1. Institute for Molecular Engineering, University of Chicago, Chicago, IL 60637, USA.
2. Department of Physics, University of California, Santa Barbara, CA 93106, USA.
3. IBM T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598, USA.
4. Ames Laboratory and Iowa State University, Ames, IA 50011, USA.