1. Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, I-56127, Pisa, Italy, and CNR-INFM, CRS “SOFT: Complex Dynamics in Structured Systems”, Piazzale A. Moro 2, I-00185 Roma, Italy
2. Naval Research Laboratory, Washington, District of Columbia 20375-5320, and CNR-INFM, Polylab, Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, I-56127, Pisa, Italy
3. Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan