1. Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235-1822, United States
2. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States
3. Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, United States
4. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, United States