1. School of Physics, University of New South Wales, UNSW Sydney 2052, Australia
2. Dip. di Matematica e Fisica, Università di Camerino, I-62032 Camerino, Italy
3. Department of Physics, Dalhousie University, Halifax, NS, Canada BH3J5, Canada
4. Laboratoire des Verres, Université de Montpellier II, F-34095 Montpellier, France