1. Dipartimento di Fisica, Università di Pisa, Largo Bruno Pontecorvo 3, I-56127 Pisa, Italy
2. CNR-IPCF, Largo Bruno Pontecorvo 3, I-56127 Pisa, Italy
3. School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
4. Institute of Natural Sciences, Shanghai Jiao Tong University, Shanghai 200240, China
5. Department of Physics, National Technical University of Athens, 157 80 Athens, Greece
6. Dipartimento di Fisica, Università degli Studi di Perugia, 06123 Perugia, Italy
7. Institute of Condensed Matter Physics, Technische Universität Darmstadt, Hochschulstr. 6, 64289 Darmstadt, Germany