[1] CEA Saclay, CNRS, URA 2464, Serv Phys Etat Condense, F-91191 Gif Sur Yvette, France
[2] Univ Montreal, Ctr Rech Math, Montreal, PQ H3C 3J7, Canada
[3] Univ Quebec, Dept Math & Informat, Trois Rivieres, PQ G9A 5H7, Canada
[4] Vrije Univ Brussels, Denst Theoret Nat Kunde, B-1050 Brussels, Belgium