1. Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2. Department of Physics, Wellesley College, Wellesley, MA 02481, USA.
3. Smart Energy Research Laboratories, NEC Corporation, 34 Miyuiga-ka, Tsukuba, Ibaraki 305-8501, Japan.
4. Physics Department and Solid State Institute, Technion, Haifa 32000, Israel.
5. Department of Applied Physics, Yale University, New Haven, CT 06520, USA.