1. Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
3. Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
4. Department of Physics, University of Hong Kong, Hong Kong, China.
5. Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
6. Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
7. Raith America Inc., Troy, NY 12180, USA.
8. Department of Electrical and Computer Engineering, Technion, Haifa 32000, Israel.