1. Physics Department “E. R. Caianiello”, University of Salerno, via Giovanni Paolo II n. 132, Fisciano 84084, Italy
2. CNR-SPIN Salerno, via Giovanni Paolo II n. 132, Fisciano 84084, Italy
3. Department of Physical and Chemical Science, University of L’Aquila and CNR-SPIN L’Aquila, via Vetoio, L’Aquila 67100, Coppito, Italy
4. Department of Physics, Technical University of Denmark, Ørsteds Plads, 2800 Kgs. Lyngby, Denmark