1. Department of Physics, University of Konstanz, D-78457 Konstanz, Germany
2. IHP—Leibniz-Institut für innovative Mikroelektronik, Im Technologiepark 25, D-15236 Frankfurt (Oder), Germany
3. Dipartimento di Scienze, Università di Roma Tre, V.le G Marconi 446, I-00146 Roma, Italy
4. Dipartimento di Fisica “E. Fermi,” Università di Pisa, Largo Pontecorvo 3, I-56127 Pisa, Italy
5. Physics and Materials Science Research Unit, University of Luxembourg, 162a avenue de la Faïencerie, L-1511 Luxembourg, Luxembourg