1. Department of Physics, Virginia Commonwealth University, Richmond, Virginia
23284, United States
2. Semiconductor
and Dimensional
Metrology Division, Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg,
Maryland 20899-8120, United States
3. Laboratory of Computational
Biology, National Heart Lung and Blood Institute, National Institutes of Health, Rockville, Maryland 20892, United
States