[1] Inst Phys, Lab Low Dimens Mat, Bhubaneswar 751005, India
[2] Homi Bhabha Natl Inst, Training Sch Complex, Mumbai 400094, Maharashtra, India
[3] St Xaviers Coll, Dept Phys & Elect, Computat Mat & Nanosci Grp, Ahmadabad 380009, Gujarat, India
[4] Chalmers Univ Technol, Dept Microtechnol & Nanosci, SE-41296 Gothenburg, Sweden
[5] Uppsala Univ, Dept Phys & Astron, Condensed Matter Theory Grp, Box 516, S-75120 Uppsala, Sweden
[6] Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, S-10044 Stockholm, Sweden