1. Department of Physics and Astronomy, Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio 45701
2. Centro de Nanociencias y Nanotecnologia, Universidad Nacional Autónoma de México, Apartado Postal 14, Ensenada Baja California, Código Postal 22800, Mexico
3. Department of Physics, The Ohio State University, 191 Woodruff Avenue, Columbus, Ohio 43210