[1] Univ Venezia, Dipartimento Chim Fis, I-30123 Venice, Italy
[2] N Carolina State Univ, Dept Phys, Raleigh, NC 27695 USA
[3] Univ Cantabria, Dept Fis Aplicada, E-39005 Santander, Spain
[4] Univ Trieste, Dept Fis Teor, I-34151 Trieste, Italy
[5] CNR INFM Democritos, I-34151 Trieste, Italy
[6] Univ Roma La Sapienza, Dipartimento Fis, I-00185 Rome, Italy
[7] Univ Roma La Sapienza, INFM CNR SOFT, I-00185 Rome, Italy