1. Department of Physics, Washington College, Chestertown, Maryland 21620, USA
2. Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87123, USA
3. Department of Physics, University of California at San Diego, La Jolla, San Diego, California 92093, USA
4. School of Engineering, Brown University, Providence, Rhode Island 02912, USA