research Center, Wright State University, Dayton, Ohio 45435, USA