1. Department of Physics, University of Calcutta, 92 A P C Road, Kolkata 700 009, India
2. Department of Physics, Raiganj University, Uttar Dinajpur, West Bengal 733 134, India
3. UGC-DAE Consortium for Scientific Research, Kolkata Centre, Sector III, LB-8, Salt Lake, Kolkata 700 098, India
4. Department of Low Temperature Physics and Superconductivity, Physics Faculty, M.V. Lomonosov Moscow State University, Moscow 119991, Russia
5. Surface and Nanoscience Division, Indira Gandhi Centre for Atomic Research, Homi Bhabha National Institute, Kalpakkam 603 102, India
6. Center for Research in Nanoscience and Nanotechnology, University of Calcutta, JD-2, Sector-III, Saltlake, Kolkata 700 106, India