1. Department of Physics “E.R. Caianiello,” University of Salerno, via Giovanni Paolo II, 132, Fisciano, Salerno I-84084, Italy
2. CNR-SPIN Salerno, via Giovanni Paolo II, 132, Fisciano, Salerno I-84084, Italy
3. Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria