1. Department of Chemical, Biological & Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700 098, India
2. Solid
State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India
3. Science
and Technology Facilities Council, Rutherford Appleton Laboratory, Didcot, OX11 0QX, U.K.