[1] Inst Romand Rech Numer Phys Mat, PPH Ecublens, CH-1015 Lausanne, Switzerland
[2] Univ Trieste, INFM, I-34127 Trieste, Italy
[3] Univ Trieste, Dipartimento Ingn Mat, I-34127 Trieste, Italy
[4] Univ Lausanne, Inst Phys Mat Condensee, CH-1015 Lausanne, Switzerland
[5] Univ Kiel, Inst Expt & Angew Phys, D-24098 Kiel, Germany