1. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Department of Mechanical and Nuclear Engineering, Virginia Commonwealth University, Richmond, Virginia 23284, USA
3. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
4. Department of Physics, Georgetown University, Washington, District of Columbia 20057, USA
5. Materials Science and Engineering Division, NIST, Gaithersburg, Maryland 20899-8552, USA