1. Department of Electrical Engineering and Computer Science; Massachusetts Institute of Technology; Cambridge MA 02139 USA
2. Department of Physics; Massachusetts Institute of Technology; Cambridge MA 02139 USA
3. Department of Materials Science and Engineering; Massachusetts Institute of Technology; Cambridge MA 02139 USA
4. Materials Science and Engineering; National Tsing-Hua University; Hsinchu 20013 Taiwan
5. Condensed Matter Physics and Materials Science Department; Brookhaven National Laboratory; Upton NY 11973 USA
6. Department of Nuclear Science and Engineering; Massachusetts Institute of Technology; Cambridge MA 02139 USA