1. Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA
2. Charles Bowden Laboratory, Army Aviation & Missile RD&E Center, Redstone Arsenal, Alabama 35898, USA
3. Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA
4. CNR-NANOTEC, Istituto di Nanotecnologia, via Orabona, 4-70126 Bari, Italy
5. Department of Physics, Duke University, Durham, North Carolina 27708, USA