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