1. Center for Nanophysics and Advanced Materials, Physics Department, University of Maryland, College Park, MD 20742-4111, USA
2. Physics Department, University of Maryland, College Park, MD 20742-4111, USA
3. Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742-4111, USA
4. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD 20742-4111, USA
5. Malibu Research Associates Inc., 3760A Calle Tecate, Camarillo CA 93012-5059, USA