1. Institute of Radio Physics and Electronics, University of Calcutta, 92 A. P. C. Road, Kolkata 700009, India
2. Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, JD2 Sector III Salt Lake City,Kolkata 700106, India
3. Department of Physics, Arizona State University, Tempe, Arizona 85287, USA
4. DOE Ames Research Laboratory, Ames, Iowa 50011, USA