1. Department of Physics, Duke University, Durham, North Carolina 27708, USA
2. Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA
3. Department of Material Science and Engineering, University of Wisconsin, Madison, WI 53706, USA
4. Institute of Nanotechnology, CNR-NANOTEC, Dept. Chemistry, Via Orabona 4, 70126 Bari, Italy
5. Department of Condensed Matter Physics and CEITEC, Masaryk University, CZ-61137 Brno, Czech Republic