1. Mehta Research Institute, Chhatnag Road, Jhusi, Allahabad 211 019, India
2. Physics Department, S.G.T.B. Khalsa College, University of Delhi, Delhi 110 007, India
3. Department of Physics, University of Calcutta, 92 Acharya Prafulla Chandra Road, Calcutta 700 009, India
4. Theory Group, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India