1. State Key Laboratory of Analog and Mixed-Signal VLSI, University of Macau, Macau, P. R. China
2. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States
3. Schlumberger-Doll Research Center, Cambridge, Massachusetts 02139, United States
4. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States
5. Instituto Superior Técnico, Universidade de Lisboa, Lisbon 1049-001, Portugal