[1] NEST, Ist Nanosci, CNR, Pisa, Italy
[2] Scuola Normale Super Pisa, Pisa, Italy
[3] Univ Konstanz, Dept Phys, D-78457 Constance, Germany
[4] Chapman Univ, Inst Quantum Studies, Adv Phys Lab, Burtonsville, MD 20806 USA
[5] Univ Salerno, CNR Spin, Fisciano, Italy
[6] Univ Salerno, Dipartimento Fis E R Caianiello, via Giovanni Paolo 2132, I-84084 Fisciano, Salerno, Italy