1. Department of Physical and Chemical Science, University of L’Aquila, Via Vetoio, Coppito, 67100 L’Aquila, Italy
2. Department of Physics, University of Salerno, Via Giovanni Paolo II 132, Fisciano, 84084 Salerno, Italy
3. CNR-SPIN Via Giovanni Paolo II, Fisciano, 84084 Salerno, Italy
4. CNR-SPIN Via Vetoio, Coppito, 67100 L’Aquila, Italy