1. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States
2. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States
3. Department of Physics and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306-4350, United States
4. Department of Physics, University of Athens, Panepistimioupolis, Zografos, 157 84 Athens, Greece