1. Department of Physics, “La Sapienza” University of Rome and CNR-INFM-SOFT, Piazzale A. Moro 5, I-00185 Rome, Italy
2. Department of Physics, University of Florence, Via G. Sansone, I-50019 Sesto
Fiorentino, Florence, Italy
3. Department of Physics, University of Perugia, Via A. Pascoli, I-06123 Perugia,
Italy