1. Department of Physics and Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, Puerto Rico 00931-3334, USA
2. CSIR-National Physical Laboratory, New Delhi, India
3. Department of Chemical Engineering and Institute for Functional Nanomaterials, University of Puerto Rico, Mayagüez, Puerto Rico 00681, USA
4. Physics Department, Oakland University, Rochester, Michigan 48309-4401, USA
5. Cavendish Laboratory, Department of Physics, Cambridge University, Cambridge CB3 0HE, United Kingdom